So whats the deal with B+ on chassis!? (Bad seinfeld impression) - Page 1
Forum section: electronics / beginners

author: algorithm
author_rank: Regular Contributor
timestamp: June 06, 2024, 08:28:05 pm
content: So I got a high voltage probe from like the late 60s for when I need to work on something containing kilovolts.Bear with me due to lack of photos.My concern is the aluminum plate the analog needle meter is on is connected to the probes + wire technically. (Through resistors)The circuit isProbe tip-400M ohm resistor-to pin 1 on a 5 pin phenolic terminal tag strip, said tag strip has a "ground leg" which is bolted to the aluminum face.Terminal strip pinout isPin 1=5.6k ohm between pin 1 and pin 3, and a 2.20M ohm between pin 1 and pin 2 "ground leg". Also a 0.01uf ceramic cap between the ground alligator clip on pin 5. And + wire to analog meter.Pin 2= mounts on the face plate and the other side of 2.20MPin 3=other side of 5.6k ohm and goes to the middle of a 25k variable resistor, the 25k variable resistor going across pin 3 and pin 4.Pin 4=aside from the variable resistor there is a 39k ohm and 27k ohm between pin 4 and pin 5Pin 5= alligator clip and ground wire to analog meter.So im just wondering if theres 10s of kilovolts going to pin 1 and thats only got 2.20M between that pin and the aluminum face plate is that unsafe? Well theres also 400M but I still have no idea on how safe that is.Because the body of the probe is largely comprised of the face plate so if its held theres a potential to grab or touch the face during test.Thanks in advance for any life saving advice.

author: aliarifat794
author_rank: Regular Contributor
timestamp: June 08, 2024, 03:20:30 pm
content: The presence of a 2.20MΩ resistor between the high voltage pin and the aluminum faceplate may cause a safety hazard, especially if the faceplate is touched during operation.

author: algorithm
author_rank: Regular Contributor
timestamp: June 09, 2024, 08:20:10 am
content: I had that suspicion as well.Yesterday I tried to do the math to be sure.Having never gone to high school I struggle with math, even using a calculator I question if ive done something wrong.Though according to my math attempts, if there was 50000 volts (the limit of meter) there would be 200+ volts on the face.Though my math also implied the amps would be 120 microamps.So even though that might not be so harmful, I suppose I should put an insulator between the terminal strip and face.Does anybody have a suggestion for an insulator? I thought of using a piece of a paint stir stick. Also thought of using epoxy. I figured the best option would be ptfe tape rated for 60kv, though from my research the only ptfe I could source locally isnt rated for 60kv or just doesnt mention breakdown voltage.I intend to use this probe on crt tvs. Ive had a tv disassembled for a few weeks relying on the probe. Im going to have to remove the flyback transformer and I want to make sure theres no highvoltage lurking before I get in there.

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2024, 09:34:05 am
content: If you put an insulator, the meter may give unreliable readings due to electrostatic attraction between the needle and the now floating faceplate. You have satisfied your self the touch current is no more than 120uA worst case. Unless the ground lead is disconnected, the voltage at pin 1 will be under 50V (actually a lot less unless the meter movement goes open circuit) so the touch current in actual use will be negligible.TLDR: Provided the 400Meg resistor and the housing round it haven't been damaged or contaminated, its safe enough as-is so just use it!