Is it possible to use low (safe) voltages to test high voltage equipment? - Page 1
Forum section: electronics / beginners

author: billbyrd1945
author_rank: Regular Contributor
timestamp: December 02, 2018, 02:03:20 pm
content: It really gets cumbersome and dangerous to check circuits powered by line voltage. Is there some way to use an AC wall wart (or something else) to do the same test allowing for changes in the values? If so, one could then switch to high voltage if there was good reason to think the component was functional. This would seem an excellent way of testing when one has a known-good board to test against a malfunctioning board. Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 02, 2018, 02:12:19 pm
content: In general? No.The activation energy of a faulty component (e.g., capacitor breakdown) can take many forms. Whether this can be tested with reduced input voltage, who knows -- the rest of the circuit may not even activate at low voltage.You might also consider an energy limiting fuse device. I built myself a range of electronic fuses for this purpose, some which allow high currents but open very quickly (microseconds), some which strictly limit current and open slowly (~100s ms).Tim

author: cdev
author_rank: Super Contributor
timestamp: December 02, 2018, 02:16:47 pm
content: T3sl4co1l,I'd like to hear more about your fuses!Quote from: T3sl4co1l on December 02, 2018, 02:12:19 pm

author: billbyrd1945
author_rank: Regular Contributor
timestamp: December 02, 2018, 02:27:59 pm
content: I agree with CDEV. I think we need some pics or diagrams. This is a very important subject in my book.

author: cdev
author_rank: Super Contributor
timestamp: December 02, 2018, 02:29:43 pm
content: Maybe what you mean is for testing antique radios, test equipment, etc?AVariacor variableautotransformerwould fulfill the lower voltage testing function (for example, its good to use a lower voltage or an incandescent light bulb in series when turning on antique equipment for the first time in years)but it wont supplyisolation. Which you also need with old equipment because some of it is not built to modern safety standards at all. Sometimes it has line level voltages in places you could easily make contact with.Dont touch anything with your body. That old radio chassis or its knob shafts could be hot. And also combine the variac with a medical grade isolation transformer.How they are connected together is important and other people here know how this needs to be done. (I never work with high voltage, period.)It may be in the site's sticky documents.Don't float your scopeor other test equipment. That kills people. Get the above setup (isolation transformer, plus variac, or in some cases instead of the variac you can use a power plug modified so that the device under test when power is first applied receives it in series with a 100 watt light bulb) and use it properly.Also install aground fault circuit interrupter socket (GFCI)at your bench so that any shorts in your equipment result in GFCI turning the power off immediately, rather than the short starting a fire or killing you. Between all these things, you should be much safer than you would be without them.Quote from: billbyrd1945 on December 02, 2018, 02:03:20 pmIt would be even more cumbersome being dead!Do some research into the right ways to handle the dangers presented by old possibly dangerous line powered equipment.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: December 02, 2018, 02:48:55 pm
content: No, not testing old radios. Actually trouble shooting good PSU against dead PSU for modern Peltier wine cooler. Requires 110v and goes up to 150v or more on some components. I appreciate the difference in cumbersome and dead, but thought there might be a third choice. :-)

author: cdev
author_rank: Super Contributor
timestamp: December 02, 2018, 03:10:39 pm
content: Try to get the PSU's service manual if one exists, or whatever other documentation may exist.These days many PSUs are rebadges of other PSUs so even if the exact model you have doesn't have its service manual online, its not unlikely that another version thats similar or identical to it exists elsewhere under another brand name and model number.By doing that I managed to get schematics for both of my bench supplies and a service manual for one of them. (and fixed the other which I bought broken, so now I don't have that immediate need I once did)

author: Vtile
author_rank: Super Contributor
timestamp: December 02, 2018, 03:18:45 pm
content: If there is transformer in equipment you can always bypass that and use array of lab supplies to feed the electronic power rails. If it works then there is odds are that the transformer of type ABC is malfunctioning.At mains level components the test voltages are in many cases much higher to verify the safety margins, one good example of this is the CAT ratings in the multimeters. Where applied voltage might be something like 12kV.

author: bc888
author_rank: Regular Contributor
timestamp: December 02, 2018, 05:43:46 pm
content: Quote from: cdev on December 02, 2018, 02:29:43 pmThank you! Concerning your very last sentence...."Do some research into the right ways to handle the dangers presented by old possibly dangerous line powered equipment." Could anyone provide any links for this?

author: cdev
author_rank: Super Contributor
timestamp: December 03, 2018, 08:13:19 pm
content: There are definitely a fair number of people here who collect old radios and test equipment and if you post your question with a title that makes it clear thats what you want to know about, you'll get a lot of responses.Anything with tubes in it (like a CRT) and likely lots of old solid state equipment too, anything that runs on line voltage, likely has dangerous voltages in it.A meter that can warn you about this (like a Uni-T UT201E clamp meter with NCV and non-invasive current measurement) is really good for that kind of situation because you can both get an audible warning if any circuit is energized (I need to see how well it warns you about caps that remain charged up )and you can measure current without making any electrical connection to it directly.

author: amyk
author_rank: Super Contributor
timestamp: December 04, 2018, 02:11:04 am
content: Quote from: billbyrd1945 on December 02, 2018, 02:48:55 pmI wouldn't really consider that "high voltage", when I read your first post I was expecting something >1kV. Probe only with one hand, ensure that you are not touching anything else grounded. Remember you will only get shocked if a current is able to flow through you.