recable a 50kv high voltage test probe? - Page 1
Forum section: electronics / projects

author: crazyhog
author_rank: Regular Contributor
timestamp: October 03, 2017, 06:04:24 am
content: Hi,I recently got a Simpson 50kv test probe (pic)for my Simpson 260 vom and 303 vtvm. I have a couple of old Tek scopes that I like to use the probe to restore and the CRT are driven with 10kv and more. While the condition of the probe and its cables seems to be good condition, I am still concerned about the condition of the cable insulation given it's age (probably 40+ years).Am I right to be concerned about the age of the cables?A coaxial cable is used to connect the probe to the positive terminal of the meter. The internal shielding on the coaxial cable, via a braided anduninsulateconductor, is connected to the negative meter terminal and a wire with an alligator clamp to ground. The uninsulated conductor seems odd and dangerous. My understanding is a multimeter is for differential voltage measurements and can use any point in a circuit for ground reference. If so, the negative termial of the multimeter could be at a high voltage too! Unless, we must always use true earth ground when measuring high voltage? Is this right?If the recommendation is to recable the probe because of the age of the cable, what kind of coaxial cable should I get? Also where? By the way, is the use of coaxial cable the best practice for high voltage measurements? It seems some high voltage probes on ebay don't use coaxial cable.Let me also clarify my plan for my Simpson vtvm with this probe. The high voltage probe is designed for my Simpson 260 vom which can only be used at 1000v range (0 to 50kv reading). However I realize my Simpson 303 vtvm has a constant impedance of 10M ohm on all all ranges. This means, the probe (980M ohm) is a 100x probe for all the ranges of the 303 vtvm! I was quite pleased with the realization that I'll have fine resolution control.Thanks all for pointing me in the right direction. I am a newbie and your help is sincerely appreciated.

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2017, 07:33:35 am
content: Understand that after the 980Meg big resistor, it's all relatively low voltage, say 1kV maximum.The probe output cable is usually RF coax or audio shielded cable in probes I have seen.If it's rubber jacket, I would replace it due to cracks. But can be very hard to disassemble the probe, if glued together.

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2017, 07:48:55 am
content: I wouldn't be so sure it's 100:1 division until you check it against a known voltage.For CRO measurement of even the 2-3KV EHT a probe with 20K Ohm/V is barely high enough impedance for the task without loading the HV and returning incorrect measurements.The PDA voltage from the scopes multiplier is only just a few mA's AND at Negative voltage so you'd have to reverse the input for a moving coil meter. This probe might not be suitable for PDA measurement but for EHT even with it's low impedance you might just get away with it.

