HomeRoast Digest

Topic: PID'ing Silvia (long) (65 msgs / 1500 lines)
1) From: Mark
Well it's not directly roasting related, but I don't follow alt.coffee much
(though I have searched the archives for PID info) and I'm certain there are
some here who've done the mod, and others who are considering it, so I
thought I'd seek some feedback here. First off, I don't know much about
electronics. I'm basically trying to copy what I've seen done without doing
anything irreversable. I purchased a used PID and a solid-state relay
through Ebay, and bought some self-adhesive type K thermocouples (I'll have
extras if anyone's interested, but I don't want to sell them until I've
completed the mod and am sure everything's working properly). Yesterday I
started the project, connecting the thermocouple and power only, just to see
what the boiler temp is using my standard procedure.
While Silvia is heating up, I notice that the temp varies from about 190 to
about 250 between cycles -- huge variation, to be sure, but the temp swing
gets smaller as the machine heats up. Going through my normal shot
procedure, minus pulling a real shot, I notice that the temp drops even
lower while I'm bleeding off the water for the start of my "temp surf", and
the displayed temp continues to drop after the boiler light comes on. After
my standard 45 sec. "surf", the temperature displays low 190's. By the time
the boiler light shuts off after approx. 1 min., the temp is around 205, and
it continues to rise to about 210. Consistency is not as good as I'd have
I had decided to use 229 as my "baseline" temp, since that seems a common
preference for those using PID's. However, it doesn't seem to want to get to
that temp on its own. So I prepare to pull a shot, deciding to use the steam
switch to bring the temp up a bit higher. I measure out my beans, grind, and
begin bleeding off water as normal. As the light comes on, I shut off the
water and allow it to heat, as I dose and tamp for my double shot. Light
goes off, temp reads 211, turn on the steam switch. Lock and load, temp
rises to 226, I shut off the steam switch, temp continues to rise. When it
hits 229, I start my shot. Everything appears as normal. Nice 25 second
double. I smack my forehead, realizing I shouldn't be trying a new blend for
this experiment. First taste, though, is good. I *think* the origin flavors
are more pronounced (hard to tell, since I haven't used this blend before --
50% aged Java "old brown" that I was trying to use up, 50% mocha Raimi). But
I've used up to 50% aged Java in espresso blends before, and the "funk" has
never been so potent. It is a bit overwhelming. The fruit from the Raimi is
there, but I get no chocolate. It seems to be hidden behind the funk. I'll
try another shot of this using my standard procedure, ignoring the temp
display, to see how it compares.
From my initial experience, though, I think I'm going to like having a PID
control. I'm surprised just how cool the temp was using my standard
procedure. I was also surprised to find that I needed the steam switch to
get to the temperature that a lot of PID users seem to prefer. I might move
the thermocouple to see if the temp reading changes significantly, but there
was no indication that the water was too hot from my first shot (no
perceived bitterness, crema not excessively dark), so I don't expect to see
a huge difference. Or I may just leave it where it is and play until I find
the temperature I'm happiest with.
Concluding, I have a lot of experimenting to do, but I wonder if my initial
results are close to what others have experienced. Or does the fact that the
temperature is reading so low after following my standard temp surf
procedures indicate a problem? Finally, among those who've had the patience
to read all of this, if there's someone familiar with electronics who'd be
willing to walk me through the rest of the mod (connecting the relay looks a
bit more complicated than I expected, but as I said above, I know little
about electronics), I'd appreciate if you'd email me. I can probably figure
it out, but I'd like to avoid making any dumb mistakes.
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2) From: EuropaChris
It sounds like you didn't disconnect the stock thermostat when you wired in the PID.  Or, did you disconnect the wrong one (steam thermostat)?.  The PID should be able to bring up the boiler to 229 without any intervention from you.  If it's stopping at 210, that's because of the stock thermostat shutting off (and it sounds like the newer, 100C thermostat to boot).
Chrishttp://www.execpc.com/~n9zes- see for PID info.
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3) From: Mark

4) From: Greg Scace
Hi Chris and Mark:
I get from Mark's post that he just hooked up the thermocouple and powered 
up the controller, and that the relay etc are not hooked up.  Am I right 
Mark?  If this is so, then I'd guess that your Silvia uses the 100C 
thermostat, not the 110C one.  One thing that you have to keep in mind is 
that you are measuring the temperature of the boiler wall just above the 
water inlet to the boiler.  You aren't really measuring the temperature of 
the water.  I'm presuming here that you have affixed the thermocouple to 
the same place as the stock thermoswitches (a reasonable choice). So things 
appear to happen that are'nt what you think.
