Probe issulation to my scope - DIY / buy? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 16, 2019, 04:00:32 pm
content: I know that I can buy differential probes to my scope but the price keeps that a dream.The only need I have is to avoid common ground without having to use two probes for one measurement.We are talking about DC and maybe AC but not HF.Is that possible to build or buy for lest then what a cheap scope costs?

author: //Matt//
author_rank: Contributor
timestamp: April 16, 2019, 04:39:10 pm
content: https://www.analog.com/en/design-center/reference-designs/circuit-collections/ltc6268-10-oscilloscope-differential-probe.html#cc-overviewThere are schematics like this one which claims a bandwidth of around 100Mhz. The lowest price for a differential probe on eBay is around 120 £.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 16, 2019, 08:13:36 pm
content: Thank you a lot, great link! :-)What enclosure would be best for this probe in a noisy environment (LED, SMPS, 2.4GHz...)?

author: not1xor1
author_rank: Frequent Contributor
timestamp: April 17, 2019, 05:24:57 am
content: Quote from: FriedMule on April 16, 2019, 04:00:32 pmif you want real insulation you need an insulated probe, a differential probe may avoid problems arising from oscilloscope ground connection but usually are not insulated

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 17, 2019, 07:27:32 am
content: Lets say I want to measure voltage on both sides of an transformer, 240V - 12VWould that suggested circuit me sufficient?What shall I make of enclosure, so the circuit is not just bare and are somewhat protected against common noise?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 17, 2019, 08:43:40 am
content: Micsig DP10013 is less than a cheap dso. isolated diff probe is much more expensive than non-isolated one like tektronix, agilent, pintek etc that still cost much more than Micsig. at least you have few Meg ohm non-galvanic isolation, better than normal probe. ps: i'm currently resurrecting my old hi-volt diff probe design for second pcb manufacturing, because although Micsig is cheap, 4 of them is not. and the design is cluttered of wiring everywhere and 50/500X attenuation is not amusing for me. so i thought i'll do my way, that design posted above is not good enough as it is ac coupled and only few Kilo ohm of input impedance. if i fail again, maybe Micsig it will be. ymmv.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 17, 2019, 09:30:01 am
content: It's looking as though anyone with long term probing intentions needs to dig deep in the pocket and buy TWO differential probes,buy once - cry once - less BANG! potential - and or 'live long and prosper...'or do it the old school frugal way with two 10x probes in the scope's differential mode,and LOSE those pesky BANG! prone croc/alligator/pirahna clipsThere are other ways to sort of 'get by' but that invites DRAMA and misfortune to the uninitiatedand I won't go there, as everyone has differingtial coin toss luck levels

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2019, 09:52:46 am
content: Quote from: //Matt// on April 16, 2019, 04:39:10 pmOne thing that would be worth doing would be to replace C1 and C2 with Y1 rated safety capacitors. I'm not saying that it would make it safe but it would make it safer.Safety relies heavily on constructional factors too.

author: //Matt//
author_rank: Contributor
timestamp: April 17, 2019, 11:39:57 am
content: Quote from: Gyro on April 17, 2019, 09:52:46 amGood idea.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 17, 2019, 11:49:13 am
content: Would a small dual compensated R/C front-end add more safety? at what cost of BW and gain? what about C6?

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2019, 03:56:45 pm
content: QuoteProbably difficult to do with a differential instrumentation amplifier configuration (don't know). Regarding C6, it ought to be ok safety-wise as the output is referenced to the coax shield anyway. Unfortunately it needs to be rather large as the circuit is intended to drive a 50R load - it would need some care in choice if the circuit is designed for 100MHz.

author: rstofer
author_rank: Super Contributor
timestamp: April 17, 2019, 05:49:16 pm
content: Quote from: FriedMule on April 17, 2019, 07:27:32 amI wouldn't use that circuit at over 12V, maybe 50V if I was desperate. The first op amp is powered at 5V and has a 100:1 divider on the input coupled with a gain of 10. So a 10:1 reduction ratio overall. No matter how you look at it, the inputs to the op amp can't go beyond the rails. Then the second stage kicks in with a gain of 10x so the output voltage is the same as the differential input voltage - but it is still limited by the 5V supply to the op amp.Think about the pins they are using for the input. Do those look properly designed for high voltage? Not a chance!It may be a differential probe but it is not a High Voltage differential probe.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 17, 2019, 05:55:41 pm
content: P = V^2 / R = 240^2 / 50k = 1W = smoking resistor

author: Cliff Matthews
author_rank: Unknown
timestamp: April 17, 2019, 06:08:38 pm
content: Quote from: Mechatrommer on April 17, 2019, 05:55:41 pmThat's why the R/C front-end question was raised.

