Driving a Pelter with DC

[Noggin]

I'm working on making a desktop drink cooler. I've purchased a 12v, 100 watt peltier device from Amazon as it was cheap. I'm running 1.5 amps at 3.3v (pure DC current, no PWM). I have the peltier sitting on a 100mm x 40mm x 20mm heat sink with a small fan. I have more fans I'm planning on using with the system, so I should get a fair amount of improvement beyond what I have right now. I have also not done any optimization yet, I may get a better temperature running at either a slightly higher or lower current. I think that really depends on my heat sink and how quickly I can remove the heat from it.

I knew I wouldn't be pushing 100 watts of power through the peltier, but I didn't expect to only be running at 1.5 amps at 3.3v. So here's my question... Would I be better off using something more like a 20 watt peltier?

 

[steveB]

One aspect you didn't mention is insulation. Insulation on the cold side is also an important factor for the power needed.

I think 20 W might be a little too low, but would probably work. Perhaps 100W is a little high, but it's better to have extra power if you need it. Probably about 40 W is what is needed to keep a drink near 5 deg C with ambient at 25 deg. C, assuming good heat sinking and and good insulation. But, on a hot day, you might need that 100 W if you want the drink to be very cold. (desktop implies room temp and maybe 5 deg C is too cold for your drink)

These are just estimates, of course, and the details matter.

 

[tcmtech]

So why are you running a 100 watt device at roughly 5 watts and expecting it to do anything?

 

[KeepItSimpleStupid]

A few warnings. If you exceed the delta T of the device it's over. Second warning, watch the cold side getting below the dew point (condensation). They are better used in constant current mode (PWM or linear). In voltage mode they could go into thermal run-away.

I know, we used them all the time.

 

[Noggin]

One aspect you didn't mention is insulation. Insulation on the cold side is also an important factor for the power needed.

Yeah, I was thinking of making an insulated chamber for the cold side. Basically, I was thinking of using a metal pipe to help distribute the lower temperatures upwards and insulate that with a drink koozie.

I think 20 W might be a little too low, but would probably work. Perhaps 100W is a little high, but it's better to have extra power if you need it. Probably about 40 W is what is needed to keep a drink near 5 deg C with ambient at 25 deg. C, assuming good heat sinking and and good insulation. But, on a hot day, you might need that 100 W if you want the drink to be very cold. (desktop implies room temp and maybe 5 deg C is too cold for your drink)

These are just estimates, of course, and the details matter.

Thanks for the input! If your thoughts are accurate, then my peltier is probably not too far from optimal that i should worry myself with it.

 

[Noggin]

So why are you running a 100 watt device at roughly 5 watts and expecting it to do anything?

I wasn't expecting exemplary performance from what I've done today. Today is only an experiment. Also, I think our thoughts are not quite the same. Basically, I'm pushing about 5 watts of power through the peltier and was achieving temperatures of around 15 deg C on my cooling plate. Right now, I'm pushing about 6.5 watts and seeing 10 deg C on the cooling plate. My thought was that based on what I'm seeing, and the temperature differential it is at, that maybe a 20 watt pelter device might have been enough and my 100 watt is overkill. Also, along those same lines of thought, achieving my goals may have been even simpler had I chosen a lower power peltier.

Right now, I have this sitting on my bench with a small fan blowing air through the heat sink. I have additional fans I plan on hooking up to it and I may increase the width of my heat sink. Again, today is just an experiment and my first chance to learn anything useful before I start digging into this deeper.

 

[Noggin]

A few warnings. If you exceed the delta T of the device it's over. Second warning, watch the cold side getting below the dew point (condensation). They are better used in constant current mode (PWM or linear). In voltage mode they could go into thermal run-away.

I know, we used them all the time.

Your post has brought up several questions:

1. Is there a rule of thumb for the allowable delta T? I haven't seen this concern before in the few forums I've looked at. My plan was to choose a relatively low temperature limit for the
2. I've seen people mention the condensation issue, but that isn't really a problem so long as I keep the peltier out of air, right? I have a metal plate mounted to the cold side through the use of thermal tape and I was going to put a bit of RTV around the perimeter of the peltier to keep the moisture away from it.
3. I've also read to stay away from PWMing a peltier as that generates additional heat and lowers efficiency. My current plan is to make a 80kHz buck supply controlled with a PIC32

Anything you can elaborate on is greatly appreciated!

 

[ronv]

Just make sure you keep the hot side cool enough. I think you can use pwm as long as the maximum voltage is below the spec limit. In other words don't use 24 volts and a 50% duty cycle on a 12 volt device.
You would benefit from some insulation between your heatsink and what you are trying to cool even if you need to make your metal plate a little bigger. Styrofoam works good.

 

[KeepItSimpleStupid]

It actually depends on the Peltier device, but I've never seen more than 60 deg. C difference.

For fun, look here: https://www.electro-tech-online.com/custompdfs/2013/03/peltier_application_notes.pdf

 

[throbscottle]

From a purely practical point of view, (well, rather impractical for you, having seen your photo, it depends on which kind of practical your point of view is from) it would be better having the peltier sitting on top of the can, so the heat can rise up from above the can and the cold can sink down.

You could make a wondrous heat transferring contraption of copper so you can slide the can in and out, and when it's in it gets lots of lovely coldness - hmmm, copper contraption, makes me think of steampunk...

 

[Noggin]

Just make sure you keep the hot side cool enough. I think you can use pwm as long as the maximum voltage is below the spec limit. In other words don't use 24 volts and a 50% duty cycle on a 12 volt device.
You would benefit from some insulation between your heatsink and what you are trying to cool even if you need to make your metal plate a little bigger. Styrofoam works good.

I'm hoping to have access to a 3D printer here at work in June. I'll be able to make an enclosure that goes between the cooling plate and the heat sink. Good point about needing something there though.

It actually depends on the Peltier device, but I've never seen more than 60 deg. C difference.

For fun, look here: https://www.electro-tech-online.com/custompdfs/2013/03/peltier_application_notes.pdf

That PDF looks like it has some great info in it! My cooling plate is stainless steel from a pizza cutter I bought this morning. I knew stainless steel wasn't as good as aluminum as far as thermal conductivity goes, but after looking at the document I got curious. The doc shows aluminium with a conductivity of 170 to 240, depending on alloy / purity. Stainless steel isn't in the doc, but I looked it up and it was freaking 16! I've gotta get me some aluminum...

 

[Mr RB]

Your heatsink should be a few times bigger, with a fan directly mounted on it (extract).

Instead of a cooling plate I would use an aluminium (or copper) tube slightly larger than the can, and the peltier on the side of it near the bottom. Then you can put an ounce of water in the tube, so when the can is inserted the water thermally couples all sides of the can directly to the metal tube.