Mark III

First microcontroller managed furnace.

Like The Mark II, the Mark III was envisioned to be a DC furnace from the start. DC is easier to control once it has been smoothed. That was the goal of this furnace.

For this furnace, the power source was the same transformer that powered the Mark 1 and Mark II, which was repurposed here and, again, rectified with a 200A full bridge rectifier which was actively cooled with a heat sink and fan. This furnace was going to be significantly more complex than any that I had built thus far. As such, it included measures to limit current, measures to monitor current and a fully automated electrode armature moved by a stepper motor and a linear screw actuator and controlled by an Arduino Uno 16MHz microcontroller.

Beginning to assemble the Mark III. This is the "upper deck" where are the controls and power smoothing takes place. The "lower deck" (not seen here, it is underneath the wood plank) houses the transformer and the full bridge rectifier and their respective cooling fans.

This furnace was the first design to actually work (kind of). Inside the furnace was some housekeeping circuitry that I put there to try to limit the current. As this furnace was controlled by an Arduino microcontroller, I could monitor the current by way of the voltage. Any time the voltage dipped below a certain level, I could infer that the current was spiking in order to maintain the output wattage. While this may have been a super simplistic approach, I was able to use that premise to keep the furnace from tripping it's breaker. The breaker was internally housed on this furnace to save myself the trip down into the basement every time I tripped it.

The approach was simple. Prior to rectification and smoothing, the A/C power was routed through a Solid State Relay. The Arduino would keep the relay energized unless the current spiked. When the current spiked, the relay would shut down and a shunt tied to the relay's input and output would shunt the current through a 600W resistor (actually a projector bulb). This was moderately effective and the current threshold needed to be adjusted via the code rather than externally, which later furnaces would resolve. The image below shows how this worked with two optocouplers standing in for the SSR.

This did limit current, but in most cases, tended to shut down the arc completely, requirement dozens of restarts per second. This can be seen in the video below where the light is constantly flashing and the arc never fully starts:

The furnace did work with this resistor system disengaged. The issue is, it would routinely ramp up much higher currents than the transformer would be able to sustain for any length of time. and would shorten the life of the transformer tremendously. As it was wound with 10AWG wire, the most it should handle is approximately 40 amps. However, given the imprecision of the system, once the arc started, it would routinely bypass 75 amps. In hindsight, some sort of inductor would have been much more effective to limit the current, but I just went on to the next design. The video below is of the Mark III operating without a resistor and an arc can be maintained in that circumstance, albeit very poorly.