When no water is flowing, you'll measure the temperature of the boiler wall 
as the boiler heats up and cools down.  Yeah it's a big swing and that's 
why yer gonna PID the thing.  Keep in mind that all machines that use snap 
action thermoswitches are gonna do this.  When you hit the brew switch, 
cold water is pumped into the boiler.  So the temperature that you are 
measuring goes down quickly because you're measuring the boiler wall 
temperature right near the cold water inlet.  What's actually being pumped 
to the portafilter is water at the bottom of the boiler, which doesn't mix 
immediately with the cold water coming into the boiler. This stratification 
keeps the brew temperature pretty darn constant as you brew your shot.
There is an easy explanation for why the temperature continues to rise 
after the heater light shuts off.  Again, you're measureing the temperature 
of the outside surface of your boiler, not the water.  The water is 
actually hotter than the outside of the boiler during heating.  As soon as 
the thermoswitch cuts off the power to the heater, the heating element 
stops heating, and the outside boiler wall temperature catches up with the 
water temp, which is not rising anymore.
For this reason, you are prolly getting your brew water too hot if you heat 
with the steam switch to 226F.  When your machine is PID'd, the controller 
will just tickle the heater to maintain the 229 F point.  So there won't 
really be much catching up of boiler wall temp to water temp.  What you are 
doing now is overheating the water, then starting the brew when your boiler 
temperature rises to 229F.  A better approximation would be to find the 
temperature at which you should shut off the steam switch that will allow 
the boiler temp to just barely coast up to 229F.  Then brew your shot.  I 
suspect it will taste better.  the 229 degree point on the outside of the 
boiler will yield Schomer's 203.5 F at the group, according to my small 
bead thermocouple measurements of brew water temp at the top of my coffee 
cake.  Schomer's number is only a number that he has found to work for his 
blend.  Other blends like different temperatures and you should 
experiment.  For example, I like Monkey blend at temps near to Schomer's, 
and I like Barry Jarrett's Decatur Street Blend at boiler wall temperature 
around 220.  Let yo taste buds be your guide.
One other thing.  I actuate the steam switch just before hitting the brew 
switch so that my heater is running full tilt when I brew.  This makes 
sense because you have cold makeup water to heat.   Brew water temperature 
studies of my Silvia showed that this was a good thing to do.  They were 
published on alt.coffee a coupla years ago and I bet Google turns em 
up.  Search for "Temperature Study of my Sylvia........looooong" or 
somesuch.  Silvia was spelled wrong in the title of my post on AC.
Now here's your quiz for the day:  What happens to the temperature that you 
measure when you open the steam valve and start frothing?  Can you explain 
this?  Have fun.
At 03:24 PM 1/6/2003 -0500, you wrote:
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5) From: EuropaChris
Cool.  Yeah, the 'new' 100C thermostat for the Silvia is actually too cool, whereas the older 105C (or is it 110C??) thermostat was too hot.  There's no chance of getting consistent shots with either.
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6) From: Mike McGinness
This list is evil, AAARRRRRGGGGHHHHHH!!! Just when I'm happy with my setup I
do the dumbest thing and read more. The reason's for adding a PID to Miss
Silvia seem to make so much sense. Maintain user selectable virtually
constant boiler temperature, eliminate the need to surf. Though the surf
isn't that big a deal to me, just knowing, really knowing, it could be
better just irks me! And Pepe's installation is so clean... though there's
something to be said for being able to revert to stock as Chris has done if
you wanted to. Oh woe is me, more to contemplate:-)
MM;-) aka Kona Krazy miKe mcKoffee
Variable Variac Rockin' Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
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7) From: Mike Gallant
Give in to the dark side, Mike... (so I don't feel so guilty about just 
placing an impulse order to ttiglobal... :)
On Monday, Jan 6, 2003, at 16:52 US/Pacific, Mike McGinness wrote:
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8) From: Rick Farris
Mike says:
No!  Mike!  Get a Livia!
-- Rick
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9) From: Greg Scace
Howzat go - a little knowledge is a dangerous thang  :)  .
come to da dark side.
