High voltage probes for oscilloscope - Page 1
Forum section: electronics / beginners

author: DisasterNow
author_rank: Regular Contributor
timestamp: December 20, 2023, 04:27:40 pm
content: Hi, with the probes of my Siglent 1204, I only take measurements on low voltage. However, I often have to carry out checks, for example on the high voltage part of the power supplies (230 AC, or around 320 DC). I would like to be able to monitor the high voltage signal on the oscilloscope. I know that there are differential probes (but they cost a lot), or a safety transformer is also used (high price).My high voltage measurements would be sporadic, so I don't want to spend too much.I would like your opinion:the idea would be to take two 100x passive probes (for example I've seen cheap: Cleqee P4100 100:1).For alternating voltage, I would use one probe at 100x on the phase and the other on the neutral (differential, without using the probe's negative crocodile), I should see the wave without major risks right?For high voltage DC, could the same thing be done?Thank you.

author: u666sa
author_rank: Frequent Contributor
timestamp: December 20, 2023, 05:16:38 pm
content: Probes aside, you need "galvanic decoupling", improper English warning.YouTube search for galvanic decoupling.

author: DisasterNow
author_rank: Regular Contributor
timestamp: December 21, 2023, 03:03:24 am
content: Thank you. However, I was wondering if anyone on the forum uses TWO normal probes (100x) to measure a mains voltage, without using the connection of the probes' ground crocodiles. Or does everyone use expensive differential probes, or a safety transformer?

author: Proxy64
author_rank: Regular Contributor
timestamp: December 21, 2023, 05:12:01 am
content: Quote from: DisasterNow on December 20, 2023, 04:27:40 pmIt’s not a high price, specially if you make one by yourself, I made one with two same transformators connected head-to-tail.I mean:INPUT > T1: 220V/110V ——— T2:110V/220V ->OUTPUTYes their efficiencies are not 100% but that is not really an issue, specially for smps repair.For 10:1 probes you should have a look to datasheet of yours, mine from Rigol are CAT II 300 V, so not enough, so I use 2KV 100:1 probes from hantek (Aliexpress).In an smps with a 310V DC bus, there is higher tension spikes on the Mosfet drain.Beaware, using an isolation transformer does not mean that there is no risk !

author: bdunham7
author_rank: Super Contributor
timestamp: December 21, 2023, 05:26:23 am
content: Yes, 2 100X probes and the math function (CH1-CH2) will work OK at mains frequencies. For high-energy circuits (directly to mains) you might want a better quality or ideally CAT-rated probes as just high resistance and voltage rating doesn't mean the connectors and probe tips will be suitable (safe) for those applications. If you really want to connect directly to a mains circuit that can't be reasonably isolated, then a CAT-rated HV differential probe is the way to go.The Cleqee P2301C has better connectors for HV, but is not CAT-rated. I wouldn't use any less on mains. Two of them run $80 or so, a Micsig DP10013 HV differential probe, CAT-rated and with some decent connectors is less than $200.

author: EPAIII
author_rank: Super Contributor
timestamp: December 21, 2023, 07:21:05 am
content: First, 230 VAC is not normally considered high Voltage. High Voltage is considered to be 1000 Volts and above. 230 VAC or 424 Vp-p is considered medium Voltage.But you asked about 230 VAC which is the RMS value. I looked briefly for the specs on your scope but had no luck. I am not going to spend more time on that. I did check the manual on my Hantek DSO and found that the maximum rating of the scope, with a 10X probe is 300 VAC, RMS. That means that the rating at the input jack for the probes is about 1/10 of the p-p value of 300 VAC or around 42.4 Volts, DC. In short, 230 VAC, with a 10X probe is perfectly safe on my scope as long as the probe is rated for 325 VDC or higher (230 V x 1.414 = 325 V).Now, what about the question of the ground connection. Every scope that I have ever used, going back at least 60 years, has had a ground pin on the power cord so the outer shell of the scope was grounded to earth potential, more or less. And the incoming power, from that same plug/socket is also referenced to that earth ground. This is the same power mains that come into your power supply. I have never had or seen a problem with ground connections and probing mains Voltages on any of those scopes and would be very surprised to see one on a DSO.One more fact: the wavelength of a 60 or 50 Hertz signal is around 6000 Km. So the distances between the ground on the power cord and on the probe ends is TOTALLY negligible. It does not matter for the line frequency and for any reasonable harmonics thereof. You should see reasonably accurate waveforms regardless of where you establish the ground connection and even if there are multiple such connections.I would not do the same with your scope without checking the specs for the scope and the 10X probe, but with my scope I can check 50 or 60 Hertz signals with no problems about where the ground is connected. Unless your power supply has some kind of floating common which is not tied to earth ground, AND that floating common IS CONNECTED to a different potential, then there should be no problems probing the mains power in that power supply with my scope with a suitable 10X probe. This is my first DSO. Before I have always had CRT scopes, usually Tektronix but I have used other brands (HP, R&S, etc.) and this would not have been a problem on any of them: WITH a suitable 10X probe. And so far, I have not seen any indication that there may be any with my scope.Now once you get on the other side of a power transformer or beyond a galvanic connection, things may change. I doubt that a common point would have a significant enough difference from earth ground to produce any real dangers, but some I have seen some low Voltage supplies that rode on top of a real high DC Voltage.But please carefully read and understand the specifications of your scope before doing likewise. I would be very surprised if there is a problem, but you never know.Understand YOUR equipment! And understand what you are doing with it. But for checking mains potentials in a power supply, I don't really think you need anything beyond normal 10X probes as long as those probes are rated for that Voltage.

