Electronics/Potentiometer question

Electronics/Potentiometer question

Max
Max

August 6th, 2012, 8:28 pm #1

I'm working on putting together a swappable charcoal to electric heat conversion for my smoker. And am wondering about how best to control the element I have for it. I'm using a 110v 1500w electric burner I scavenged out of an electric BBQ that takes the smoker up to high. 300 deg F if left alone. I was thinking it would be best to set it up like an electric hotplate which uses a Potentiometer (right?) Can one of you tell me is that the correct part (a pot) and some advice as to what sort of voltage/wattage/amperage ratings I should be looking for to keep from burning the thing up. I'm hoping this is a radio shack part since one's around the corner from me but if I knew what I was looking for I suppose I could order one or find one somewhere in town as well. Thanks-Max
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sniper1rfa
sniper1rfa

August 6th, 2012, 9:46 pm #2

no.

A potentiometer, when used to control a load directly (actually a rheostat in this case) works by shunting the power past the load in directly to the sink.

If you think about it like filling a glass of water: Turn the faucet on, and then hold the glass partly under it. The water that hits the rim and enters the glass is controlled, and the rest passes the glass and goes down the drain. That means you're always using the same amount of water, or power in the case of the burner, with most of it being wasted.

Now, you could do that. You'd need a 1500W rheostat, probably 0ohm to some large multiple of the resistance of the burner - say 10x. Resistance could in this case be measured simply with a multimeter since it's purely a resistive heater and thus follows reasonably ohm's law, even though it's powered by AC.

A much, much better solution would be to use a potentiometer to control a TRIAC. The simple, pre-packaged version of this is a dimmer switch for lights. 1500W ones look like they run about $20-40 for cheap ones.
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sniper1rfa
sniper1rfa

August 6th, 2012, 9:51 pm #3

Sorry, my explanation was wrong. Trying to multitask.

If you run the pot in series rather than parallel like I described, the power gets wasted as heat. Not as bad as the glass in the sink scenario, but still pretty damn inefficient.
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sniper1rfa
sniper1rfa

August 6th, 2012, 10:54 pm #4

no.

A potentiometer, when used to control a load directly (actually a rheostat in this case) works by shunting the power past the load in directly to the sink.

If you think about it like filling a glass of water: Turn the faucet on, and then hold the glass partly under it. The water that hits the rim and enters the glass is controlled, and the rest passes the glass and goes down the drain. That means you're always using the same amount of water, or power in the case of the burner, with most of it being wasted.

Now, you could do that. You'd need a 1500W rheostat, probably 0ohm to some large multiple of the resistance of the burner - say 10x. Resistance could in this case be measured simply with a multimeter since it's purely a resistive heater and thus follows reasonably ohm's law, even though it's powered by AC.

A much, much better solution would be to use a potentiometer to control a TRIAC. The simple, pre-packaged version of this is a dimmer switch for lights. 1500W ones look like they run about $20-40 for cheap ones.
The dimmer-switch approach is the dumb, simple version. If you're willing to fiddle a bit at first to find the right setting and afterwords never change it again it will work perfectly well.

The next step up would be a thermostat controlling a relay to turn it on and off. The thermostat will turn the relay on at some low set point, and back off at some high set point.

The last, "smartest", option would be a controller with a thermocouple in the oven and a electronically variable resistor controller the dimmer pack. If I were to do it I'd probably use a thermocouple, a PID temp controller from automation direct, and a hot-wired dimmer switch or some other dimmer pack.

These solutions obviously get progressively more expensive, with the PID coming in at probably around $200-300.
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Max
Max

August 7th, 2012, 3:16 am #5

I'm looking at also doing a DIY sous vide with a crock pot and a temperature controller. Maybe I can set them up so that I can use one controller on both. If I get one that's programmable I can just swap the thermocouple and power leads between the two and set it wherever I need. I don't want to spend a bunch of money on the smoker alone. If I were putting several hundred dollars into it I might as well just buy a quality pellet smoker and junk the cheapo cabinet I have now. but if I were able to use the parts in more than one system it would help justify the expense of the superior system.
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Maker of Toys
Maker of Toys

August 7th, 2012, 3:45 am #6

The dimmer-switch approach is the dumb, simple version. If you're willing to fiddle a bit at first to find the right setting and afterwords never change it again it will work perfectly well.

The next step up would be a thermostat controlling a relay to turn it on and off. The thermostat will turn the relay on at some low set point, and back off at some high set point.

The last, "smartest", option would be a controller with a thermocouple in the oven and a electronically variable resistor controller the dimmer pack. If I were to do it I'd probably use a thermocouple, a PID temp controller from automation direct, and a hot-wired dimmer switch or some other dimmer pack.

These solutions obviously get progressively more expensive, with the PID coming in at probably around $200-300.
So a co-worker also does a lot of smoking, and we ginned up a parts list for a similar project:

Solid State Relay: Digikey:
CKRD2420 Crydom Co. CC1518-ND
(about $50; includes heat sink.)

you could lower costs here to about $20 by going to an ordinary 20A electromagnetic relay; you lose a bit in reliability, though.

