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Topic: Peltier-based dehumidifying cabinet (Read 2 times) previous topic - next topic

PaulS

Which are you trying to control - temperature or humidity?

Have you researched humidors? An oversize humidor seems like what you are trying to create.

AdrianLopez


Which are you trying to control - temperature or humidity?

The goal is to control humidity, but the cooler needs to be at the right temperature for water vapor to condense without freezing over.

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Have you researched humidors? An oversize humidor seems like what you are trying to create.

Humidors are meant for humidification rather than dehumidification. I want to remove moisture from the air, not add it.

I'm looking to build something like this, but less expensive and without the humidifier component (which I don't need).

Chagrin

So what you want to do is set the cold side of the peltier to the dewpoint so that you can collect the condensation and remove it from the cabinet.

You're going to attach some sort of aluminum plate/heat sink to each side of the peltier. The hot side will need a fan to turn on whenever the peltier is active. On the cold side you will also need a temperature probe of some type (e.g. LM35 or DS18B20) to measure the temperature of that side. Use the PID library to control the power on/off to the peltier (via relay or mosfet, preferably mosfet) to keep the dewpoint temperature of the cold side stable.

There's no PWM needed here, or putting it another way there's no reason why you'd need to switch the peltier on/off more than a couple times a second to keep a reasonably stable temperature.

AdrianLopez

There's no PWM needed here, or putting it another way there's no reason why you'd need to switch the peltier on/off more than a couple times a second to keep a reasonably stable temperature.


What about thermal stresses, as noted by jroorda? His reply is consistent with this FAQ, which states:

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"Significant precautions must be employed with PWM, however. First of all, the PWM should be at a high enough frequency to minimize thermal stresses to the TE devices. While we like to keep the frequency in the low killihertz (Hz) range, in many applications these days we must compromise at around 120 Hz for the sake of electromagnetic compatibility. Another important issue is the potential for generating electro-magnetic interference (EMI) in the wiring to the TE device. If you are using PWM, you may need to shield your power wiring or keep it away from any sensitive electrical signals."

Chagrin

That FAQ and all of their technical specifications provide pretty much no detail as to thermal stress considerations, and if it were a big issue to be concerned about I would expect that they would list some specification as to the number of switching cycles it can handle. It also contradicts itself with its discussion of using mechanical relays by suggesting that the relay fails before the peltier junction does? After digging around with Google I'm not denying that PWM would be preferable, but I still can't find any decent numbers as to how much of a concern this is.

Stick with the K.I.S.S. principle. A peltier junction is ~$10, and if it does wind up failing too quickly then you can revisit how you're driving it. Drive it at a low voltage and tweak the voltage a bit until you get a reasonably low switching rate. You don't really need that much cooling; At 70F (room temperature) the dewpoint is only 50F for 50% humidity and that should be a much much lower power requirement than these devices are capable of.

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