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Old 08-01-2011, 09:24 AM   #1
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Default Ideas for alternate temperature controller that uses built-in fridge circuit

It seems that by far the most popular temperature controller design is the Ranco/Love type that "overrides" refrigerator controls by providing it's own thermal sensor and killing power to the fridge. They are a bit pricey, and it seems just a tad convoluted to go about things in this way. The fridge has a perfectly good thermostat inside it already, and the only problem is that it's not in the range we'd like it to be in. Right?

I would think it would be possible to cut the leads going to the thermister in the fridge's own thermostat unit, and add a small piece of circuitry (e.g. a pot, and maybe a few other components, honestly I haven't thought this through completely) to provide a manual 'offset' which could be tuned to push the operating range of the fridge's thermostat into the right zone for beer fermentation. Then, a bit of trial and error, a thermometer, and a large jug of water could determine exactly what setting on the fridge's thermostat dial (usually not given in degrees anyway) corresponds to what temperature. Of course this wouldn't be as fancy as a digital display, but it might be possible to implement for a few bucks in parts.

Has anyone tried this? I'd love to get some feedback from the electrical savvy folks on here. I suspect the exact design might depend on the specific unit to be re-wired, but these fridge thermostats can't be all that different from each other can they?

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Old 08-01-2011, 09:49 AM   #2
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Assuming it is a modern electronic fridge then they work on the thermocouple principle that is when two metals are joined together they generate a voltage based on the temperature of the junction. This is then referenced against the temperature of the other junction where they connect to a Amplifier.

The amplifier is typically a high impedance input design as any current draw would not be supported by the incredibly small voltage generate at the junction. By putting a pot in the path you would either block the current altogether or have no effect.

You could design an high impedance differential input amplifier that could provide a veriable gain around unity or slightly less then this would work, but trust me it is a lot more work than just making your own thermostat from scratch. I have several based around a DS18B20. It is neat IC that measure temp and talks to a microprocessor, I have one working on my fermentation chamber. It is a giant cooler with 4" of foam insulation with a insulated hose attached to the freezer on the beer fridge. When it needs to cool the fermentation chamber a fan draws cool air from the freezer.

I'm trying to get some guys at work to make me a thermowell with one of these ICs at the end, I need his tig welder to seal up the tube I'm using. I'm also working on a inline thermometer controlling the cooling water of the counter flow wort chillier.

Clem

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Old 08-01-2011, 10:08 AM   #3
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I think I see what you're saying. I was under the impression that fridges used thermistors instead of thermocouples but I may be wrong. In the thermocouple case, the design becomes more complicated as you suggest because the opamp input is based on the voltage provided by the thermocouple... However, with a thermistor design, I was thinking the pot would go in parallel with the thermistor to tune its resistance (which is of course is part of a circuit that provides an input to an opamp as well, but easier to 'mod' I would think).

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Old 08-01-2011, 10:30 AM   #4
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There are a few builds around here replacing the built-in thermostat with a wider-ranging unit. I built a fridge regulator from a microcontroller and a temp sensing IC, though I still switch the input power like on a Johnson, not the thermostat leads.

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Old 08-01-2011, 12:58 PM   #5
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I thought most fridges had an adjustment screw that you could use to set the coarse range, then use the knob inside to set it to a narrower temp range.

I don't know, I have a Johnson on mine.

But I've seen one before when I was young and I thing it looked just like your average control with the contacts and springs.

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Old 08-02-2011, 09:49 AM   #6
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Quote:
Originally Posted by Homercidal View Post
I thought most fridges had an adjustment screw that you could use to set the coarse range, then use the knob inside to set it to a narrower temp range.

I don't know, I have a Johnson on mine.

But I've seen one before when I was young and I thing it looked just like your average control with the contacts and springs.
Actually there's an awesome thread about this here. Wish I found it before I posted this, it's a better way to go if it works since it's freeeee.

http://www.homebrewtalk.com/f51/guide-set-internal-chest-freezer-thermostat-32f-eliminate-external-control-249612/
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