Solder Pot Controller

In 2006 to be RoHS compliant Parallax Inc moved toward making all products lead-free including the BASIC Stamp Modules which they manufacture in-house. This required a new Lead-Free Solder Pot to dip the modules in. The new solder pot did not come with a temperature control. Instead I created a custom solder pot controller which keeps the temperature at a specified set point.


Based on input from Parallax manufacturing certain features were implemented. The temperature of lead free solder must be maintained at a fixed temperature. The solder pot controller does this by comparing the current temperature to the set temperature. The current temperature is obtained via a Parallax thermocouple kit and a pin probe which goes into the solder pot itself. Previous solder pot controllers that Parallax used caused premature wear of the connectors and contacts due to current spikes during switching on / off. This unit solves this problem by using a solid-state relay with a zero-crossing detector so that it switches during the zero-crossing point of the AC cycle. There are also no mechanical contacts to wear out as previously.

The solder pot controller was prototyped on the Parallax Professional Development Board. I even used the 7-segment displays with an MC14489 before I got a separate serial 7-segment display. The set temperature is obtained by pressing a red safety button (which also displays the current set temperature) and rotating an encoder. The allowed range is 450° – 550° F, but could be changed in code.

Once the desired set temperature is reached, releasing the button stores that value in EEPROM and the solder pot returns to comparing the temperature and turning the solder pot on / off as needed to maintain that temperature. The optimum temperature for the lead-free parts is about 520° F.

The heating element in the Solder Pot is switched on via a Crydom H12D4850 Solid State Relay. This relay is rated at 240/480V @ 50A and can switch the element on / off without any noise. This allows for differential gap control without deteriorating contacts as was had with the previous temperature controls, which broke down from apparent arcing. This new system does not exhibit these symptoms.

Prototyping was done at room temperature ranges for safety. Whenever the SSR is on an LED is also turned on to indicate the heating element is on. In the above photos the set temperature is 78° F and the threshold is 1°. This means that when the current temperature is one degree above or below the set temperature the solder pot controller will turn the heating element off or on as needed.

Here is a close-up of the display.

One request the manufacturing department had was a way to know when the solder pot was up to temperature without having to keep going over and checking it (it takes a little while from a cold start to heat up). A Piezo Buzzer is connected and when the Solder Pot first reaches its set temperature it beeps with three long beeps. After this point the LED activity is much like a Soldering Station with the unit turning on / off at whatever rate is required to maintain the temperature and the buzzer is quiet.

Anything above or equal to the set temperature turns off the SSR. Anything less turns it on. The differential gap threshold is 1° by default but could be set higher if needed. For this application it is not necessary due to the slow change in temperature in the solder pot due to density.

As usual I chose the Super Carrier Board (#27130) for this project.

Things were a little cramped in the box. A cut-out was provided for the DB-9 connector on the Super Carrier Board. This way I could work on the code while the unit is in operation.

The box is metal and grounded, however the Super Carrier Board is actually isolated from that ground to avoid problems with the needle probe in the solder pot. This was causing some problems originally until I isolated the grounds. This was due to the way the thermocouple probe was configured.

Completed Project

Here is the completed unit running and doing its job at Parallax Inc. The plant dish with sand in it is there to catch solder spillage.

Schematic Diagram

A schematic of the Solder Pot Controller is available as well as the BASIC Stamp Code. The code is available AS-IS! The schematic and controls were well thought out, but the code was basically the Thermocouple Demo Code with what I didn’t need pulled out and what I did need put in. Essentially this is hacked code and not what would normally be distributed. But it does work and has been for years now.

The display is a Rentron Serial LED Display that I happened to have on hand. There is also a FUSE on the incoming AC rated for just over the maximum current expected. This can be seen in the photos but was omitted from the schematic for clarity. Due to the 1-Wire interface this will only work on a BS2p, BS2pe or BS2px model BASIC Stamp. Unfortunately the schematic shows a BS2, which will not work.

The BS2px was chosen for the faster speed in reading the Quadrature Encoder, which is quite difficult for a slower BASIC Stamp Model. The download package includes the source code, schematic and BOM (in Excel format).

Final Thoughts

The enclosure is not too important. I chose a metal box for grounding purposes and to help heat sink the SSR. You could use a plastic one just the same, though you may need to make considerations for the SSR.

The piezo buzzer was just one I had in my parts bin. I think the resonant frequency was ~3kHz and 5 VDC. It was sufficiently loud enough for the manufacturing department to hear. This is not a piezo speaker, but a self-contained buzzer. When voltage is applied it makes sound.

For safety reasons (accidental changing of the set temperature) the push button was isolated from the rotary encoder. You could just as easily use an encoder with an internal button if you like.


Solder Pot Controller by Chris Savage is licensed under CC BY 4.0


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