HomeRoast Digest


Topic: controlling a 240v roaster heating element with PID (35 msgs / 806 lines)
1) From: Michael Wascher
Tom,
You really don't need the neutral. From hot to hot is 240 volts. From
hot (either one) to neutral is 120 volts. Your stove or dryer has all
three wires run to it. Will connect the heating element from hot to
hot for 240 v, uses the hot to neutral 120 V to run controls,
displays, light bulbs.
How many connections to your heating element? Most likely it is
connected from hot to hot, and the SSR is just in series. It doesn't
matter which side you use. Current flows in one hot line, through the
heating element, and out the other hot line.
If your heating element is "center tapped", the ends connect to the
hot & the center connects to neutral, then you need to switch both hot
sides. Just use an SSR in each hot side, tie both controls together
(e.g. use the same controller to run both of them).
BTW, the two sides are more than "slightly" out of phase. They are
inverted versions of one another (180 degrees out of phase).
This'd be easier with a pen & paper! Would a phone call help?
--MikeW
On Sun, Apr 13, 2008 at 5:04 PM, Tom & Maria - Sweet Maria's Coffee
 wrote:
<Snip>
-- 
"Excess on occasion is exhilarating. It prevents moderation from
acquiring the deadening effect of a habit." --W. Somerset Maugham
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2) From: Bob Hazen
Tom,
The 240V 3-wire service you have has two hot lines that are not just 
slightly out of phase but 180 deg out of phase with respect to the neutral. 
This is how you get 120V on each relative to neutral, but across both hots 
its 240V.  When one side is at its positive maximum of the sine wave, the 
other is at it's minimum (negative).  If they were exactly in phase, you'd 
get a net zero.
Since you're not using neutral, you could use an SSR on one side to control 
the circuit.  The return path is through the other side and you only need to 
break one side.  Remember, though, that even with the SSR off, you'd still 
have voltage present from the other side relative to ground in your heater. 
No conductive path through the heater, but one through YOU to ground should 
you put your mitts on it.  So for safety's sake, use a proper dual-pole 
breaker with the circuit.
If you use SCRs, bear in mind they will latch until their current passes 
through zero during each cycle.  If you have enough inductance in the heater 
you may have trouble getting them to turn off when you want them too.  I 
don't think you need to control every cycle or half-cycle as you likely 
don't need response times in the tenths of seconds, so why bother with SCRs 
and having to sync them to the line.  I'd skip the SCRs and go with Rich's 
idea of SSRs.  Depending on the thermal mass and how much variation in 
temperature you can live with, control cycles of 10's of seconds are 
probably fine; maybe approaching a minute.
If you want to sketch something up and send it my way, I'd be happy to look 
it over.  Likewise, if you have questions, feel free to ask.
Bob

3) From: raymanowen
I'm hate to unnecessarily complicate this, but the pair of SSR's or pair of
Diac gated Triacs using a 4 - 20 mA PID controller might give the smoothest
control of any regulation scheme.
For a turnkey setup, contact an Omega Engineering
application engineer.
Instead of On/ Off switching, The PID controller would quickly "learn" the
hysteresis factor and go to steady state power (including gas valves)  for
any temperature or ramp profile you define.
A multipoint chart recorder could plot temperatures at several different
points in the roaster during a roast, as if you needed it for roast-to-roast
comparison.
Expensive way to go? Not if the alternative involves the destruction of a
rare roast, your equipment or your building.
Cheers, Mabuhay -RayO, aka Opa!
On Sun, Apr 13, 2008 at 3:38 PM, Bob Hazen  wrote:
<Snip>
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita (ex- NYC Paramount) WurliTzer- 1976
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4) From: Tom & Maria - Sweet Maria's Coffee
Thanks so much Michael Bob and Rich - I get it now, and a bit more 
too. AC isn't rocket science  but I never seem to grasp it ... of 
course, in this case the way I should wire the SSR is so simple that 
NO WONDER I couldn't google the answer. Ok, I gave it a test and an 
seems to work. Yes, my burner has just 2 hot wires, and I can test 
each at 120, or across them at 240 (actually, it runs a bit hot: 248 
or so). Since Sweet Maria's was basically a tire retreading shop, we 
have some fairly nice electric service here, but always a b it on the 
hot side. (a good problem to have than the opposite). Thanks for 
explaining the SCRs and Micahel - the 180 opposite phases.