author: Ian.M
author_rank: Super Contributor
timestamp: October 03, 2017, 08:02:34 am
content: EHT voltage measurements are dangerous, especially when using vintage test equipment that does not meet current safety standards. *YOU* must assess the risks for yourself, and a second qualified opinion on your risk assessment is always a good idea.If you do not understand the risks *STOP* and seek qualified advice in person.My comments below are provided on a 'best effort' basis without assuming any liability.The uninsulated connection isn't a major problem. You *CANNOT* use such a probe safely to make a floating measurement as the meter case will only be rated to withstand a voltage of its maximum voltage range so one terminal of the meter *MUST* always be within a safe voltage range of ground and the Black (ground) lead of the probe *MUST* be connected to a point that guarantees that. Also, you *MUST* check the ground lead continuity *EVERY* time you use the probe - if its broken the 980Meg resistor will allow the meter internals to charge up to the EHT voltage you are trying to test and the insulation somewhere is almost certain to fail, even though the charging current is limited to max 51uA by the 980Meg resistor in the probe.The meter is *NOT* touch safe while the HV probe is being used - non-shrouded plugs and exposed grub screws mean you need to set it up on the bench and read it 'hands off'.However if it bothers you and you expect to need to make measurements of an EHT supply relative to a HT rail, disassemble the black plug and see if you can detach the braid and sleeve it with two layers of clear heatshrink before reassembling the plug. (Clear so its integrity can be verified by visual inspection).Its *NOT* a x100 probe. With a 10Meg meter its actually a x99 probe - if the meter has an accurate 10Meg impedance. You'd need to add a series resistor of 101K at the meter end to make it a x100 probe. You should measure the meter input impedance and calculate the divider ratio to derive a correction factor for that meter. With its intended 20K/V meter on a 1KV range, that's 20Meg making it a x50 probe (as its intended to be - 50KV FSD on the 0-50 scale on the DC 1KV FSD range). Also note that its a DC probe - Depending on the cable capacitance and the meter input impedance it could under-read significantly at frequencies as low as a few hundred Hz!Rewiring it may not be an option - it says "The resistors are sealed in a special compound ..." and if that includes the cable end of the 980Meg resistor you are SOL. However the knurled collar where the cable emerges *may* simply unscrew (be gentle) and if so, cable replacement may be possible. Probably the best course of action would be to dismantle the red plug so you can examine the cable inner insulation for any signs of degradation, then Hi-Pot test it between the red plug and the braid to 1KV (+10% if possible), and between the braid and the outside of the jacket over the whole length of the probe wire to pick up any nicks or cracks in the jacket. Also examine the probe body itself under high magnification for any signs of cracking - I really don't fancy trusting 40 year old moulded polystyrene!P.S. post a scan of the whole of that instruction leaflet - I cant find the manual for theSimpson part no. 0509 50,000V high voltage probeon Google.

author: crazyhog
author_rank: Regular Contributor
timestamp: October 04, 2017, 04:24:09 am
content: Hi all,Thank you all for your thoughtful and detailed feedback. I now have a better idea of the issues and what to research further.I've look inside the probe and verified it can be rewired. The 1000M resistor is sealed but the two ends have metal contacts that presses against the cable via a spring. As such new cable can be refitted. The measured resistance of the resistor is 1050.67M ohms. A bit of a bummer that it has drift this far.I've attached a scan and OCR copy of the voltage probe's manual.

author: Ian.M
author_rank: Super Contributor
timestamp: October 04, 2017, 05:06:53 am
content: So forget about using it with the Simpson 260, unless you are happy with it underreading by 7%. OTOH you could bring it back into calibration for use as a x100 probe with your VTVM - simply measure the VTVM's nominal 10Meg input resistance on the same meter you used to measure the probe and add a series padding resistor at the red plug to bring the total of the meter + padding resistor up to to 10.613Meg (1050.67/99). As at the full rated 50KV there is only 500V at the VTVT, the additional resistor will only ever have a few tens of volts across it so doesn't need to be anything special.The only issue would be insulating its body from the nearby braid and figuring out how to mount it to be mechanically robust. Making a new longer shell for the red plug might help there, or it might be possible to do something at the probe end with a small SMD resistor sandwiched between two disks of copper foil covered with Kapton tape on the inside apart from a slot in each to solder the end of the SMD resistor to, and the rest of the space between the foil disks filled with an insulator, to produce a miniature 'hockey puck' resistor that will fit between the nominal 980 Meg resistor that's gone high and its contact to the cable.

author: crazyhog
author_rank: Regular Contributor
timestamp: October 06, 2017, 07:08:08 am
content: Doh, I just realized my measurements of the 1000meg ohm resistor is unreliable! It's not simply connecting the resistor to the positive and negative terminal of my hp 3456A. The accuracy according to the spec is +/- 11%!. Attached is the hp spec.How do I get a more precise measurement of 1G ohm resistor?Hmm, I can accurately measure the vtvm impedance. I could hook the high voltage probe to the vtvm and use vtvm to measure a known high voltage souce. I can measure the voltage drop of the probe and the voltage drop of the meter with the hp3456A. From that I can calculate the probe's actual resistance. Do I need to worry about environmental effects? Other ideas to measure very high resistance?

