Wednesday, 26 October 2016

Power Supply board, Part 2

After the previous less-than-completely-successful circuit board fabrication effort, we now have a board that is 100% correct. I am using the "press 'n peel" method of printing the layout onto a plastic sheet then using a hot iron to transfer the design onto the copper side of the PCB, before then putting the PCB into the Ammonium Persulphate etchant to dissolve away all the copper not covered by the resist (=transferred printer toner)

The transfer process was time-consuming and not particularly effective, it required multiple passes with the iron and then careful peeling off to see how much of the resist was transferred before then carefully re-ironing.... rinse and repeat several times until it seemed no more resist would be transferred. Then it was a case of filling in the missing parts with a sharpie, fortunately not much was needed.

From there it was into the chemical bath for 10 mins being constantly agitated until all the remaining copper was dissolved off.

Then... drilling, cleaning, then stuffing and soldering.

The stuffing and soldering took around 2 hours, this included testing the delay circuit to make sure the timer circuit (my design) was working. Happily it was.

The timer circuit takes the 6.3v AC heater supply, feeds it through a bridge and then into a 5V regulator and then to a 555 Timer IC with suitable component values to give around a 30sec trigger delay before closing the relays and applying the high-voltage AC to the rectifier diodes. This is to give the valves a chance to heat up and be ready to draw current before any HT is applied.

This is the completed supply:

On this board we have:
  • Timer delay circuit including relays
  • Rectifier (Diodes in series because of peak inverse voltage exceedance) plus balancing capacitors
  • Main capacitors (4x, in series-parallel configuration, with balancing resistors)
  • Separate supply for input valves (x2, 300V)
  • Separate supply for Driver valves (x2, 360V)
  • Separate supply for Negative bias voltage for output valves (X4, -80V)

Next step: testing

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