At 04:52 PM 1/6/2003 -0800, you wrote:
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10) From: Mark

11) From: Greg Scace
Gee, dunno why you need this (a snubber).  The purpose is usually to 
eliminate electrical noise produced by a solenoid or other coil being 
energized.  That brings me to a question for you which is "Does your PID 
have a mechanical relay instead of a solid state relay on the back of 
it?"  You want a solid state relay because the mechanical ones have a finte 
lifetime and don't like to be cycled quickly.  For this job, you want to 
shorten up the cycle time to a nice short time like 5 seconds or less.
That location is reasonable.
Use something small like 1 Deg of less.  That isn't hysteresis really, if I 
understand it right.  Doesn't that particular setting tell the controller 
when to begin instituting PID control?  For instance if set to 1 Degree, 
the controller will be full on or full off unless inside the 1 degree 
window, when it will pulse the current according to the control parameters 
set by you or the controller.
So you would think.  And of course the steam temperature drops as the 
pressure drops within the system, but the boiler wall heats up as the steam 
leaves for the steam wand due to increased convection heat transfer.  So 
the indicated temperature rises a few degrees at first.
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12) From: Mark

13) From: Greg Scace
You're not switching an inductive load.  Your setup should be a PID 
controller with a solid state relay option (attached to it, usually inside) 
that switches a larger solid state relay that can handle 10 amps.  The 
larger relay switches the heater on and off.
None of that stuff is an inductive load so you don't need no stinkin 
snubber :)) .
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14) From: Rick Farris
Greg wrote:
I used to design/maintain machines with thousands (literally) of relays in
them.  I can tell you that modern relays have life cycles in the millions of
closures and cycle times in the milliseconds.
Not only that, but their "on" resistance is orders of magnitude less than
the on-resistance of an SSR.  In this application, where there is a fairly
large current flow, remembering that P=I*R, I'd be looking for a mechanical
relay to reduce the heat buildup in the PID.
How long *is* 5,000,000 seconds, anyway?
-- Rick
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15) From: Rick Farris
Mark wrote:
Maybe so, maybe no.  If your heater is a "heating coil," then yes, there
will be an inductive component to your load.  I can't think of a way,
offhand, to measure the inductance without either expensive equipment or a
fairly high "twidget" factor.
-- Rick
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16) From: Rich Adams
P being Power rated in watts.

17) From: Mark
That's good to hear. I should be able to finish this up tomorrow then.
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18) From: Mark

19) From: Greg Scace
P=I^2*R.  P=i*E refers to a tastee desert. :))  Yummmmm.
At 05:00 PM 1/7/2003 -0600, you wrote:
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20) From: dewardh
No.  The snubber is to protect the output device in the PID, not the load.  In 
any case the PID is *not* going to be connected directly to the heater, but to 
the input of a SSR, so any heater inductance, real or imagined, doesn't matter.
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21) From: David Lewis
At 3:31 PM -0500 1/7/03, Greg Scace wrote:
You don't. The snubber is needed when switching inductive loads. 
Otherwise, the kick-back from the load will, due to large di/dt, 
couple enough current through the parasitic capacitors in the SCR to 
keep it firing so you can't turn it off. Ask me how I know this... 
With a resistive load like a heater, that shouldn't be a problem.
"To announce that there must be no criticism of the President, or 
that we are to stand by the President right or wrong, is not only 
unpatriotic and servile, but is morally treasonable to the American 
     -- Theodore Roosevelt
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22) From: Mark

23) From: Greg Scace
I was mulling over your project the last day or so and I realized that you 
might want to set that threshold at which proportional control starts to a 
number larger than some arbitrary small one.  Here's why.
When you're heating up your machine, the water inside is a few degrees 
hotter than the boiler wall temperature.  If you turn off the heater, the 
boiler wall temp continues to rise for a few degrees as it catches up with 
the water temp.  Eventually the boiler wall temp becomes in equilibrium 
with the room and the hot water.  If you set the threshold value too small, 
then when proportional control starts, the boiler wall temp is gonna 
overshoot the setpoint by a fair chunk.  I'd bet that the right value for 
the threshold is several degrees.  Seems to me that the way to figure it 
out is to warm up your machine, then watch what happens temperaturewise 
when your heater cuts off by the thermoswitch.  Note the temperature when 
the heater cuts off, then monitor the temperature rise of the boiler 
wall.  Eventually the temperature is gonna stop rising and start to 
drop.  Record that maximum temperature and set the threshold value to the 
difference between the two temperatures or just a little larger.
