A Peltier Module Temperature Chamber

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One of our interns built a “hot box” out of wood a few years ago.  What’s a “hot box” you ask?   Beyond something you’d find in a Paul Newman prison flick,  its pretty much a poor man’s temperature chamber. Except the temperatures only go up from ambient.  I’ve been surprised how often we use it, and how well it has held up, considering it was put together with wire-wrap and  off-the-shelf development boards.  But I’ve decided to take that old axiom “if it ain’t broke don’t fix it” and throw it out the window.

I’ve decided to go ahead and re-design it.  I guess I could list a few reasons for embarking on the project but it really comes down to a desire to explore some technology I haven’t designed with before.  The heart of my temperature control box will be Peltier modules.

Not too long ago we were working on a laser based system in a food production facility.  The factory floor where our system lived was subject to a wide range of temperatures and humidity.  It was apparent early on that the laser electronics could reach shutdown temperatures during hot summer days.  So the electronics needed to be cooled.  To keep the laser running a thermoelectric air conditioning system was installed.  This was a great solution because it didn’t require chemicals (something you need to keep away from food), controlled a small chamber’s temperature, and had access to all the electrical power a factory could provide.  But we didn’t design the cooling system.  We just suggested it as an option, and our customer purchased and installed it.

I wanted to bring some if this “coolness” to our “hot box”, and maybe give us an option to cycle our electronic designs through temperatures from 0-100C, instead of just higher than the ambient office temperature.  Peltier modules allow you to generate a hot and cold side based on the direction of current flow.  You can also vary the voltage across them to control how hot or cold they get.    In a sense they are like dc motors, where forward/reverse and speed are analogous to heating/cooling and the differential in temperatures of the two surfaces of the Peltier module.

One option to control a Peltier module is to use an H-bridge where the pulse-width-modulation output is filtered through a large capacitor (creating a proportional DC voltage).  Since temperature changes pretty slowly, I opted to replace the H-bridge with relays.  Rather than control the voltage across the Peltier module directly, I’ll control the duty cycle of the relay “on” time.  For example, during heating I might heat for 45 seconds, out of every minute.  This will allow the heat sinks and fans on the modules to keep the temperature differential between the hot and cold sides below the maximum the module allows.  The schematic below shows a portion of the temperature chamber microcontroller that controls the voltage polarity across the Peltier module.

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