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High Voltage Tester
- Thread starter AudioShockwav
- Start date
I cringed when I saw the clip leads with that much voltage on them. They typically are made with only 600 volt rated wire. Back in tech school days I determined that a television we had in the shop needed a new fly-back transformer. For some reason the instructor told me to wire the new one in with clip leads first to check it. There was almost 28,000 volts, yes twenty eight thousand volts, and it worked. I shut it off and as I went to disconnect the clip leads a couple had fused together. The plastic insulation had broken down under the stress of that much voltage. God only knows how many microseconds we were away from frying it. LOL
I’d like that kit. Would be handy for hi pot testing some of the doorknobs and vacuum capacitors I have laying around. Wonder if he sells kit or has schematic.
Definitely more compact and lightweight than mine. We built one based on Rich Measures' design. Uses a neon-sign transformer with appropriate current limiting. And a meter to display leakage current.
Had to go to the Wayback Machine to find it.
https://web.archive.org/web/20121017082937/http://www.somis.org/BVT.html
If Tom's had a current meter as well, I'd like it a lot better than mine.
73
Had to go to the Wayback Machine to find it.
https://web.archive.org/web/20121017082937/http://www.somis.org/BVT.html
If Tom's had a current meter as well, I'd like it a lot better than mine.
73
Exactly. With a meter to measure leakage current, you can start to see it at the microamp level. With only an LED, you are well into the milliamp range before you can start to detect leakage. While that's fine for tubes, more sensitive components like capacitors, can have their insulation damaged with higher current flowing. I also have to wonder how hard it could have been to stop the LED from burning out. Some 1N400X diodes to limit voltage drop and a resistor to limit current from there? It's the same thing we do with the IR LED in an optocoupler, to prevent it from burning out in a fault condition.
Ah, if you put a 1 Meg resistor in the circuit, and a neon lamp in parallel with it, you'll see 1 Volt of drop for every millamp of leakage current. Most neons won't ionize and glow below 70, 80 Volts or more. Would make the minimum reading 70 uA. The meter on mine pegs out at 50 uA. The tube tester uses a higher resistance than 1 meg for this test, most likely.
The actual current to light a neon glow lamp doesn't need to be much, so long as you shade it from room light. To make it bright still takes a few milliamps minimum.
73
The actual current to light a neon glow lamp doesn't need to be much, so long as you shade it from room light. To make it bright still takes a few milliamps minimum.
73
This is what we used back when I worked for a short time at a TV shop. Color sets had an over voltage protection circuit on the fly-back to prevent people from being x-rayed. This type of probe was used almost daily in the shop and in the field to make sure the HV was within limits.
Not so Nomad. Check out the datasheet for the old NE-38S here: https://www.mouser.com/datasheet/2/423/NeonLampsRev3-1222934.pdf The specified current to light this neon indicator is only 300 microamps. The 90 volt drop is insignificant when dealing with the high voltage associated with this tester.
I also wouldn't place any resistor in parallel with the neon light because that would reduce its sensitivity and still not provide a fixed current limitation to the bulb. That's where I would place a Zener in parallel with the bulb, with a voltage just above the point where it lights. That would protect the bulb regardless of the voltage setting or leakage current, without increasing the current required to light it.
I’d have to sit down later and do some math. But why couldn’t you float the B- to a certain point and then use the B- to measure current like in a tube amp? Instead of trying to measure or read something on the B+? I’m asking, not saying you should. Then maybe you could use 100uA meter. Still need a series resistor on B+ to limit to say 100uA as a safety precaution. I’m just asking, as I’m not sure this would work? I’m happy to take feedback from those with more experience.