Seems to me that you do want that threshold to be as small as you can 
get.  But the system is gonna dictate just how small that is.  Hope that 
PS - Hey, any of you controls guys (Rick Farris?) out there.  Got any 
suggestions to help Mark get his control parameters right besides looking 
up PID tuning methods on Omega.com?  I'm really not a controls guy.  I 
pretty much fake my way thru it after reading the instruction book.  RTFM 
you know.
At 10:08 AM 1/8/2003 -0800, you wrote:
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24) From: Rick Farris
Greg wrote:
These days, the devices I control are satellite communications equipment.
I'm fascinated by PIDs but have no experience with them.
-- Rick
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25) From: Dan Bollinger
There are dozens of internal PID settings on those gizmos.  There are two or
three levels in the menus. It will drive you nuts.  Here is what I do:
First, set it for a 'heating' application (not 'cooling').  Then, if your
unit allows, set if for AutoLearn or SelfLearn.  And, set it for a FAST
response.  When you do this, the unit will set its own PID values (at least
the LOVE controllers I use do).  Then, set the cycle rate for how often you
want the unit to make changes.  I'd start with 5-10 seconds on little
boilers.  Dan
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26) From: Mark
Well I did something really dumb. Yesterday I did hook up the relay and
powered everything up. It was working as expected. Turned on Silvia, boiler
doesn't come on, turn on the PID, and it starts to heat up -- for a few
seconds anyway. Then, sparks start shooting out of the relay.
After immediately unplugging everything, I examine the relay more closely.
Input voltage should be 3 - 32 V. Well, I can't believe I did something so
stupid, but as you might have guessed, I was using 110V. Anyway, I just won
an auction for another one on Ebay, a whopping $2, and this one's rated for
90 to 280 VAC. For now I'm back to using the PID's temperature readout only.
Fortunately there was no other damage. I'll be trying again when the new
relay arrives.
I did discover one other thing. I had noticed that the temperature was
fluctuating more and more, and didn't seem to be getting as hot; when I
opened Silvia back up, I found that the self-adhesive thermocouple had come
loose. It is now mounted under the screw that holds the stock thermostat in
place, so I won't have this problem again.
Thanks to those who've given feedback/advice so far. Hopefully I'll be
posting better results soon.
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27) From: dewardh
Be *sure* that's an *input* rating, and that you've allowed an ample safety 
margin on the current rating (I'd use nothing less that 15 amp) . . . check 
*all* the ratings carefully.
Also be espacially careful about mounting to the heat sink . . . it's hot in 
and around the boiler .  See the link below for heat sink information . . .http://www.ciicontrols.com/hints/ssrhints.htmDeward
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28) From: dewardh
Maybe I should expand on that . . . I don't remember ever seeing a SSR rated 
for 120V *control input* . . . (doesn't mean they don't exist, but . . .). 
 Were you using the "relay" output of the PID to switch 120V to the SSR control 
input?  Or were you "trying it out" with the SSR in free air to see if it 
worked before mounting to the heat sink?
Most PID controllers will have a 5V output sufficient to drive "hockey puck" 
SSRs directly
Most power SSRs will switch fine (often zero-crossing) with just that 5V drive
No SSR passing 10 Amps (or more) will last more than a few tens of seconds 
without a heat sink.
Maybe we're getting too "technical" to continue this on-list ? ? ?
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29) From: Jeffrey A. Bertoia
dewardh wrote:
They do exist....  if required.
Hockey puck SCRs generally require a particular amount of current to 
energize the gate.  The voltage really
has nothing to do with it....
This is not true.  It depends on how the SCR is rated.  Most of the 
hockey pucks that I deal with wouldn't
break a sweat at 10 amps regardless of the heatsink that is provided.
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30) From: dewardh
You're right . . . I went looking and found some listed.  Still don't remember 
ever *seeing* one, though .  And why would one use one with a PID 
controller?  Again going just by what I've seen (mostly Omega) I've never seen 
a 120V output on a PID . . . they're all, in my experience, either 5V, 4-20, or 
NO/NC relay contacts.  You'd have to go needlessly (and dangerously) out of 
your way to  get a 120V "control" signal out of them (and then in any real 
world application you'd then have to treat the control line like a power line . 
.. . conduit, mechanical protection, all that "stuff" that a low voltage control 
line doesn't require.  what control engineer worth his salt is going to want 
all those extra headaches? It's bad enough dealing with all the other problems 
without having to put your control lines in the raceway with the power . . .).
break a sweat at 10 amps regardless of the heatsink that is provided.