Tom
<Snip>
--
                   "Great coffee comes from tiny roasters"
            Sweet Maria's Home Coffee Roasting  -  Tom & Maria
                      http://www.sweetmarias.com                Thompson Owen george_at_sweetmarias.com
     Sweet Maria's Coffee - 1115 21st Street, Oakland, CA 94607 - USA
             phone/fax: 888 876 5917 - tom_at_sweetmarias.com
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5) From: Michael Wascher
Excellent!
I should be vacationing in SF next month, I hope to pick up an order
of beans. I'll check your work while I'm there!
;)
--MikeW
On Sun, Apr 13, 2008 at 7:35 PM, Tom & Maria - Sweet Maria's Coffee
 wrote:
<Snip>
-- 
"Excess on occasion is exhilarating. It prevents moderation from
acquiring the deadening effect of a habit." --W. Somerset Maugham
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6) From: Jim Gundlach
Tom,
     Your wiring has two hot 110-120 circuits and one ground.  When  
you connect the two hot ones you get your 220-240 volt circuit.  Just  
put the relay on either one and it will open and close the 220-240  
volt circuit.   You DO NOT want to connect the two hot 110's and tie  
them to the ground.  That may seem like the logical way to get 220,  
110+110"0 right?   It does not work that way,  doing it that way  
burns wires and throws circuit breakers.
        pecan jim
On Apr 13, 2008, at 4:04 PM, Tom & Maria - Sweet Maria's Coffee wrote:
<Snip>
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7) From: Dave Bush
<Snip>
Tom,
I've only recently subscribed to this list despite being a customer  
for a couple years and just sat down to submit my first question when  
I saw your posting. It appears that we may be working on very similar  
projects. I'm building a Sivetz-like fluid bed roaster using a 4750W  
240V clothes dryer heating element controlled by a PID. How about  
yours? As an electrical engineer I'll also vouch for the accurate  
advice you've received on how to hook up the SSR. Let me add for  
safety's sake that you make sure all metal parts are grounded and that  
you use a GFI breaker for extra protection. Great coffee is no reason  
to risk getting hurt.
My first question is how hot should the air be as it's entering the  
bean container? And secondly, what bean temperature profile(s) would  
you recommend programming into the PID? I've been looking, but haven't  
found much so far. Since I expect to be roasting 1 to 1 1/2 pounds at  
a time I don't want to do any more experimenting than necessary.
Thanks,
Dave
<Snip>
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8) From: Michael Dhabolt
Tom,
Easy to get confused when thinking about the common/ground wire in a
220/240 V circuit.  Best thing to do is to trace that wire and you'll
find that it hooks up to the frame or structure of the machine
(possibly the heater housing). Now forget about it for a while.  The
other two wires have an electrical potential of 220/240 volts across
them, interrupting that circuit in any fashion will turn the circuit
off.  Put the SSR into the circuit at either end of the heater element
- one wire from a hot wire at the current source to terminal one on
the SSR, terminal two on the SSR should feed one leg of the heater and
the other leg of the heater goes directly back to the second hot wire
at the current source.  Terminal three on the SSR should be hooked to
the positive output terminal from the PID and terminal four on the SSR
should be hooked to the negative output terminal from the PID.
I would expect that you are familiar with some local shops who have
PIDed their espresso machines (I've been doing LM Lineas in this
area).  A quick gander at one of these machines will put all this in
perspective......they have 220/240 V heaters and have the PID, SSR and
heater hooked up exactly the way you are talking about.
Back to that pesky common/ground wire.  Different folk have different
attitudes about it and how it is used and blended into a circuit,
codes differ and marine versus landlubber uses and safety attitudes
can confuse anyone. Primarily always remember that if you complete a
circuit between either of the 'hot' wires to the common/ground you
have half of the 220/240 V ..... 110/120 V.  If you complete this
circuit with a portion of your body - you'll light yourself up.  You
can use this 110/120 V to provide power to something like the PID you
are using if it needs the lower voltage.  Most PIDs function with
either voltage supplying the instrument, and you are always better off
making things as simple as possible (stay with 220/240 V for
everything).  Most european circuits use three wires and that common
wire becomes ground at the machine, a lot of american circuits use
four wires and the fourth is ground (that third common wire would not
be hooked to anything unless you needed the 110/120 V for something).