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2019, 06:11:17 pm
content: Quote from: Mechatrommer on April 17, 2019, 05:55:41 pmAlthough you'd need to be a few orders of magnitude above mains frequency to push that amount of power through the impedance of a series 2n2 capacitor (otherwise a lot of us would be smoking too).High frequency, high voltage would certainly cause dissipation issues though, for instance straight across the switching transistor of an off-line high frequency SMPS.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 17, 2019, 09:16:24 pm
content: Quote from: Gyro on April 17, 2019, 03:56:45 pmThe edge of C6 is under the nylon tie, but this board (w/ bodge) doesn't look like something AD would sell..

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 18, 2019, 08:39:58 am
content: The problem is that I cant afford ~200$ for a probe and if it can be made for fare less, I'd be down for that.My goal is that I can probe where I want, when I want in <250V RMS but I do not have enough knowledge to decide if one circuit is safe or usable.

author: tszaboo
author_rank: Super Contributor
timestamp: April 18, 2019, 09:31:53 am
content: Quote from: FriedMule on April 18, 2019, 08:39:58 amLook, if you are not sure if it is safe or dont know how to do these measurements, then you shouldn't be doing these measurements. There isnt too many things that needs to be measured on a 12V transformer. Preatty much 230V 50Hz AC goes in 12V Hz AC comes out. You can measure all this with a multimeter.

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2019, 11:59:49 am
content: Quote from: FriedMule on April 18, 2019, 08:39:58 amI understand where you're coming from, I can't justify the cost of a high voltage differential probe either, I just don't do any off-line SMPS primary stuff any more.Unfortunately the safety dilemma comes up from time to time - particularly in the Beginners section...Theoretically, anything that's behind a couple of Y1 (test voltage 8kV) is safe. Most SMPSs put lesser Y2 Caps (5kV test) between the mains and the non-earthed output for EMC filtering. Two problems arise:1. Safety of construction - we have no control over the care you take in construction. Whether you safely insulate the Y cap connection, whether you use proper wire andstrain relieveit properly. Keep the correct clearances between the Cap pins etc. Whether you keep it battery operated etc. The AD example PCB obeys very few of these precautions as far as I can see.2. What happens when you start using it on nasty waveforms - a Y cap will only pass a minimal current on a mains frequency sinewave, but hit it with some sharp high voltage edges and the caps will pass far more current. Posing a safety (shock) and cooking hazard.Perhaps the biggest issue, from your described useage, is performance. Looking at the input, you have 2 x 2n2 capacitors in series (= 1n1), with 100k resistive load. That gives a response that is already 3dB down at 1.5kHz. It will be hopeless at 50/60Hz. Same with the output - 1uF feeding 100R (50R series + 50R scope input) is also 3db down at 1.5kHz. Combine those two high-pass filters and its low frequency response is truly horrible. I don't know what sort of common mode rejection performance you could expect either.The circuit is designed to show off the high frequency performance (10s of MHz) of AD's Opamp, it is useless at low frequencies. You can't increase the input capacitors due to safety concerns. If you increase the input resistance to, say, 10Meg then you will loose high frequency performance and probably stability too. You're in a bit of a trap with this circuit.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 18, 2019, 01:28:36 pm
content: Great answers:-)First as stated before my knowledge about electronic is minimal but my knowledge about 230V+ is a bit greater, not pro level but have made installation together with electricians.Maybe my example about 230V could be misleading (was only to tell the upper voltage limit).Since my knowledge about electronic is a bit small and I am trying to learn, I think that the possibility to probe several places in a circuit, without common ground, would be fantasic. F.ex. probing on both sides of a rectifier bridge or both sides of a capacitor bank, to learn what is happening and where.To do that, I have to work with insulated probes so I do not unaware transfer AC from the one side of the bridge, to the other side or I am not getting strange results, because I am shortening some ware.I am not thinking of working with HF yet so we are talking about 0-xMhz.