I'd like to see some part numbers for power SSRs that don't require any 
heatsink . . . (links to their data sheets would be particularly nice).  10 
Amps means close to 15 Watts dissipation in the device . . . where's that heat 
going to go ? ? ?  All the manufacturers data sheets that I've seen are notably 
emphatic about thermal considerations.  To quote from the link which I 
Adequate heatsinking, including consideration of air temperature and flow, is 
essential to the proper operation of a solid state relay (SSR). It is necessary 
that the user provide an effective means of removing heat from the SSR package. 
The importance of using a proper heat sink cannot be overstressed, since it 
directly affects the maximum usable load current and/or maximum allowable 
ambient temperature. Lack of attention to this detail can result in improper 
switching (lockup) or even total destruction of the SSR. Up to 90% of the 
problems with SSRs are directly related to heat.
All solid state relays develop heat as a result of a forward voltage drop 
through the junction of the output device. Beyond a point, heat will cause a 
lowering (or derating) of the load current that can be handled by the SSR. 
"Heatsinks" are used to create a method of removing heat away from the relay, 
thus allowing higher current operation.
With loads of less than 4 amperes, cooling by free flowing convection or forced 
air currents around the unit is usually sufficient. Loads greater than 4 Amps 
will require heat sinks.
If you know otherwise do post documentation . . . I'm always open to new and 
better devices.
energize the gate.  The voltage really has nothing to do with it....
The Crydom SSRs which I generally use specify control *voltage* . . . "typical" 
input current is noted, but is just as notably *not* the "turn on" 
specification . . . see the spec page below for a "typical" device family:http://www.crydom.com/pdf/cs.pdfIn my experience that's pretty much the industry "standard" way of specing 
things.  What devices do you use that are speced by input current? (not 
counting devices with a 4-20 "signal" line input . . . that's a whole different 
kettle of fish)
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31) From: Tom Gramila
On Thu, 9 Jan 2003, dewardh wrote:
Not 10 A, perhaps, but rated for 7A in "free air mounting. -- ie no 
heatsink.  (10A with a heatsink) :
http://www.magnecraft.com/products/section2_9_13-14.pdfGrainger sells these under a different label.
I am using the "25 amp style" to source 15A with no heat sink, and have 
had no problem.  (at $20, it was much cheaper than the cryodoms I could 
	Tom G.
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32) From: Rick Farris
Deward wrote:
Which, I might point out, is why I suggested a mechanical relay...
-- Rick
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33) From: dewardh
heatsink.  (10A with a heatsink) :
http://www.magnecraft.com/products/section2_9_13-14.pdfYou believe in living dangerously .  I would *never* run a device rated 7A 
max. at 38C in free air at 7A in free air (unless the "free air" was outdoors 
in a gale in the Arctic in the winter ).  And every other curve, on every 
other graph on the page, is *with* a heat sink. And, in the lower right corner 
of the first page:
And in the "General Specifications" *every* device has a "Suggested Heatsink 
I'd take that as Gospel . . .
Ps. there *was* a time when I was less "religious" about such things . . . I 
hate callbacks . . .
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34) From: Dan Bollinger
4-20, or
fyi: The LOVE Fuzzy Logic PIDs we use a work have 120 or 240 outputs that
drive 50A 3-phase relays.  Using line voltage for controls has the advantage
of less components (no bulky control transformer) and smaller wire size. Dan
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35) From: AlChemist John
Sometime around 17:43 1/9/03, dewardh typed:
Unless other objections are raised, I like the discussion about PID's and 
enjoying this.
John Nanci 
AlChemist at large
Roasting and Blending by Gestalt
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36) From: Jeffrey A. Bertoia
Tryhttp://www.ciicontrols.com/products/ss.htmTheir 40A runs 10A at room temperature and their 75 Amp model is _WELL_ 
But significant heat sinking can be gained simply by bolting it to a 
metal case.
dewardh wrote:
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37) From: EuropaChris
I have a 120V control input SSR on my Silvia.  My PID has only relay outputs, so I have to feed the output relay with 120V which then gets sent to the SSR to turn it on and off.  
So far, almost two years of use and it works flawlessly.  SSR Drive output PID's are nicer (no relay clicking) but either one works very well for our application.
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38) From: Jeffrey A. Bertoia
dewardh wrote:
That depends on what process that you are trying to control.  In several 
cases I have used a
110V N.O. contact out of PID controllers to control my process directly 
with no additional components.
See other post with Continental Industries link and info.
OH...  Your I misread your original point here.  Note that I was 
addressing hockey puck SCRs  not
panel mount SSRs.