It is really a good idea to use a double pole switch (switches both of
the hot wires) at the power source for this type of circuit.  A single
pole switch (switches only one of the hot wires) leaves the entire
circuit at a potential of 110/120 V between itself and ground.
I hope my simplistic explanation doesn't offend any folks with in
depth knowledge of this.  I also hope that if any of those folks see
any safety concerns I've not addressed, they will do so.
Mike (just plain)
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9) From: raymanowen
"Just put the relay on either one and it will open and close the 220-240
volt circuit."
No electrical code would allow that set-up. Even though it will work fine on
paper, safety engineering has to account for the worst case failure.
One of the two heater legs is unswitched and always has power applied.
The failure will happen that one of the ceramic insulators will break,
allowing the Nichrome heating wire to short to ground.
If the unswitched leg grounds out- no problem. It will just trip the breaker
with no further damage caused.
If the switched leg grounds out- no problem again. The breaker will trip as
soon as the relay closes and the current surges.
If the heater grounds near midpoint, the section between the unswitched leg
and the ground will heat at moderate to high current and power, so the
breaker won't trip. It will continue to heat uncontrolled on that half.
As soon as the fire inspector sees the home brew power switching
installation, operations would be halted and the fire insurance invalidated
until the installation was corrected.
Bob* called in a panic one day (normal mode- he never called until something
was completely broken) and said "Help, Ray- the fire marshall just shut me
down..."
*Not his real name-
It does happen- better that than a devastating fire, and I've seen that,
too.
In "220V" electric ranges, The broiling, roasting and range top heat
elements always have both legs switched. Switching a single leg would work,
but the potential exists for a grim visitation.
The one-armed disconnect switch boxes open both or all three legs in single
phase or 3 phase circuits. Same reason- Off means No blue fire when you
leave for the day, and no jolt when troubleshooting.
Cheers, Mabuhay -RayO, aka Opa!
Don't have time/ can't afford to do it right? Ooh- Kay!
Uhhh, Mr. Owen- uhhh- can you do it right, ASAP?
On Sun, Apr 13, 2008 at 5:53 PM, Jim Gundlach 
wrote:
<Snip>
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita (ex- NYC Paramount) WurliTzer- 1976
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10) From: raymanowen
The GFCI depends on no current in the ground wire, and equal current flow in
the Line and Neutral wires. Since the GFCI has been integral with bathroom
appliances and mandated on new bathroom construction, it has reduced organ
donations.
Cheers, Mabuhay -RayO, aka Opa!
Got Grinder?
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11) From: Ira
At 04:35 PM 4/13/2008, you wrote:
<Snip>
It's OK to use 1 SSR to control the heat, but only if you also have a 
big double pole switch to shut the machine down. A single SSR always 
leaves the element electrically hot and dangerous to touch even when 
it seems off.  If it was me I'd use 2 SSRs driven by the same signal 
to switch both sides at once and a nice big switch or relay to turn 
off the power to them when I don't need the heat.
Ira
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12) From: Mike Chester
Tom,
I hope that you are using two SSRs, one on each leg of the 220.  One will 
turn the element on and off, but it would be very dangerous as there would 
always be power present on one side of the heater.  This would be an 
accident waiting to happen.
I have worked with electricity all of my life and the goal should not be to 
simply get something to work, but to get it to work safely. I don't fear 
electricity, but I do respect it.  To take this to the ridiculous,  I could 
theoretically run bare wires up the outside of my walls to power a ceiling 
light and it would work.  Obviously that would not be very safe, but it 
would work.
Please don't take this wrong, but you seem to have only a rudimentary grasp 
of electrical circuitry.  This is the level of knowledge where you are most 
likely to hurt yourself or someone else.  A person who knows nothing and is 
afraid won't touch anything and thus not get hurt.  A person who is educated 
about circuitry will know enough to do it properly.  If you are not sure 
what you are doing, it might be better to not try to do this yourself.  Many 
of us on the list are educated enough about electricity to set up what you 
want and may even dismiss it as very basic and easy, and for us it would be, 
but that does not mean it would be that way for you.  When a friend of mine 
was a new apprentice, he caused an electrical explosion that he was very 
lucky to survive.  The journeyman that he was working with (very foolishly) 
quickly explained how to do the job and left him alone to work on a live 480 
volt bus bar.  The apprentice thought that he understood and was following 
instructions, but ended up sticking a screwdriver into the bus duct and 
shorting out the bars.  This created a large explosion and fireball that 
just missed his face.  He was very lucky and only suffered a temporary 
hearing loss and ringing in his ears for several weeks.  I could tell many 
more scary true stories, but I think you get the idea.
