HomeRoast Digest


Topic: protecting the heater (8 msgs / 208 lines)
1) From: dewardh
Adding fan speed control to the Caffe Rosto (or any other air roaster, for that 
matter) raises the issue of protecting the heater against inadvertent power-on 
with the fan not running.  Just cascading switches (putting the heater switch 
in series with the fan switch) is not enough . . . there remains the 
possibility of powering the heater with the fan speed control turned all the 
way down.  Blowing the thermal fuse or frying the heater by so doing is 
obviously not a "warranty repair" (and is a real pain-in-the-***).  Apart from 
*caution* here are two possible "solutions", either of which you might want to 
implement in your control box:
1) put a mechanical "stop" on the speed control dial so it cannot be turned to 
the "full off" position (and bypass any switch on the speed control that might 
inadvertently turn the fan "off")
2) wire a diode (250V, 3A minimum) in parallel with the speed control.  this 
causes the fan to run at no less than half speed regardless the setting of the  
 "dimmer" or "motor speed control" since the control is switching only 
alternate half cycles of the supplied "mains" power (the diode, which can be 
wired either polarity, turns the other half cycle "always on").  it is unlikely 
that you would ever want to run the fan at less than half speed (with or 
without the heater "on") . . . and this "solution" expands half-speed to 
full-speed over the entire range of the controller dial.  it should work with 
any dimmer or triac motor speed control.
Deward
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2) From: Mike McGinness
From: "dewardh" 
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that
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power-on
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switch
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the
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from
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want to
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I'd realized the potential problem but hadn't come up with a solution.
Thought of using an ac triggered relay but hadn't addressed the issues of
triggering it with variable voltage AND the heater current it would have to
handle. The later would likely make it expensive. (used ac relays for amp
selector I built for bedroom speakers over a decade ago, one pair speakers
fed from two different amps. Relays triggered by plugging into bedroom amp
socket. Benefit: go to sleep to one source sound level, wake up to another
single pair speakers! Nothing like it was marketed at the time. Built a
couple for relatives too. All still working fine. I believe Niles Audio now
has a more 'elegant' device that does the same thing. Oh, obviously both
bedroom and house main sound systems also on timers...:-)
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turned to
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might
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Yes, the Lutron fan controller dial turns to a fully off position. Potential
oops there. Haven't looked into to disabling it. Won't until I incorporate
into combined unit, now use it for the fan on/off.
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this
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the
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be
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unlikely
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with
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Much simpler (and I'm sure cheaper) solution than ac triggered relay. I
agree limiting the slow down of the fan should not be an issue. How should
it be wired specifically? From one of the ac out lines from the trimmer to
the fan or both?
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
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3) From: dewardh
Mike:
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I'm not familiar with the Lutron device, but most dimmers are two wire gadgets, 
replacing a two terminal switch.   One wire goes to the supply, the other to 
the load.  If there is a third wire (in addition to "ground") it is bonded to 
the neutral, which is common to both input and output, but most dimmers don't 
have a terminal for that.  Think in terms of a simple switch with a diode 
across the contacts . . . depending on the switch position it is either half-on 
if the switch is open  (alternate half cycles through the diode) or full-on if 
the switch is closed (all power through the switch). The dimmer replaces the 
switch . . . the diode keeps the fan half-on, the dimmer takes it the rest of 
the way.
Deward
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4) From: Ken Mary
<Snip>
You have solved one of my problems with simple dimmers, the useable knob
rotation is sometimes only 90 degrees. I was going to play with the dimmer
pot by adding resistors to expand the required knob rotation.
I was also looking into a fan off or low flow detector to shut down the
heater. Nothing seems easy to me, do you have any thoughts?
--
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5) From: dewardh
Ken:
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heater. Nothing seems easy to me, do you have any thoughts?
Reliable flow detection/measurement is always hard to implement . . . things 
have a tendency to clog or jam.  With the Rosto there's the "thermal switch" 
(not the "fuse") on the heater that might be sufficient "interlock" . . . (but 
I'd rather not push my luck trying to test it, thank you ).  While several 
"is it spinning?" detectors for the fan come to mind they're all too 
complicated to be practical.  Even the "half speed" trick might not be enough 
to fully protect the heater with a full load of beans restricting airflow, but 
at least you'd hear the fan laboring, and have a few moments to correct the 
problem.
Deward
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6) From: Mike McGinness
From: "dewardh" 
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things
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switch"
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(but
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several
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enough
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but
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the
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I was mistaken on the price of a 15A 120vac relay. Picked one up today for
$2. I'm going to test it's low limit threshold to see if it'll work as a
heater safety. In other words, if it'll trigger from say around 100v to
relay power to the heater switch then it should work as heater protection.
This would not protect against a fan failure of course, just insure heater
not powered unless power fed to fan... I'm not going to worry about going to
that extreme!
Course, what ticked me off was the same place had 25.2v 2A transformers for
$3, same as I paid Radio Shack $10! But then, they're out of the way, Radio
Shack was around the corner when I needed the part...
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
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7) From: Mike McGinness
From: "Mike McGinness" 
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to
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Just did a test. Triggers at 72.5v so I'm going to try using it as heater
switch enable from fan dimmer. It's a double pole, I'm thinking to wire just
one side of ac line in parallel to distribute the heater load, just like a
switch breaking just one side of ac circuit. Result will be if voltage to
fan drops below 72.5v the heater will be disabled.
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

8) From: dewardh
Mike:
<Snip>
fan drops below 72.5v the heater will be disabled.
I hope you are talking about a *mechanical* relay ("double pole" implies that) 
.. . . otherwise you are setting yourself up for several potential problems here 
(and the first applies to a "mechanical" relay as well as a SSR . . . it will 
behave differently driven by a dimmer than it will driven by a variac).  First, 
the peak voltage to the motor will not fall below 75V until well below "half 
power" . . . in fact at the "half power" point the dimmer turns "on" right at 
peak voltage.  It's a toss up what a mechanical relay will do in that 
circumstance, but you can have the fan set very low and still have a SSR 
trigger.  Motor brush "noise" may trigger it at lower settings than you expect, 
too.  Second, don't forget the heat problem discussed in the Silvia thread. 
 The relay will be passing as much as 12A (at variac "boosted" heater voltage) 
.. . . that current through the 1.6V forward voltage drop of a SSR will produce 
almost 20 Watts of heat in the SSR, and requires a heat sink. That's not an 
issue with a mechanical relay, of course.
Deward
Ps. I gather you've found a local electronics "surplus" outlet . . . worse even 
that Harbor Freight, eh?
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