As regards panel mount SSRs you are correct.  
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39) From: Tom Gramila
On Thu, 9 Jan 2003, dewardh wrote:
(about SSR's):> Tom:
	In this case, the "callback" is not such a big deal, its just a 
walk to the back porch....  (g)
	I would agree that for a commercial installation, using a 25A 
relay to run 15A without a heatsink is C-R-A-Z-Y, but if I took an 
industrial bombproof approach to modding up my popcorn popper, it would 
likely never see beans! 
 Of course, your approach would have probably saved me the destruction of
my first popper, whose heater ended up being turned on while the fan was
off, when my attention was focused on replacing the 20A fault interrupt
outlet that failed at 11:30 PM on christmas eve.  Having an "fan-on"
interlock on the heater wire would have been nice, but then I would have
missed all the exciting smoke and all!
	I expect to eventually put a 6" aluminum plate on the ssr -- 
probably when I get around to using a boost/buck transformer to step up 
the voltage supply (re - your earlier posts) so I can free up the now 
inefficiently used 20A variac.
Tom G.
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40) From: jim gundlach
While I have not participated in the discussion, I have learned a lot 
from reading the discussions.  I would vote for your continuing the 
discussion here so I can go to the archive and re-read this discussion 
if and when I decide to play with this kind of electronic control.
Jim Gundlach
roasting over pecan wood fires.
On Friday, January 10, 2003, at 07:10 AM, AlChemist John wrote:
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41) From: dewardh
Because of no shield?  Why else?
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42) From: dewardh
But where do you have "room temperature"?  Probably not inside the box that 
keeps your fingers off the terminals.  Which makes more sense . . . a proper 
heat sink, or hanging the device over the kitchen sink with a fan pointed at 
In device ratings "ambient" doesn't mean the temperature in the (air 
conditioned) front office, or that the weatherman is reporting, or even on the 
shop floor . . . it means the temperature *in the immediate environment of the 
device*, generally in a box in or on (or very near to) the device controlled. 
 And "free air" doesn't mean 1/4 inch clearance around the gadget, it means 
that the device itself is actually exposed to at least some convective flow.
Yes . . . unless the "metal case" is part of a Silvia, in which case it might 
be more a "source" than a "sink" .
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43) From: dewardh
which then gets sent to the SSR to turn it on and off.
That makes sense . . . it does spare you having to provide a control voltage, 
since the 120 is already there on the back of your PID.  It's a perfectly 
reasonable way to deal with what you've got . . .
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44) From: dewardh
cases I have used a 110V N.O. contact out of PID controllers to control my 
process directly
with no additional components.
Well yes . . . but few PIDs include a relay rated for a 10-15 Amp load (like 
the Silvia heater), and it's considerable trouble and expense to open up the 
PID to replace a fried relay.  And it's not uncommon to want to (electrically) 
isolate the control panel from the device controlled . . .
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45) From: dewardh
my first popper, whose heater ended up being turned on while the fan was
off, when my attention was focused on replacing the 20A fault interrupt
outlet that failed at 11:30 PM on christmas eve.
We all "learn by destroying (doing?)" . . . (someone will probably chime in and 
say it's a "boy thing" ).  I learned long ago to "remember the (generalized) 
lesson, forget the incident" . . . (except for the really funny (or really 
disasterous) ones).
I'd probably "soften my image" if I posted some of my more spectacular 
"learning experiences", but they'd all be way "off-topic" so I'll spare myself 
the embarrassment  . . .
I just don't want to see someone (with a boiler machine) bypass the 
pressurestat (for "better control") and then fail "on" an SSR . . . potentially 
sending shrapnel across their kitchen.  However small the risk is one should 
still never underestimate the danger of steam, or the fallability of a whole 
raft of "safety devices".  I've seen pressure releases jammed with scale, and 
even burst diaphragms clogged with sludge or installed with a too-small line to 
the blowdown tank.  I count myself lucky that I've not seen someone seriously 
hurt . . .
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46) From: Dan Bollinger
Higher voltage, less current, smaller wire size for the same task.  A glance
at your 12V jumper cables shows why.  By the time I get done stuffing an
enclosure with wires every little bit helps.  I'd rather string 22 or 18ga
than 16ga. It doesn't seem like a big deal until you get 20 wires in a
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47) From: Mark

48) From: dewardh
I thought we were talking control wires here, not power leads . . . Cat5 would 
work fine regardless the voltage . . .
