EHT measurement probe

[Dave Moll]

Doctor Who's thread about EHT in his television has set me to wondering (again) about whether anyone still sells EHT probes - the ones which incorporate an electrostatic meter.

I own (though I've been very reluctant to try using it) a probe that purports to measure EHT from about 5 to 30 KV by adjusting the gap between two metal spheres until an arc is formed. I would, however, feel much easier about using the electrostatic meter type of probe - and I do like to be able to measure things.

[ppppenguin]

I have used my home made 25kV EHT probe for many years. It's 500M of resistors in a paxolin tube, all neatly soldered. There's an earth guard ring and a lead to feed a 50uA meter. The actual resistors are VR37 series, 33M with a padding resistor to make it exactly 500M. There's also a resistor (100K?)across the output lead to stop it floating when the meter is disconnected.

The resistors do need to be rated for high voltage, hence VR37 series.

I did calibrate my probe with a DVM and a 1kV supply but it's going to be within 5% even if you don't do this.

I suppose that 50uA at 25kV is more than an electrostatic meter takes but I doubt that will be a problem. These days I use a DMM on the uA range and convert to kV in my head, remembering that each uA is 500V.

[SPCh]

Your "ad hoc", Jeffrey, is almost identical to at least one (previously) commercially available EHT probe: a paxolin tube full of megohm resistors (but without the no-load safety tie-down). I used to use an EHT discharge resistor (off an old Ekco) in a paxolin tube, and do the arithmetic in my head !

[ronbryan]

Hi

I've got a Brandenberg EHT meter which measures up to 30kV DC. It uses 3 x 15G ohm resistors in series, mounted in a potted housing located in the instrument itself, with a large ribbed feed-through insulator for the HV probe flying lead. The series resistors then feed a 3140 op-amp, which drives a large mc meter with mirror scale. It claims a current of less than 1uA with an accuracy of 1%. How that compares with the spec. of an electrostatic meter is unclear - presumably even the best electrostatic instrument will have some leakage current.

The instructions include a warning not to connect the probe to HV supplies capable of giving more than 5mA without the supply being switched off. Seems a sensible idea to me!

Ron

[Skywave]

Originally Posted by ronbryan

Hi

I've got a Brandenberg EHT meter which measures up to 30kV DC. It uses 3 x 15G ohm resistors in series, mounted in a potted housing located in the instrument itself, with a large ribbed feed-through insulator for the HV probe flying lead. The series resistors then feed a 3140 op-amp, which drives a large mc meter with mirror scale. It claims a current of less than 1uA with an accuracy of 1%. How that compares with the spec. of an electrostatic meter is unclear - presumably even the best electrostatic instrument will have some leakage current.

The instructions include a warning not to connect the probe to HV supplies capable of giving more than 5mA without the supply being switched off. Seems a sensible idea to me!

Ron

Yes, I own one too - from my CTV servicing days. Good, aren't they? The one feature I DO appreciate is the big red guard so that your hand doesn't slip down the probe handle onto the EHT source!!! The "Test Batt." switch position is a worthwhile feature too.