author: abraxa
author_rank: Frequent Contributor
timestamp: April 18, 2019, 02:10:53 pm
content: Honestly, if you say your "knowledge about electronic is minimal" then you should stay away from probing any circuit above 24V until you *know* what you can do and cannot do. We can tell you hundred things on this forum but the one-hundredth-and-one thing you try that we didn't warn you about might kill you or a loved one.I don't know you, so I can only speculate as to why you think differential probes solve the issue for you. From my point of view, they pose only a threat in your case because you might overload the differential probes when you use them with a "I can do whatever because they keep me safe" mind set.My advice? Always keep your scope's ground to the low voltage ground of whatever you examine, set your probes to 10X and be happy. If you think you need more, you're either trying to do things that should be done with a multimeter or you're trying to do things that shouldn't be attempted by people with minimal knowledge - e.g. trying to measure a transformer's primary side with a scope.Yes, I may sound a bit harsh but my intention is to make sure you approach this stuff with the correct mind set. I understand the lure of wanting to probe anything and everything but if you just want to observe and learn then I suggest you use a simulation tool like LTSpice instead. You can model a linear power supply there and look at the waveforms while modifying the parameters of the supply and see what happens. Much safer than doing this on a live supply with a non-isolated scope. Honestly, I've never had to probe the primary side for anything, and I highly suggest you don't do it either.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 18, 2019, 03:38:32 pm
content: abraxa you are right in that 25-30 and up is not to play with. It is also mostly noy intent to measure on the high side, most of the time. What I want to is to probe circuits and see what things do. Without insulation I can "only" probe where the ground is the same.The "high" voltage side, the deadly side is what I have the best grasp on and what I am least interested in.

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2019, 06:18:53 pm
content: Quote from: FriedMule on April 18, 2019, 01:28:36 pmI can certainly understand your frustration with being tied to the probe ground clip... and it is the reason for the many 'how not to blow up your scope' threads. The wish to measure the signal between two points often comes from moving from a DMM only world to a scope world. There's no denying the need for isolated differential probes for certain applications, for example, primary side SMPS development as I've already mentioned but that's a fairly niche application. A couple of suggestions that may help:1. Avoid the mains - I know it's what you're more familiar with, but as others have said therereallyisn't anything interesting to see there. If you want to monitor spikes, voltage variations etc. then you can get almost all the way there by indirectly monitoring with a small mains transformer, for instance a little 24V toroidal (approx 10:1 ratio), where you can ground one side of the secondary.2. Use the differential measurement capabilities built into your scope for free. Assuming that you have two channels or more, set one channel to' Invert' and the mode to 'Add' (the two channels). You then have two probes that act as a differential probe. This works well, particularly at DC to low MHz frequencies. Be careful to insulate (preferably unplug) the ground clips so that they don't dangle into your circuit. Theoretically you could do this with high voltages too but it would be safer to use x100 probes (don't trust cheap probe voltage ratings) and insulating the ground clips becomeswaymore important. It's really best avoided.Suggestion 2 will get you a long way in practice and as I said, it's free.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 18, 2019, 08:28:15 pm
content: I had mentioned this in another of the 'don't blow your scope up' threads but I'll mention it here as well. For my occasional use of an isolated probe, I had saved an isolation board from a weird (and useless) piece of high voltage equipment that had some nice high voltage isolation amps I could use. Everything is a compromise and these amps can be used from D.C. through audio or slightly higher at up to about 3500 volts, so pretty low frequency. This is a case of 'I have them, I should find a use for them' rather than having a need and designing something that suits that design. They would have been expensive to buy but I got the board with 6 identical circuits for peanuts so what the heck. maybe these will give you some ideas you could use.I tested one out and it works as expected. Now to build it into a case and make an input circuit to measure different voltages. Here's a photo of the board being tested and the specs.

author: not1xor1
author_rank: Frequent Contributor
timestamp: April 19, 2019, 05:29:04 am
content: Quote from: FriedMule on April 18, 2019, 03:38:32 pmIf you limit the usage to low voltage circuits, you might make a simple yet cheap differential probe with an instrumentation amplifier.AnAD8429, set to unity gain, has a -3dB bandwidth of 15MHz and with few external components might be a good beginner project.You might also use different input probes to extend its input voltage range.There are also various kinds of insulation amplifiers IC, which might allow to measure with little risk high voltages, but they are more bandwidth limited.E.g. an ISO124 is limited to 50kHz, an AD210 to 20kHz so would not be much useful in probing a switching PSU.

author: f1r31c3r
author_rank: Newbie
timestamp: May 27, 2021, 06:34:29 pm
content: I know this is an old post however i thought maybe someone searching through the forum or coming across this post may want this design from an article in the CircuitCeller magazine.https://circuitcellar.com/research-design-hub/high-voltage-differential-probe/The circuit above with the placements of the capacitors blocks DC and you may want to measure DC but have a high resistance to reduce the current to a safe level as not to damage the scope. This is why a set of series resistors are used, mainly so that if one of the resistors fail the rest wired in series still provide enough resistance to reduce the current to the scopes front end.Hope this helps someone out.