Ramping DC for Peltier question

[Raul]

I want to drive a couple of 12 Volt peltier devices. Maybe 30 or 60 watts and I want to ramp down the DC power to the peltiers on shutdown over a period of around 5 minutes or so to zero volts / amps. The ramping time period is not terribly important. I just want the cooling fan on the peltier to be able to draw the residual heat off so it doesn’t slam back into the thing I just cooled. So I’m guessing 5 minutes ought to be OK. Maybe 10? I have not purchased the peltiers yet so I have no empirical data.

I have learned that I can use a switching PS like an ATX unit to drive the peltiers so long as there are no other loads matching one PS to one Peltier.

I also don’t know whether it’d be more straight forward to build a ramping device for each peltier or for a group of them. Prolly one each would be smartest, I’m thinking.

Any help? Know of any circuit designs that will do what I am looking to accomplish?
The simpler the circuit the easier for me to breadboard it together.
Elegance is not my goal.




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Some background info you may or not care to know:
I've looked at two ways of doing this:
1.) Mineral oil pumped up from a reservoir in another room (cellar) where I can heat or chill it using a water heater element or an old converted Dehumidifier respectively then into a heat exchanger coil of SST tubing inside the Fermentor.
2.) peltier units mounted to an aluminum platen that has one side machined to conform to the radius of my Fermentor and mounted to it.
What's my app?
Beer. 12 gallons of beer.
I have a Fermentor for which I want to regulate the temperature.
I figure I have 40 watts of passive load if I don't insulate the Fermentor and 20 if I do. That's something that 2 or 3 peltiers in the 30 - 60 watt range can handle easily
The thermal range I want to bring my beer to lies between 64 degrees and 78 degrees F, specific temp' of course depending on the brew the yeasts and the result I want.

The up side to peltiers is that I don't pierce the Fermentor with anything that might harbor bacteria. They are reversible so they can heat as well as chill. Controlling them would be VIA a SSR which would be switched by a computer based unit called the BCS 460 or simple PID units and thermocouples. Reversing polarity for warming the brew is a thing I can do manually because I either want the brew above ambient or below. But, I won't need to be driving temperature up and then down in the same brew cycle.
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But I suck at electricity. I mean, I can wire a dual pole set of light dimming switches, but household electrical work is about the apex of my accomplishments.

 

[ronv]

12 Gallons of beer is a lot to cool with peltiers. They are not very efficient. I would just put some fans on the heatsink side and leave them on or put them on a timer as opposed to switching the peltiers. This would help you get the most out of the peltiers as well. Maybe someone will come on that can figure out how many it would take to cool a half tank of beer.

 

[Raul]

12 Gallons of beer is a lot to cool with peltiers. They are not very efficient. I would just put some fans on the heatsink side and leave them on or put them on a timer as opposed to switching the peltiers. This would help you get the most out of the peltiers as well. Maybe someone will come on that can figure out how many it would take to cool a half tank of beer.

One company is doing it with peltiers.
**broken link removed**
Theirs is pretty basic strictly On / Off no ramping up or down of the DC to let the fan draw heat off the hot side VIA the heatsink.

I did a work up on a 30 watt peltier with one inch of insulation on my liquid volume. and it'll just about do it with a few BTUs to spare. I'm looking at something a tad more robust than a single 30 watt unit. The goal is to go from ambient which I calculate to be about 70 Degrees F down to about 64 F and up to Ambient depending on the yeast the brew. The trick in fermenting beer is to hold a temp not to be driving it up and down. When I pump the wort from the brew kettle to the fermentor it's already been cooled down to about ambient by an integral heat exchanger. So there's not a lot of Delta T to go.

But leaving them on is not an option since you gotta maintain a fairly precise temperature and just running 'em all the time would likely crash the yeast.

 

[ronv]

Just leave the fans on for a while, after the peltiers are turned off. Not the whole thing.

 

[Raul]

Just leave the fans on for a while, after the peltiers are turned off. Not the whole thing.

Yah I am probably going to power the fans independently.
However, the hot side is really hot - so I am told - and the peltier is a really good conductor. ERGO the idea about ramping it down.

I believe that PWM technology is the most likely candidate but ya know this is the internet-S and my being able to say the words "PWM technology" doesn't serve to make me any more informed about it.

I've looked at PWM modules power supplies and peltier cold plates from on manufacturer. They sell individual configurable modules not turn key systems and even in that format they are more $$ than I want to throw at it. Driving a lousy 30 watt peltier ends up costing me about $600. I'd just buy the PWM module but they made it to only run on goofy voltage so on can't just use any peltier or power supply.

I do know that a peltier can be throttled back in steps to idle where it doesn't conduct but rather insulates one side form the other.

I've been to the Arduino web site. I wonder if there's something there I can use. It's all pretty much over my head. So if there is, there's an attendant learning curve.

 

[Nigel Goodwin]

Like others have suggested, simply turn the Peltiers off, and leave the fan running for a while.

 

[Raul]

Like others have suggested, simply turn the Peltiers off, and leave the fan running for a while.

I may. I have so many things on my plate right now with this brewery. I prolly don't need to pile on.

 

[zipdogso]

This circuit has been used successfully by several people. It will ramp down the voltage to the TECs as they get colder.
You can use a different NTC thermister if the one suggested does get the result you want -just alter the resisters in the bridge to suit.
It has been tested for voltages between 12v and 24v - you need at least 12v to run the MIC502.

I am sure the experts on here could possibly even suggest alterations to it.

 

[ronv]

You might look at this design. I used something similat for my golf cart air coditioner (Keeps the back of your neck cool). The heat sinks run the bill up but sometimes you can get a deal on ebay. Note in this design the 1 inch metal spacer on the cold side. I thought it was just to allow space for insulation between the heatsinks, but maybe it addresses your concern with the larger cold mass as well.

 

[ronv]

Here is a good read from a contributor here.
**broken link removed**

 

[Raul]

This circuit has been used successfully by several people. It will ramp down the voltage to the TECs as they get colder.
You can use a different NTC thermister if the one suggested does get the result you want -just alter the resisters in the bridge to suit.
It has been tested for voltages between 12v and 24v - you need at least 12v to run the MIC502.

I am sure the experts on here could possibly even suggest alterations to it.

Definitely on my winter projects list.
Mili Grazi.