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49) From: Dan Bollinger
CAT5 is what, 22ga?  That's too small for me. Relays have high inrush
values. Also, what is the insulation rated at?  I'd be surprised if CAT5 was
600V.   Plus, I prefer to us stranded wires.  Almost everything I make is
attached to a machine and vibration is a consideration. Dan

50) From: dewardh
That should work, although 10A is awfully close to the actual load current of 
the Silvia heater, isn't it?  Modest safety margin on rated current makes 
device cooling more important.
the relay mounted inside of Silvia, where it definitely does get hot.
I'd look for the largest, coolest, outside exposed metal surface, and mount on 
that, if I couldn't put the device outside the box altogether.  Don't forget 
the heat sink compound, and consider a backing plate to keep the sheet metal 
from warping away from and losing contact with the back of the device when you 
tighten it down (a backing plate would provide some additional sink, too). 
 Another alternative might be to mount a heat sink on the back panel (fins, or 
the whole thing, outside the box) and mount the SSR on it (through a clearance 
Since I already have 110 to power the PID, that's what I'm using.
Got it . . . Chris posted a good explanation of why that makes sense . . .
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51) From: Marchiori, Alan
I'm not a controls person, but I am an EE.  I checked out the SSR and am
somewhat worried if it is big enough.  It says "24 to 330 VAC 10 Amp".
Which is interesting becuase they don't say at what voltage it is rated for
10 amps at.  Without that spec I would error on the side of being safe and
assume it's rated for 10 amps at 24 VAC.    Now P=iv, so the max rated power
is 24*10$0 watts.  I don't know the specs on Silvia's heater but I thought
I was reading something like 10-15 amps, which I assume to be at 110 volts.
So Silvia's heater going to pull something around 10*110 = 1,100 watts.
Now, you can see the problem here if the relay is only rated for 240 watts.
If I have made some mistake in my reasoning, please someone correct me.
So, you really need a relay rated for at least 1100/24 = 45.8 amps at 24
VAC.  Or 10 amps at 110 VAC.  etc etc etc...

52) From: dewardh
22 or 24, I believe . . .
Mechanicals, yes, SSRs, no . . .
Mostly rated "service" to 225V, breakdown to 600-700, or something like that, I 
believe . . . (but remember, I'm not a fan of 120 volt control lines ).  I 
find a surprisingly wide variety of insulation ratings . . . "Plenum" cable 
tends to be rated to match whatever might be pulled along with it.
Cat5 is available solid or stranded . . .
Vibration resistance can be a problem with any device or any wire . . . I'd 
think twice about putting a mechanical relay directly on a vibrating machine 
(SSR would be more reliable), but wouldn't wire it with #12 solid either.  The 
flexible connector you use at the machine is not necessarily the wire you want 
to use all the way back to the panel (either power or control).
We're getting a long way from the Silvia here . . .  . . .
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53) From: Dan Bollinger
You ARE talking about a relay here and not a SSR, right?  If so, then you
should know that relays are sized by the inductive load they can carry.  The
resistive load they can carry is about 20% higher.  In this case, the relay
can probably handle a 12A heater.  If you get the relay's specs, this is
mentioned in the fine print.  Dan
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54) From: dewardh
The current rating applies over the entire range of rated voltage.  The primary 
factor involved is the forward voltage drop (when on) across the device . . . 
that's typically about 1.6 volts at max current, regardless of supply voltage. 
That voltage drop is where the heat comes from (P), and dealing with the 
heat is what determines the rating of the device (right down to heat transfer 
and the size of the junction on the Silicon itself).  The key issue with the 
devices, as long as you don't exceed their rated current or breakdown voltage, 
is keeping them cool.
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55) From: dewardh
No . . . we're talking specifically SSR.  Look at the e-bay link that Mark 
posted . . .
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56) From: Tom Gramila
	Your mistake is using the wrong voltage in P.  the voltage 
should be the voltage drop across that circuit element, and not the 
total voltage in the circuit.  The voltage drop across the SSR doesnt 
change very much as the supply voltage is varied, so that 10A works for 
a wide range of voltage.  
	Its not like a transformer where all the current and voltage
matter, more like two resistors in series:  suppose we put 110V across a
series combination of 10 ohms and 100 ohms. The total power disapated is
110 Watts, but only 10 W are disapated in the 10 ohm resistor.  (The
analogy fails in some details: the SSR is not a resistor, -- its voltage
drop doesnt scale like the current, But I think this example illustrates 
the idea...)
