I recently decided I needed to upgrade my garage electronics bench to include capability to work on surface mount components. I ordered a hot air reflow station and some no-clean solder paste from SRA.
Solder paste is a little tricky to handle, because most solder paste needs to be refrigerated at 32-50°F to maintain it’s shelf life. Stored at room temp, it tends to get tacky and dry out within a few weeks. Solder paste is also toxic (it contains lead among other things) so it’s not something you want to put in the fridge with your meatloaf.
I had an old beer cooler sitting in the garage that Kylie picked up on the street a while back. It uses a Peltier thermoelectric cooler to cool the inside and can achieve sub-freezing temperatures.
Since I didn’t want to leave the cooler on constantly, and below freezing is actually too cold for solder paste, I decided to add a PID controller to the cooler to create a solder paste fridge for the garage. To do this, I needed the following items:
- Love Controls 16A PID Controller found at a surplus store. This one happens to run on 12V and also has a 15V output which made things simple, but other configurations are possible and almost any PID controller should work provided it supports “direct” mode (for cooling instead of heating) and has a relay or switched voltage output.
- K-type thermocouple to measure the inside temperature of the cooler
- power MOSFET capable of switching 12V @ at least 5A, lower on-resistance is better.
- 1k bleed resistor to ensure the MOSFET turns off when it’s supposed to
- a small heatsink for the MOSFET (may be unnecessary, mine doesn’t even get warm)
Since the PID controller happens to run on 12V I was able to use the existing 12V power supply for the cooler to power everything. I configured it so that if the desired temperature is below the current temperature, the PID controller turns on the MOSFET which supplies power to the Peltier cooler and it’s associated fans.
The only hangup I had was that at first I didn’t place the 1k resistor across the output of the controller, and the cooler would stay on constantly. It turns out that because MOSFETs have almost no gate current, once the PID controller turned off it’s 15V output, the gate of the FET would continue to float high. The bleed resistor to ground ensures that this can’t happen, and the FET turns off properly.
Here’s a picture of the finished solder paste fridge complete with PID controller (click for a larger version).
You can check out a bunch more photos of the cooler on flickr. It should be possible to perform this modification on a more conventional mini-refrigerator as well for better control over the temperature, provided it uses a Peltier cooler, or maybe you could even build the whole thing from scratch using a Peltier cooler off ebay.