[Sticky] New project: Capacitor reformer

Posted by: Reelman

BTW a very workmanlike piece of kit that’s earning its keep and a great write-up. I already feel a hand reaching for the iron to build my own...

Sorry had not spotted the circuit pictures when I asked for the .pdf.

Peter

Thank you Peter for your kind words 🙂
A lot of the parts were from RS. The transformer was quite hard to find. The section of the wafer switch that deals with the ammeter is MBB (make before break). The other 2 sections are BBM (break before make).
If you have difficulty in getting parts, let me know and I'll dig out the suppliers and part numbers for you.
Thanks again.... Have fun 🙂
All the best,

Tony

Hello Tony,

As has been said, a nice piece of kit.

Just one point, you have the capacitor discharge circuit "upstream" of the current meter. This means that if you switch to discharge with a charged test capacitor connected the discharge current will flow backwards through the meter. The diodes mentioned in previous posts will help protect the meter but it may be better to have a push button (changeover) discharge function connected directly across the capacitor.

Peter

Posted by: electronpshr

Hello Tony,

As has been said, a nice piece of kit.

Just one point, you have the capacitor discharge circuit "upstream" of the current meter. This means that if you switch to discharge with a charged test capacitor connected the discharge current will flow backwards through the meter. The diodes mentioned in previous posts will help protect the meter but it may be better to have a push button (changeover) discharge function connected directly across the capacitor.

Peter

Hello Peter,

Welcome to forum 🙂

You are entirely correct about the reverse flow through the ammeter, when the voltage selector is set to Discharge. I could move the ammeter to another part of the circuit, which would make sense. It would only need me to shift a few wires over.

What I was thinking of doing was adding a normally closed momentary push switch across the ammeter, which would act as a 'Push to read' switch. At the moment, I only use the ammeter to briefly measure the current through the test capacitor, once the neon stops flashing. If I then take readings every once in a while, I can plot the progress of the reformation without having the ammeter constantly in circuit. This is the way I had always intended to utilise the ammeter. Putting a 'Push to read' switch across the ammeter would then protect the ammeter at all times, except for when necessary to obtain readings.

As it happens, I picked up a TV for restoration today, so the reformer will probably be in action over the weekend. I may well have a tweak of the circuit, if the chance occurs.

Thanks again Peter, for taking the time to offer such sound advice. It is appreciated 🙂

All the best,

Tony

Hello all,
I've burning the midnight oil again and getting busy with the revamped reformer, as below

I've added a small power supply and fan to help cool the components, which I felt was needed if the unit would be on overnight, say.
I've also added a "Push to read" switch, across the ammeter. The switch is biased to the normally closed position, so that it keeps the ammeter safe until such time as it's needed for a reading.
There's also a couple of germanium diodes (thanks Marc!) across the ammeter to deal with any overloads. They do interfere with the calibration of the ammeter, especially on the 100uA range. For now, I've left them off, until I can find a happy medium. The ammeter is heavily protected now with the extra switch.
As you can see, I've left the ammeter in the same position as before. If I put it in between SW2b and SW2c, the voltmeter will draw some current, skewing the readings. The best place I can see the ammeter give a true reading of the leakage current through a test capacitor is exactly where it is, so it stays 🙂
I'll post up a few photos of the finished bit of kit when I get some time.
The reformer will be getting used again shortly... I have another restoration to do, but before that, there's another piece of kit that I'm building. Details soon!
All the best,

Tony