author: crazyhog
author_rank: Regular Contributor
timestamp: October 06, 2017, 07:19:17 am
content: Argh, I cannot accurately measure voltage drop either because the impedance of the hp 3456A is not high enough! The act of measurement will change the results.I suppose I can treat the hp voltmeter as a parallel resistance to the vtvm and calculate the affective resistance.Will try tomorrow.

author: Someone
author_rank: Super Contributor
timestamp: October 06, 2017, 07:25:12 am
content: Quote from: crazyhog on October 06, 2017, 07:08:08 am...high voltages, bringing us back around in the circle. If you can measure a several hundred k ohm resistor accurately, use that as the low side of the divider and attach the probe to the highest voltage you can otherwise measure and generate at the other end.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 06, 2017, 10:05:11 am
content: Quote from: crazyhog on October 06, 2017, 07:08:08 amYou need an electrometer like an older Keithley 614 or 617 or a modern 6517BAlternatively, even if you would apply 1000V to your resistor from a reliable voltage source, you would need to measure the current precisely to less than 1uA to get good results.Be extremely careful, this is dangerous stuff.Especially when you have no experience with high voltage

author: crazyhog
author_rank: Regular Contributor
timestamp: October 06, 2017, 03:19:49 pm
content: Hah! I forgot I had a voltmeter with infinite impedance!It is my glorious Fluke 887AB differential voltmeter! I brought this voltmeter from ebay and repair it after I saw a similar model on youtube. It is a very interesting piece of electromechanical gear. It has a mechanical chopper that makes a clicking sound and you have to work all the dials to get the measurements. And it has 6 digits! Although I didn't have the patience to hunt down all the resistors that have drifted out of spec and therefore it cannot be calibrated to factory accuracy.. Some day I'll get to it.I never thought it is still useful, never mind besting the hp 3456A! To infinity and beyond!!

author: crazyhog
author_rank: Regular Contributor
timestamp: October 06, 2017, 04:12:44 pm
content: I thought I should try to verify the infinite impedance of the fluke 887ab and not take the theoretical claim at face value. It really is infinite!Here is the test setup. I am using the hp3456a to measure the resistance of the fluke 887ab. I am also using the 887ab to measure the resistance test voltage of the hp. The hp can measure 1G ohm. As the pic shows, the resistance of fluke is too much for the hp to measure! Also note the hp resistance test voltage is 5v.I know this is not definitive proof since the hp 1G ohm < infinite. Nevertheless, this is enough supporting evidence for me given the theory of operation of the fluke.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 06, 2017, 06:48:57 pm
content: You can replace your 887ab with any meter, a pot, and a low-voltage power supply. See here:https://www.eevblog.com/forum/blog/eevblog-584-what-effect-does-your-multimeter-input-impedance-have/msg396860/#msg396860For testing, if you use your 3456A to measure any convenient voltage (maybe 50 - 100 V), you can then set up a voltage divider with your probe and a 10M resistor. Your 3456A will be able to read the voltage (0V5 to 1V0) with its 10G impedance.Use the 3456A to precisely measure the 10M resistor and you can calculate the value of the probe.But you don't really have to do this testing because if you add a 10M resistor in series with the probe and the 3456A, magic happens! The probe changes from 980M to 990M and changes from a 50:1 probe, i.e. (980 + 20)/20 = 50, to a 100:1 probe, i.e. (980 + 10 + 10)/10 = 100. The maximum voltage is still 50KV, but the output voltage is now <= 500V. If the probe resistance has drifted, you will probably still be able to calibrate it by picking the right resistor.You must be careful when using this probe. The 100:1 division only applies if the meter has a 10M impedance. If the meter autoranges to a low-voltage, high-impedance range you could then put many thousands of volts into your poor meter! Granted it will be current-limited, but still....Ed

author: crazyhog
author_rank: Regular Contributor
timestamp: October 09, 2017, 08:16:24 am
content: Thanks all for your help. My previous direct measurements of the hv probe was indeed way off. The correct resistance of the probe is 978.8M ohms (vs spec 980M) which is excellent.Simpson: Instruments that stay accurate!