Neon based capacitor leakage tester

[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.