IIRC, Digikey also offers a potentiometer controlled solid state current control unit if you don't mind 'human in the loop' controls. . . I'm too lazy to look it up at the moment.

Temperature controller: Omega Engineering
(both less than $120, there are others; I've used the 7800 series quite a lot lately.)
http://www.omega.com/ppt/pptsc.asp?ref=CN4000_Series

or

http://www.omega.com/ppt/pptsc.asp?ref=CN7800

a somewhat overkill thermocouple: but for less than $30. . .Also by Omega.

BLMI-INC-K-116U-12 (you'll probably want to extend the leads a bit. . . .)

Omega makes one they advertize for $9, but it might not do well in the dirty environment you're talking about. Besides, it's a little short for your purposes.

Any number of people will sell you thermocouple wire, and you can then make your own thermocouple in whatever shape and length suits you. . . twist the ends together and crimp them in a butt splice or hit them with a oxy-fuel torch or tig. It really is that easy. Type K wire is cheap. (something less than $1 a foot(?) last time I bought some)

Total cost, $150 or so, not including incidentals and overnight shipping. . . and thats NIB.

So, yeah, with a project box, wire nuts, and a cheap extension cord, $200. But you probably have a cord already, wire nuts are cheap, and Ammo Cans and old tackle boxes actually make pretty good enclosures for this sort of thing. . . Flea-Bay and various other surplus sources can probably knock half of the remainder off.

Post pictures!
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Bruce Bergman
Bruce Bergman

August 7th, 2012, 5:09 am #7

The dimmer-switch approach is the dumb, simple version. If you're willing to fiddle a bit at first to find the right setting and afterwords never change it again it will work perfectly well.

The next step up would be a thermostat controlling a relay to turn it on and off. The thermostat will turn the relay on at some low set point, and back off at some high set point.

The last, "smartest", option would be a controller with a thermocouple in the oven and a electronically variable resistor controller the dimmer pack. If I were to do it I'd probably use a thermocouple, a PID temp controller from automation direct, and a hot-wired dimmer switch or some other dimmer pack.

These solutions obviously get progressively more expensive, with the PID coming in at probably around $200-300.
Variacs are a variable tap auto-transformer and they don't waste a lot of energy as heat - they just feed the heating element 30V or 75V instead of the full 120V or 240V (depending on the model), each notch bumps it up 2 or 3 volts...

Warning: They are Not Cheap if you go buy one new, especially the larger ones that can handle a full 20A load. Look for surplus.

That'll give you rough temperature control and get the heat in the right neighborhood - especially useful if you've got a pan of wood chips directly over the heating element, and you don't want it to burst into flames every time the heating element cycles on...

Then you hook up the Programmable Temperature Control to cycle the power between the Variac and the element, and give you the spot-on temperature you want. It cycles the element at 80% duty cycle between dull red and off, rather than 20% duty cycle between Full On and Nothing - and often overshooting the target temperature in big swings.
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Renegade_Azzy
Renegade_Azzy

August 7th, 2012, 5:16 am #8

I'm working on putting together a swappable charcoal to electric heat conversion for my smoker. And am wondering about how best to control the element I have for it. I'm using a 110v 1500w electric burner I scavenged out of an electric BBQ that takes the smoker up to high. 300 deg F if left alone. I was thinking it would be best to set it up like an electric hotplate which uses a Potentiometer (right?) Can one of you tell me is that the correct part (a pot) and some advice as to what sort of voltage/wattage/amperage ratings I should be looking for to keep from burning the thing up. I'm hoping this is a radio shack part since one's around the corner from me but if I knew what I was looking for I suppose I could order one or find one somewhere in town as well. Thanks-Max
But why not hit up the junk yard for the controls from an electric stove, or just a simple hot plate?

Better yet, grab a used hotplate down at the local goodwill and call it a day.
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Sleepydawg
Sleepydawg

August 7th, 2012, 1:25 pm #9

Go to Wally world, buy a $19 toaster oven, gut it and put the elements in your box. It already has a built-in temperature controller and a relay that can handle a few hundred watts. You can upgrade the relay if you need the full power of your 1500W element from the electric grill.

Don't worry about trying to vary the power to keep the thing from going full power like everyone else is suggesting. Just get a nice big hunk of cast iron and put that between your heating element and the wood chips for smoking. The metal will give you enough thermal mass that you don't need to worry about the element blasting things when it comes on full power. The simple bang-bang temperature control from the toaster oven is quite sufficient to keep the temperature steady on the other side of the cast iron. It'll take a little longer to heat up, but you're going to be doing low and slow cooking all day anyway, aren't you?

-Zzzzzz
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Nighthawke
Nighthawke

August 7th, 2012, 3:03 pm #10

But why not hit up the junk yard for the controls from an electric stove, or just a simple hot plate?

Better yet, grab a used hotplate down at the local goodwill and call it a day.
Better yet, a toaster oven since it has a higher output and no one hardly uses those outside of one or two attempts to make toast, then gives them away.
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