On Fri, 10 Jan 2003, Marchiori, Alan wrote:
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57) From: Dan Bollinger
My mistake, I got lost in the PID avalanche!  Dan
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58) From: Marchiori, Alan
Thanks it's all making sense now.  Not to deviate too much from coffee
roasting but, let me think outloud for a minute... So the max rated current
& voltage are pretty much unrelated (to the SSR at least, not the load).
The forward voltage drop is dependant on the current.  Power dissapation in
the SSR=Vdrop*I.  Vdrop is proportional to current so the power dissapation
in the SSR is proportioanl to I^2 and has nothing to do with the system
voltage.  So the SSR will dissapate the same amount of heat at 10A as
10A, even though more power flows at 330 volts.
Ok that was fun, back to my digital world of 0's and 1's.

59) From: Greg Scace
According to the seller, the AC control part is 90 to 280 VAC and the thing 
is rated for 10 Amps.  Silvia draws 800 watts at 115V.  Doesn't that mean 
that the heater element has impedence of 16.5 Ohms?  So if one only ran 90 
VAC thru the heater, the current would be only 5.4 Amps.  At 120 VAC, the 
current would be 7.26 Amps.
I question the 24V to 330 V part.  What's the output of the PID controller 
RE heat sinks and relays, I have successfully used a Crydom 10A hockey puck 
screwed to the side of an aluminum box that houses my controller and the 
relay.  According to Crydom's applications guys, that is a sufficient heat 
Mark:  Make sure the output of your PID matches with the relay both in 
voltage and AC or DC.  Make sure the relay can handle 10 amps of 120 VAC on 
the load side.  If you have any questions about use of a relay, call the 
manufacturer's applications engineers.  They will be happy to help 
you.  You can usually get the phone number for applications from 
manufacturers websites.  Don't be bashful. I'm really clueless on lots of 
stuff and have learned that calling the manufacturer usually sets me straight.
At 01:54 PM 1/10/2003 -0600, you wrote:
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60) From: Angelo
This subject is very interesting to me, as I would like to PID my Silvia. 
However, as with most topics on this list, the discussion gets to the point 
of the pinhead-dancing angels . What I, and others who wish to make this 
modification, need is a basic "parts list"...I realize that there are too 
many manufacturers and models to give specific recommendations, but if you 
knowledgeable guys could come to some agreement on necessary specs/features 
of an acceptable PID and relay.
It would be nice to have a check-list with which to compare the various 
devices. If people could recommend specific manufacturers/models, this 
would be crema on the espresso...
It would also be good if the "nice-to-have-but-not-absolutely-necessary" 
features could be listed as such....
Thanks in advance,
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61) From: Mike McGinness
I couldn't have said it better Angelo. I'm so confused! Course I'm currently
playing with variacs and transformers and potentiometers for roasting
control so PID'ing Miss Silvia will just have to wait:-)
MM;-) aka Kona Konnaisseur miKe mcKoffee
Variac Rockin' Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
From: "Angelo" 
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62) From: Rick Farris
Alan wrote:
The power rating of the control device is how much power that *it* must
dissipate, which is a function of the current through the device and its
resistance.  If the forward resistance of the device is 10 ohms, for
instance, then at 10 amps, the control device will be dissipating 100W, no
matter how much the supply voltage is.  (That's why for high-current
applications I recommend a mechanical relay, where the forward (and
reverse!) resistance will probably be in the milliohms.)
-- Rick
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63) From: David Lewis
At 1:54 PM -0600 1/10/03, Marchiori, Alan wrote:
Ah, a subtle error, grasshopper. The dissipation depends not on the 
voltage across the load (the 330 V only refers to the off-state 
voltage the device will withstand), but on the voltage across the 
device in the relay when it's on and carrying current. The power 
dissipated is therefore 10 Amps times the 1-1.5 Volts dropped across 
the pass device.
	David (the *old* EE)
Less than 0.1 percent of the U.S. population gave 83 percent of all 
itemized campaign contributions for the 2002 elections, according to 
the Center for Responsive Politics.
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64) From: Dan Bollinger
Or 10-15 Watts, right?  Dan
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65) From: Mark
(fins, or
That sounds like a good plan. I ordered a heatsink and some thermal compound
from an electronics surplus place that looks like it'll work. I'll cut a big
hole in the back of the enclosure, mount the heatsink on the outside, and
mount the SSR to the heatsink. Should have all the stuff sometime this week.
Your advice is much appreciated.
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