I am sorry for going on about this, but I don't want to see you get hurt.
Mike Chester

13) From: Michael Wascher
The electric code addresses external wiring, it is for main power or
fuses/breakers. Yes, there should be a main switch that breaks both
sides (both hots). Yes there should be fuses/breakers on both hot
lines. But the code was never intended for internal circuitry in a
piece of equipment.
There should be an on/off switch on the unit, first thing that the
incoming power lines are wired to, that switches both sides off. But,
an internal control like an SSR does not need to switch both sides.
--MikeW
On Sun, Apr 13, 2008 at 11:38 PM,   wrote:
<Snip>
-- 
"Excess on occasion is exhilarating. It prevents moderation from
acquiring the deadening effect of a habit." --W. Somerset Maugham
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14) From: Michael Dhabolt
Not Tom here.
Depending on the PID you are using, I may have something you can use.
email me direct michael.dhabolt if you are interested.
Mike (just plain)
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15) From: Bill
Dave, welcome to the list!!  You didn't submit your question, at least I
didn't see it!
So, what have you been roasting?  How you gonna brew it?  Can't wait to
hear!
bill
welcome again!
On Sun, Apr 13, 2008 at 6:04 PM, Dave Bush  wrote:
<Snip>
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16) From: Brian Kamnetz
Bill,
Here are Dave's questions:
My first question is how hot should the air be as it's entering the
bean container? And secondly, what bean temperature profile(s) would
you recommend programming into the PID?
Brian
On 4/14/08, Bill  wrote:
<Snip>
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17) From: Michael Mccandless
Or . . .
Look for a DPST Solid State Relay.
DPST=double Pole Single Throw.
One relay will switch both legs.
McSparky
On Mon, Apr 14, 2008 at 12:56 AM, Ira  wrote:
<Snip>
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18) From: Michael Mccandless
For example:
Relay
McSparky
On Mon, Apr 14, 2008 at 2:24 PM, Michael Mccandless 
wrote:
<Snip>
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19) From: Mike Chester
<Snip>
It is NEVER OK to leave one end of a load hot when the device is supposed to 
be off.  In over 30 years of designing and servicing electrical devices, I 
have never seen that done on a professionally designed device.  Aside from 
the danger of leaving live wires out there which could be touched, what 
Ray-O pointed out earlier is spot on.  If the element or wiring should 
develop a ground , the element may come on at a time when it is not supposed 
to which could cause a fire.  Use 2 SSRs or a 2 pole SSR or a 2 pole 
contactor to control the heater.
Mike Chester
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20) From: Rich
Somehow I do not believe that Tom is building a device that will be sold 
to the masses.  Depending on the type of heater, there is next to no 
risk in controlling it while in operation at one leg only.  "No user 
serviceable parts inside."
Mike Chester wrote:
<Snip>
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21) From: Ira
At 02:37 PM 4/14/2008, you wrote:
<Snip>
But that's a 10 amp relay, not really useful.
Ira
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22) From: Michael Wascher
It's not "off". The SSR is being used to modulate the current to
control the heat.
There has to be a switch, which should be DPDT, to turn the unit
"off". This turns everything "off", the SSR, the control circuit,
everything.
Both sides of the line should be fused. Look in your fuse box, the 240
V circuits have two breakers that are mechanically linked. When one
breaker blows they both open.
--MikeW
On Mon, Apr 14, 2008 at 5:39 PM, Mike Chester  wrote:
<Snip>
-- 
"Excess on occasion is exhilarating. It prevents moderation from
acquiring the deadening effect of a habit." --W. Somerset Maugham
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23) From: Michael Mccandless
It's a safety issue - common sense.
Always better safe than sorry (dead).
McSparky
On Mon, Apr 14, 2008 at 3:07 PM, Rich  wrote:
<Snip>
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24) From: Tom & Maria - Sweet Maria's Coffee
That would be interesting to use 2 SSRs since the PID can output to 2 
relays. This is a device just for my use, but I am being very 
careful. I have the appropriate 2 pole breaker nearby, and since it 
is a plugged machine, I can simply unplug it and know that I won't 
get unwanted electro-shock therapy. I am having a bit of trouble with 
my older Omega CN8500 PID but just downloaded the manual off their 
site - what a great company...
