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Xtreme Cooling LN2, Dry Ice, Peltiers, etc... All the usual suspects

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Unread 09-20-2003, 01:39 PM   #26
Since87
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Quote:
Originally posted by wymjym
I have NEVER seen a manufacturers specs sheet that indicated pwm was a bad thing for controlling peltier power, rather over the past few years I have seen recommendations from some that say they are okay as long as they are designed to work within some frequency (which I forget at the moment) range.
So, if you can post a link (from any peltier manufacturer) that would indicate that pwm is bad, I would appreciate it.
wj
I've seen recommendations to avoid PWM below 1 KHz.
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Unread 09-20-2003, 01:44 PM   #27
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where?
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Unread 09-20-2003, 01:49 PM   #28
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OK. There PWM controller takes DC and chops it up. Giving a square wave output. That is not as destructive.

Most (inexpensive) dc motor controllers that have a 120/220 AC input and drive a 90/180VDC motor have a waveshape that is anything but a squarewave.

The thing that destroys pelts from a bad power supply is thermal cycling, as I understand it. With a pure square wave all you are doing is turning the power on and off quickly. As you get away from a square wave the current starts to have more and more AC ripple. Which causes thermal cycling within the pelt.

Someone like Since87 could do a much better job of explaining this than I.
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Unread 09-20-2003, 01:57 PM   #29
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Quote:
Originally posted by wymjym
where?
wj
Wish I knew offhand. I found this paper which basically says, 'If PWM does significant damage to TEC's at low frequency, then our test wasn't long enough to prove it.'

Edit:

This link briefly discusses the deleterious effects of ripple from rectified AC sources.
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Unread 09-20-2003, 02:03 PM   #30
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I understand what you have said and basically that is similar to what I've read.
My rant here is that on the surface the statement indicates that pwm is not the way to control a pelts power.
I spent a lot of time trying to determine the correct way to control my pelts and found various pelt controllers, almost all of them use pwm. I'm not saying that pwm cannot hurt a pelt but just like aspirin, if you follow the directions and it was prescribed (designed) to do the job...it can without negative concequences.

So is it fair to say that pwm CAN BE an effective way of controlling a pelt rather than saying it should be avoided as a means of control?
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Unread 09-20-2003, 02:11 PM   #31
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I'd say, 'Try to use a PWM controller that switches at 1 KHz or higher to be on the safe side.'

PWM shouldn't be dismissed all together, because it is by far the cheapest and most efficient method of controlling a TEC.
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Unread 09-20-2003, 02:11 PM   #32
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I just read the paper, thanks.
The conclusion was that the degradaton was insignificant, inconclusive and that the testing would need to be run up to 10,000hours for a more accurate determination to be made.

I have been running a pelt chiller for around 14 months, 12~16 hours per day, 5~6 days a week. ( around 5,000 hours) Using a vpc controller, I am still using the same pelts and to date there have been no problems.
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Unread 09-20-2003, 02:18 PM   #33
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By all means, yes.

All I was saying was that a PWM MOTOR controller (edit:w/AC input) is not designed for controlling power to a pelt.

From TE's website:
Quote:
10. What is the best way to power a thermoelectric cooler?

a) Ideally, thermoelectric coolers should operate on purely direct current for the best performance. However, a ripple factor of 10% will only result in 1% degradation in temperature difference. Most power supplies have better filtering than that, so ripple is not likely to be a concern.

b) Care should be taken not to overpower the cooler. Overpowering the cooler could lead to inadvertently exceeding the temperature ratings and causing damage to the cooler.

c) The input power for maximum efficiency of a cooler does not correspond to its maximum operating voltage and current for (Vmax and Imax). When maximum efficiency is desired, the applied power is typically 1/3 to 2/3 of its true maximum power rating.

d) If a temperature controller is used, it should be of the linear type or the pulse-width-modulated (frequency at least 400 Hz) type to minimize any detrimental effects of temperature cycling.
So the manuf. says all of what everyone has said basically.
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Unread 09-29-2003, 08:09 PM   #34
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Isnt there something called a line conditioner?
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Unread 09-29-2003, 08:23 PM   #35
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Heres an article I ran across. I'll apologize ahead of time if its a double post.

http://radiation1.mit.edu/Publications/j14.pdf

Would adding a cap to the PWM signal smooth the signal enough?
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Unread 09-29-2003, 10:23 PM   #36
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What about using a car battery and having a charger on the battery all the time – The battery will act like a filter and the charger should keep the battery powered up.
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Unread 10-08-2003, 01:45 AM   #37
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Quote:
What about using a car battery and having a charger on the battery all the time – The battery will act like a filter and the charger should keep the battery powered up.
That is called float charging. There is no free energy so you input power must equal output power or the batteries will drain. Though, car batteries do do a good job, as capacitors
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Unread 10-08-2003, 03:33 AM   #38
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Ill try and find a link, but one of the worlds biggest tec manafacturers (in china iirc) sell tecs that are able to withstand pwm control although the price is like 300% more than the same module that cant.

The problem with pwm cycling isnt the module itself its the ceramic incasing it.

Ive got a tec with surface cracks all over the surface which i pwm controlled for a few months. Agreed the higher the frequency the better it should be but i would think you want to be hitting it with 100khz+ rather than 1000hz.

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Unread 10-08-2003, 09:10 AM   #39
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Quote:
Originally posted by sevisehda

Would adding a cap to the PWM signal smooth the signal enough?
Adding a (large enough to be meaningful) cap in parallel with the TEC would overload the power source when the PWM controller switched power to the TEC. (The instantaneous current draw at the moment of power application would be enormous.)

Adding an appropriately spec'd inductor, in series with the parallel combination of TEC and cap, would effectively give you a crude switching DC regulator. This would be a good solution, but to the best of my knowledge, the inductor you would need is not something you can really buy 'off the shelf'. You could probably connect several inductors available from Digi-Key in parallel to get the desired performance.

Edit:

Also, the added inductor only becomes practical at high PWM switching rates. (100 kHz+) The inductor becomes too large at low frequencies.
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Unread 10-24-2003, 03:32 AM   #40
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Anybody considered resonant power converter technology? I did my research on such for my BE. Coupled with a resonant power factor correction preregulator you can gain very good power densities.
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