Neon based capacitor leakage tester

[Alistair D]

As a comparison, the attached file shows the equivalent circuitry from a Hunts CRB3.

[radioman]

Thanks guys - I should add that it might just be me... I seem to have problems trying to visualise circuits purely from a description.

Andy

[Mike Watterson]

I think I've got this right. The 100n across the neon doesn't need to be 630V. The neon strikes at no more than 150V, probably under 90V,  so a 250V capacitor will be fine. The neon and 100n could equally be in series with the pair of 1M resistors rather than the -ve of the HT.  If you are certain that the voltage rating of your resistors is greater than the applied HT then a single 2M2 resistor can replace the pair of 1M.

Yes, you could even use a 100V capacitor. The important aspect is no leakage! A 630V polyester is only for leakage. Most modern 150V plastic dielectric caps would do. A ceramic part might do, though at 100nF few are rated at more than 50V, though most of my modern larger size stock 100nF (50V and 100V) do pass at 600V! I wanted to be able to also measure very high resistances by timing the flashes.

The small 1/4W general purpose resistors are rated at about 200V to 300V depending on type. No problem in most locations even on a mains valve set. Larger size resistors not only look less ridiculous in a vintage set, but typically are 350V to 500V rating. I have some smaller resistors that are even lower voltage rating. It may be exacerbated by partial helical trimming. I have some new 33M parts that are still smaller than most vintage set resistors that are EHT rated, I forget how much; I bought them to make an EHT probe. They are a similar size to modern 1/2W parts.

The two resistors, rather than one, and in series with HT+ is to totally insure safety if the + terminal and 0V is grasped! It's not like resistors are expensive. I use pairs in series for HT capacitor bleed resistors. Also if I'm putting two electrolytic capacitors in series for higher voltage rating I put bleed resistors that use about x5 the worst likely leakage current. Mismatched leakage means one capacitor may be running close or above the rated voltage. Ceramic & plastic parts don't need the balancing resistors.

The Hunts circuit is the same really, but there is no HT isolation.

[ppppenguin]

The high voltage resistors are probably VR37. Used to be rated at 2500V when they were made by Philips but they seem to have had an upgrade to 3500V under Vishay. Excellent parts. I made an EHT probe from a string of 33M resistors over 40 years ago. I still use it.
https://www.vishay.com/docs/28907/vr25vr37vr68.pdf