Maria and Ben and I are leaving for Guatemala tonight (Ben's first 
coffee trip!) so the project will be on hold for a week...
Tom
<Snip>
--
                   "Great coffee comes from tiny roasters"
            Sweet Maria's Home Coffee Roasting  -  Tom & Maria
                      http://www.sweetmarias.com                Thompson Owen george_at_sweetmarias.com
     Sweet Maria's Coffee - 1115 21st Street, Oakland, CA 94607 - USA
             phone/fax: 888 876 5917 - tom_at_sweetmarias.com
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25) From: Ira
At 04:11 PM 4/14/2008, you wrote:
<Snip>
You don't need 2 outputs. You need 1 double pole relay but they tend 
to be more expensive so 2 single pole relays with the same control 
signal is just perfect.
Ira
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26) From: Brian Kamnetz
On 4/14/08, Tom & Maria - Sweet Maria's Coffee
 wrote:
<Snip>
Safe travels, Tom. I look forward to the photos!
Brian
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27) From: Bill
Not that I want to get into a discussion about another's post (although I AM
responding, so i must in some small way...), but he was going to post a
question before he saw the post from tom...
so i suspect he still has a question.  That would be wildly eerie if his
question was about air temp in the roaster he was building...and tom just
posted about that... but that could be it... that would be crazy!  Hope Dave
comes back and lets us know!
we'll see...
bill
And Dave, still welcome!
On Mon, Apr 14, 2008 at 2:28 PM, Brian Kamnetz  wrote:
<Snip>
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28) From: Bill
Safe Travels, Tom, Ben, and Maria!  Brian beat me to it, but I'm excited
about the photos, too!!!  (I beat him with more exclamation points!!!)!!!bill
 !!!
On Mon, Apr 14, 2008 at 5:11 PM, Tom & Maria - Sweet Maria's Coffee <
sweetmarias> wrote:
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29) From: Brian Kamnetz
On 4/14/08, Bill  wrote:
Brian beat me to it, but I'm excited
<Snip>
Drat!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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30) From: Brian Kamnetz
Bill,
Sorry, that slipped by me.
Brian
On 4/14/08, Bill  wrote:
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31) From: Michael Mccandless
You might consider the dual relay I mentioned earlier.
It's 2 in one - one control input.
You will be able to switch both legs of the 220 with one relay.
Lower parts count means higher reliability, less complexity, & decreased
requirement on the control side.
McSparky
On Mon, Apr 14, 2008 at 4:11 PM, Tom & Maria - Sweet Maria's Coffee <
sweetmarias> wrote:
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32) From: Bill
Nope, just read Dave's post in another thread, and Brian you were spot on.
 I'm still a little weirded out by it... the same day a new lister comes on
to ask about inlet temps is the same day that our host is asking questions
about wiring... how nuts is that???  So props to you, Brian, you nailed it
the first time.  But I'll still beat your on exclamation
points!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  bwah ha habill
On Tue, Apr 15, 2008 at 7:50 AM, Brian Kamnetz  wrote:
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33) From: Michael Mccandless
That would handle 2400 watts.
McSparky
On Mon, Apr 14, 2008 at 3:12 PM, Ira  wrote:
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34) From: Ira
At 12:17 PM 4/15/2008, you wrote:
<Snip>
It would, and unless it's a really small roaster, that's probably to 
close for comfort.
Ira
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35) From: raymanowen
"... an internal control like an SSR does not need to switch both sides."
 You're right- they don't have to...
The fire marshall was highly interested in Bob's* homebrew apparatus that
had no certification tags on it. That's what piqued his interest; the
"birdcage" wiring and the use of plastic wire nuts to join high temperature
stranded nickel wires to larger copper wires really raised flags.
*[Not his real name]
Electrical codes are for building wiring. It can still be unsafe, even if
it's "code." The safety of machine wiring is left to the designer
(Enjanear?)
The Electrical Code may be for building wiring, but fire men have to
extinguish the flames, regardless of their origin. Flames still have the
same effect on building height.
Cheers, Mabuhay -RayO, aka Opa!
Got Grinder?
On Mon, Apr 14, 2008 at 8:30 AM, Michael Wascher  wrote:
<Snip>
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