Solder Hotplate

To perform low temperature soldering of laser diodes a hotplate is being designed and built. It consists of two parts, the heater and the controller.

The heat source is four 5 ohm 25 watt resisters attached to a half inch aluminum plate.
The base of the heater was milled from a solid block of maple. An NTC is glued into a hole in the back of the plate for temperature feedback.

The controller is sporting an AtMega8 running from it’s internal RC oscillator at 8MHz.
It features a PID loop for fast acquisition and control of temperature.  An LCD as well as serial (RS232) connectivity are being used for debugging and visual feedback.

Currently it is being used as a thermal data logging test platform to investigate the behavior of heat transfer, thermodynamics…


1) Schematic  2) CAD model  3) Resistive heating  4) Hotplate surface
5) Sensor Calibration  6) Experimental data  7) Simulation data  8 ) PCB

Soldering Hotplate Version II

It was found that the initial version took an undesirably long time to reach a given temperature as the 50VA transformer limited the applied power. It was decided that the DC power scheme be revised to eliminate the bulky transformer and AC phase angle control be implemented instead.

A hairdryer consisting of 3 heating coils and an axial DC fan was salvaged for version two. A new heating plate was designed and CNC milled to accommodate one of the salvaged heating coils. An 8mm trough, 300 mm long, lined with fire cement for electrical insulation, now holds a 25 ohm coil. Utilizing phase angle control, this should equate to a maximum power of 575 watts.

The axial DC fan will be added to the design to assist in rapid cooling for faster solder setting times.  As well, a new temperature sensing system will use two thermocouples, one internal and one external to provide more accurate feedback via a pair of MAX6675?s. The circuit layout is currently under review and parts are awaiting arrival.

2 Responses to “Solder Hotplate

  • “Mr Coffee” type coffee makers have a hot plate kinda thing in them. I never throw things away without checking them first for “neat” stuff!

  • That should work but I’d be wary of operating at a higher temperature, 60/40 solder would probably need at least 200C.
    At least give it a burn in test to get the nastys out of the paint.

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