How to safely probe high voltages - Page 1
Forum section: electronics / beginners

author: jdutky
author_rank: Regular Contributor
timestamp: November 09, 2020, 04:57:31 pm
content: I have an old Tektronix analog oscilloscope that I am evaluating to see what, if anything, needs fixing, and I’ve come to a part of the process that scares me: checking the high voltage supply rail to the CRT (-2450 V!).The highest voltage setting on the scope is 100 V/div, which I guess technically means I could measure signals up to 4000 V with a 10x probe, but I am, I think understandably, hesitant to do that for fear of damaging the scope taking the measurement (or myself). I’ve seen that there are high voltage 100x probes on Amazon at quite reasonable prices, but they don’t look any different from a regular probe. I’ve also seen high voltage multimeter probes that look like they offer some real protection from high voltage (big ceramic insulator wands).Aside from “don’t” what should I know about probing these kinds of voltages without killing my scope, multimeter, or myself?UPDATE: after reading the service manual again, I see that the high voltage rail does not need to be checked for ripple, just a simple multimeter check, so I guess I'm only concerned about killing my multimeter or myself, not my oscilloscope (my OTHER oscilloscope which I am using to check the ripple on the power rails).

author: tggzzz
author_rank: Super Contributor
timestamp: November 09, 2020, 05:56:22 pm
content: See the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Note carefully that not only do you have to not overstress the scope, you also have to avoid overstressing the probe. And the only way to avoid that is to carefully readallof the probe's instructions before purchase.

author: sandalcandal
author_rank: Unknown
timestamp: November 09, 2020, 09:19:05 pm
content: At this >1kv level you'll definitely want to use a specially designed probe that can safely handle and attenuate the voltage without breaking down. Most 10x probes are rated 300V CAT II and many 1x probes (and digital oscilloscope inputs) are rated to 150V CAT II. Fortunately, HV probes are not too expensive:100Mhz 4kV scope probe $US67:https://au.mouser.com/ProductDetail/Cal-Test/GE3421?qs=vgjKjNJexThwHr9aiUQ2Yg%3D%3D40kV DMM probe $75:https://au.mouser.com/ProductDetail/BK-Precision/PR-28A?qs=nqVuAx8duSTGQb3VR3fFRQ%3D%3DIt is also feasible to build your own HV DMM probe using high voltage resistors:http://archive.siliconchip.com.au/cms/A_111937/article.htmlIf you use properly rated equipment (that aren't dodgy imports) within specification there's no significant risk of harming your equipment. Using improper probes you could potentially get the full voltage arcing into your scope/DMM. Good quality DMMs typically have some sort of beefy MOV or TVS to attenuate over voltage input but such a level of protection is typically not present in scopes. Only gotcha I can think of with breaking properly rated HV equipment is that very fast rise times (V/ns) can cause breakdowns or bypass attenuation but given you're only probing a crt supply and not some something like a spark gap or military radar I think that's unlikely to be an issue.

author: daedalux
author_rank: Contributor
timestamp: November 11, 2020, 11:19:41 am
content: The classic tool to measure CRT high voltage is a 400Mohm series resistance probe with a long insulator for handling. You measured the HV determining your multimeter impedance and doing the math. You wired the probe ground to ground, and the probe had another two output wires to connect the multimeter without nothing extra to make a divisor. Other probes had a built in needle indicator as not much accuracy was needed for troubleshooting.Making such a probe is not trivial and has safety concerns, you should make a multiple resistor ladder with measures to prevent arcing and build it in a tube about 20cm long. And you should a megohmeter afterwards to test the result for safety and determining the probe exact value.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: November 12, 2020, 06:12:21 am
content: Don't use the scope! As you've found in the manual you just need to check the voltage, and for this you should use a proper HV multimeter probe on your DMM. I would not recommend trying to build one yourself, for a bunch of reasons such as safety and economy.When you actually go to do the measurement, turn the equipment off, discharge relevant capacitors, securely connect the probe to chassis ground and to the HV test point, make sure you are clear of everything, and then power up the scope under repair. The idea is to avoid groping around and trying to make connections while the HV is present.

author: jdutky
author_rank: Regular Contributor
timestamp: November 12, 2020, 04:53:52 pm
content: Quote from: alsetalokin4017 on November 12, 2020, 06:12:21 amRest assured that I have no plans to either try to probe the HV section of my scope with another scope, or to build my own high voltage probes. I am postponing the measurement of my scope's HV section until I have a proper HV probe for my multimeter (I'm pretty sure that I already have a high voltage probe for my multimeter somewhere in storage, but, failing that, I will simply order a new one: they're not nearly expensive enough to worry about and risk serious injury or death)

author: vk6zgo
author_rank: Super Contributor
timestamp: November 13, 2020, 04:54:14 am
content: Some scopes have a built in voltage divider from the high voltage, giving a test point voltage of around3-4 hundred volts.Double check whether yours does this.Of course, if you had an old AVO Model 8, you could directly measure up to 2.5kV with the early ones, increasing to 3kV with the later versions.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 15, 2020, 07:00:45 pm
content: Quote from: jdutky on November 09, 2020, 04:57:31 pmAre the control voltages that need to be examined by an oscilloscope on a CRT so large?The high voltage on the CRT is enough to check with a voltmeter, I think.