Dewheater and Peltiers

[MarkS]

Quick question.

I'm thinking about controlling the peltier on my Canon because I rarely need 35C of cooling drawing 2.5amps of battery power.

I've read that PWM (pulse width modulation) is the preferred way of controlling a pelt.

My dew controller uses PWM and can easily handle 3A.

Is it suitable for controlling the peltier?

[Ian]

Is 3A enough to run the pelt at full pelt? (he he...  ) See data sheets

Pelts are 100% resistive loads so will not present unreasonable loads to a PWM power supply, provided the max current available from the PSU will be enough to get the heat transfer you're looking for.

Be aware of electric noise however. Dew heaters are usually away from cameras minimising noise pickup, but your pelt will be much closer to the camera. Give it a go and see what happens.

[Mike]

If that doesn't work you could easily make a PWM controller with an MCU and a high-speed transistor.

[MarkS]

Is 3A enough to run the pelt at full pelt? (he he...  ) See data sheets

The pelt I'm using only draws 2.5A at 12V

Mike, what's an MCU?

[Ian]

well as long as the output of the PSU is at 12V, you should be good to go

[Mike]

Mike, what's an MCU?

Microcontroller

[Ian]

a couple of 555 timers would work as long as your not too fussed about accurate control and stability. Which for a pelt or dew heater, you're not...

[MarkS]

Pelts are 100% resistive loads

Is that true?  During operation, if the power cycles off there must be a back-EMF from the thermocouple effect.

Mark

[Ian]

interesting thought. Yes, there will be a voltage across the pelt, in reverse, but that doesn't necessarily make the load in any way reactive. A reactive load is one where the changes in voltage and current are not co-incident. If the load is reactive, then voltage either leads current or trails behind it depending on if it's an inductive load or a capacitative one. I think it might be the other way round, I'd have to look it up...

[Mike]

Nothing a high rating diode won't fix.

[Ian]

hmm, I'm not sure if that's a good idea or not. Yes if the transistors need protection from the back EMF, no if you want the heat to stay where it is when the power is removed. The thermal behaviour of the pelt changes if current is allowed to flow when there is a temperature difference across the different sides, effectively making the pelt more thermally conductive. A diode would effectively short the pelt out and you'll get a much less efficient cooler.

This could prove to be an interesting read, if anyone's looking at building a TEC PSU. http://www.datasheetcatalog.org/datasheet/maxim/MAX1968-MAX1969.pdf

This is quite interesting too, particularly when discussing what efficiency actually means when using a TEC http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=6325

[MarkS]

Dew controller worked a treat - gave me full control over the peltier.

Reducing consumption from 2.5amps to 1.5amps still gave me a 20C temperature drop (from ambient) - more than enough for most situations.  And it should reduce the likelihood of the CCD misting up.

Now if I will build a similar circuit but with a thyristor or similar I could get proper thermostatic control.

[MarkS]

This could prove to be an interesting read

Thanks - I'll take a look.

Mark

[Ian]

A thermistor rather than a thyristor would be useful.

The control circuit will need to compare the resistance seen at the thermistor against the setpoint reference resistance. You'll have to take a look at the thermistor datasheet to work out what that should be, and provide a way of trimming.

Then you would control the duty cycle of the SMPS to deliver the temperature. I would also have an ambient temperature measurement to ensure that you're not driving the TEC beyond it maximum efficiency... The current drawn by the TEC heats it up, so you get diminishing returns over a certain supply current.

[MarkS]

A thermistor rather than a thyristor would be useful.

That's what I meant