author: DisasterNow
author_rank: Regular Contributor
timestamp: December 21, 2023, 04:13:23 pm
content: Quote from: Proxy64 on December 21, 2023, 05:12:01 amDo you have a diagram of how to build this safety transformer using two transformers?Thanks for the advice on the 2k probes.Would the transformer then power the circuit to be analyzed or the oscilloscope?Quote from: bdunham7 on December 21, 2023, 05:26:23 amHowever, I need to understand if my Siglent 1204 has the channel sum function. Of course if I were to use this method, combined with two 100x CAT probes for voltages above 400 volts. Thank youQuote from: EPAIII on December 21, 2023, 07:21:05 amI'm not at home now. As soon as I get back I will check the specifications of the oscilloscope inputs and the 10xc probes supplied. If I remember correctly they are guaranteed for 300v, but the peaks in the power supplies are slightly higher. Thanks for all the explanation, very carefully read.

author: bdunham7
author_rank: Super Contributor
timestamp: December 21, 2023, 05:35:38 pm
content: Quote from: DisasterNow on December 21, 2023, 04:13:23 pmAssuming you have the SDS1204X-E, then yes it definitely has the channel sum/difference option in the MATH menu.The probes included with the scope are rated for 600V when used in the 10X mode, but you wouldn't be able to fit the signal on the screen because the scope has a maximum input setting of 10V/division and 4 division, so with a 10X scope that is 100V/division or 800V from the top to the bottom of the screen. A 230VRMSsignal might just barely fit on the screen. I don't recommend using the included 1X/10X switchable probes on mains voltages--not because they can't work, but because the risks of an error or mishap are pretty high.

author: DisasterNow
author_rank: Regular Contributor
timestamp: December 22, 2023, 02:30:18 am
content: I checked, the original probes supplied with the Siglent are certified at 10x no more than 300V, they are CE-catII (in the probe manual they reported 600 Vpp...); while the input channels are guaranteed no more than 400 Vpk.However, perhaps it would be more appropriate to use a 100x probe.Therefore, for a measurement on the mains voltage (230AC) and on the DC voltage, you need to have at least 100x probes.I will look on Aliexpress for a good quality 100x probe.---I finished my research: as suggested by a user above, I could get an entry level differential probe, like the micsig DP10013, which costs around 200 euros. Among low-cost differentials, is it one of the best choices? I see it has to be powered by USB? However, my Siglent has a USB port (perhaps the one on the back for connecting to a PC, so that the one in front remains free for saving on a flash drive). I accept advice on the choice. Thank you

author: MarkT
author_rank: Frequent Contributor
timestamp: December 23, 2023, 11:36:27 am
content: Cat II is the minimum for single phase mains, though specialized cat III high voltage probes are usually ergonomically safer, keep your hand well away from the action. Multimeters are often better rated than 'scope probes, note.You have to be wary with mains because it can carry transient spikes well above normal mains voltage (kilovolt range), and that's what can do the damage (cause flashover / arc-flash) in inadequately rated equipment. Once an arc is established hugh amounts of power are available to do damage. It only takes one dodgy fridge relay or fluorescent light choke or lightning storm to trigger such a transient.

author: DisasterNow
author_rank: Regular Contributor
timestamp: December 23, 2023, 11:49:29 am
content: Quote from: MarkT on December 23, 2023, 11:36:27 amThanks Mark. My choice will be to buy a probe at least Cat II. Consider that when I measure mains voltages, I always use electrician's insulating gloves for safety.I'm evaluating a Micsig probe, I'm undecided between 4 models (I opened a specific topic for the choice).Thanks again.