I had a PCB custom made for my capacitor multiplier–as they said in Flashdance, What a Feeling!

by

I had built a capacitor multiplier for use in my Hiwatt Custom 50 amp. The idea was to reduce the 60 cycle hum it had. One of my frustrations with the task was how fussy the cap multiplier was to wire, and it looked uncomfortably homemade. I recently noticed that there is free software on the internet that allows one to design printed circuit boards (PCB) and have them built by prototype companies–I settled on KiCad as the design program.

The minimum PCB order quantity is three at oshpark. I ordered three PCBs from oshpark for $31 dollars, and received them today. I assembled one and and have been testing it on my bench. The first thing I did to verify my design was to assemble it according to the printing I put on it and see if the circuit is correct–it was. I would not have been surprised if something was wrong with my first try, and I was prepared to go through a few cycles of prototyping. A properly working design right out of the gate is on the high end of possible outcomes.

On the test bench I have around 500 volts going into it. I have a 1 watt (470k) resistor for a load and its drawing 1.1 ma. It has about 1.5 volts drop from in to out. Perhaps because the low is so low, its dramatically reducing the characteristic sawtooth wave that emerges from a linear high voltage guitar amp type rectifier and capacitor circuit. I note that sawtooth has some high frequency noise riding on it, not sure where that is coming from.

Since the power supply on my Hiwatt can emit in excess of 600 volts. Any apps using small size capacitors need two caps in series, as 400 volts is the voltage limit on readily available capacitors–500 volts is the general limit, and that does not allow for generous over-engineering to make it more reliable. I get my electronic parts from digi-key and mouser. The active device is a BUL416T NPN transistor which can handle 800 volts across its collector-emitter. These are made for fluorescent light ballasts and cost about $3.60 each.

I want to acknowledge Merlin Blencowe for his wonderful book “Designing Power Supplies for Valve Amplifiers” (2010) for the idea and circuit.

I ordered some high voltage FETs, and I am thinking of laying out a PCB using a mosfet in a capacitor multiplier and see how that works…

This is the device itself, very small–around 2×3 inches. Looking at it, I can see that I could have packed the parts more densely for a smaller device. I hope it will fit in the Hiwatt. I noticed the ones I built by had were smaller. For instance, the resistors and diodes were vertically mounted and the caps were closer together. I could also put the silkscreen message with my name on the other side.
Maybe due to the low load, but the output is surprisingly smoother than the input. The idea of this device is to use a transistor in a circuit which simulates an extremely large filter capacitor–roughly speaking. I see some sort of high frequency noise on that sawtooth waveform, not sure where it is coming from. Note that I had to use a 50 times higher range to show any ripple at all in the output signal. I believe under full load, the output ripple will be maybe 1/10 of the input sawtooth wave.
Here is the test setup. The meter on the far left is measuring the dc voltage coming out of it, 502 volts. The meter on the far right is measuring the current through it, 1.1ma. And the scope on the top is measuring the waveform going in and coming out of it. Smoother is better. The device itself is buried under a bunch of wires in the center of the workbench. There is also a bunch of junk on the workbench not related to this activity.

I am currently working on the layout of a more complicated device–the circuitry for a complete high voltage power supply which I will use to experiment with guitar amp circuits.

There is the schematic of the device as I entered it into the KiCad design tool. The high voltage made it simpler to put two capacitors in series. The resistors by the caps are to ensure equal distribution of voltage among the two despite the inevitable differing leakage currents in the caps. The diodes are there to protect the transistor from power on and power off voltage transients. Other than that, it’s an extremely simple circuit but it does the job surprisingly well.

This is what it looked like in the KiCad PCB layout tool.

Leave a Reply