Are differential probes what I need? - Page 1
Forum section: electronics / beginners

author: BorisRap
author_rank: Contributor
timestamp: October 27, 2022, 06:54:24 am
content: Hello, I have to work on a PFC that means high voltage (400V-450V) but also some "fast" signals (up to 500khz).I also need it to be isolated from mains ground.My oscilloscope is not rated for such a high voltage, and I was looking into buying a differential probe.I've seen models like this ones:And they go normally up to 1.4kV. But they don't seem to have a BNC input for a shielded probe. The bandwidth of these devices is normally up to 100MHZ.Is this the type of probe I'm looking for? Are this able to measure short switching events with this kind of unshielded tip?Thank you!

author: PartialDischarge
author_rank: Super Contributor
timestamp: October 27, 2022, 07:24:55 am
content: If you are going to use the probe for a low-side measurement then maybe they can be ok. But for high-side where there is a common mode don't expect to make meaningful measurements, because these probes have long ringing with fast common modes. An isolated probe would be better although of course they are more expensive.

author: jonpaul
author_rank: Super Contributor
timestamp: October 27, 2022, 08:12:11 am
content: Design or debug of mains PFC is not trivial. If you can detail l the PFC circuit, PCB or problem, we can suggest the best solution for probes.In decades of consulting, design and manufacture of SMPS, electronic ballasts and PFC, We never had use of the diff probes you pictured.`Generally, current waves are more useful than differential voltages.99% of power electronics work can be without a diff probe.We used HV probes like 1000 x TEK P6015A, current probes like P6021, P6022 and X10, X100 probes.A 1:1 240/240 1..5 KVA isolation transformer and variac/variable mains transformer is much more useful.Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 27, 2022, 03:23:04 pm
content: I would always recommend to invest in a current probe first.For example one of the PEM range of CWT probes.http://www.pemuk.com/products/cwt-current-probe.aspx

author: jonpaul
author_rank: Super Contributor
timestamp: October 27, 2022, 05:05:03 pm
content: PEMUK probes; costlyDC unneededBrexit makes any shipping very costlyhttp://www.pemuk.com/products/cwt-current-probe.aspxUsed TEK P6021, P6022 far easuier to get off ebayJon

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 27, 2022, 06:56:12 pm
content: P6021 is a good one to have for the low power side, gate pulses etc.but check ratings on Manual Page 8 and bore diameter, max DC component page 10.

author: EPAIII
author_rank: Super Contributor
timestamp: October 27, 2022, 07:18:57 pm
content: You don't seem to actually need differential measurements. At least your post does not mention them as a requirement.What you need is one or more probes that are rated for the Voltage you are working with. And that will REDUCE those Voltages to an acceptable level for your scope.Without the specs for your scope, I can only guess that it is rated for a few hundred Volts. So a 10X probe may provide the needed Voltage reduction. 450V / 10 = 45V. If that is not enough of a Voltage reduction, then you can consider a 100X probe which would drop 450V down to 4.5V. In either case, you need to purchase probes that are rated at 500 VDC at the very least. A higher Voltage rating would be better as there may be spikes that go higher than normal operating levels.10X and 100X probes will have a greater frequency response than a 1X probe. So they can take better advantage of the response of your scope which may be the limiting factor. This is the primary reason why people use a 10X probe as their first choice and only use a 1X probe when there is a particular need for it, like low signal levels.That leaves your need for isolation from the AC power line. Again you need to consult your scope's specifications. But lets assume it is not isolated and it will have a power line ground. That ground may already be common to the circuit and the scope. This means that a 10X or 100X probe will divide any Voltage it's tip is connected to. And you will have a workable arrangement as far as the "high" Voltage level of the circuit is concerned. But, if the probe's ground is not connected at the circuit you may, and probably will, have ringing introduced.One possible arrangement to help the ringing would be to use two 10X probes with the scope providing the differential between them. Most scopes have this feature. I have done this in the past for waveforms that are not too prone to that problem. But it may not always work.Another possible method would be to use just one 10X probe and use a capacitor between the probe's ground and a ground point in the circuit. This will provide DC isolation but a low impedance to the AC components of the signal being measured. Of course, the capacitor should be selected to have a low impedance at the frequencies of interest and higher AND be rated for at least 500V in your case. A 1KV rating would be better.This low impedance is in relation to the probe's/scope's input impedance, usually 10MOhms and 1MOhm respectively for most scopes and 10X probes. Example: For a 10KOhm Xc at your 500 KHz:C = 1 / (2 Pi f Xc) = 1 / (2*3.141*5*10^9)C = 3 pfSo a 0.01 uf rated at 1000V would be great.But do you really need the isolation between the circuit ground and the scope's ground? In most properly designed equipment this is not the case.

author: bdunham7
author_rank: Super Contributor
timestamp: October 27, 2022, 07:21:41 pm
content: Quote from: BorisRap on October 27, 2022, 06:54:24 amHV differential probes aren't magic, and while they have their uses I have found mine to be problematic in this type of application. They aren't actually 'isolated' in the literal sense as each probe can have as little as 2M input impedance to ground. As others have mentioned the lead arrangements can have ringing issues and many of the lower-cost models may not meet their already marginal CMRR specs. You may have difficulty picking out your desired signal from the noise and various other artifacts. 100MHz bandwidth is a fairy tale IMO on the cheap units.As others have suggested, looking at current might be one solution. However, you still may want to look at voltage and solutions that have worked for me include careful use of an isolation transformer for the DUT, 100X low-capacitance ordinary single-ended probes with a normal oscilloscope or an isolated channel oscilloscope (e.g. Fluke ScopeMeter, Tek TPS series).

author: alm
author_rank: Super Contributor
timestamp: October 27, 2022, 10:33:16 pm
content: Quote from: EPAIII on October 27, 2022, 07:18:57 pmDon't count on a modern 10x probe being rated for 450V. Back in the days of tube equipment 10x probes like the Tek P6006 10x might have been rated for 600V, but these days 300V is common. Keep in mind frequency derating if you're measuring fast pulses (it's not clear to me if 500 kHz is bandwidth or repetition rate).

author: EPAIII
author_rank: Super Contributor
timestamp: October 28, 2022, 07:09:40 am
content: YES, you do need to insure that the Voltage spec. is there. I said that. And Tektronix is one good source for quality probes. But they will be more expensive. There are others.Quote from: alm on October 27, 2022, 10:33:16 pm

author: jonpaul
author_rank: Super Contributor
timestamp: October 28, 2022, 09:42:08 am
content: Probe 10X have derating of the max V with frequency, the 400 V applies to DC to perhaps some MHzWe use may TEK P6137 for 2465/7/B TEK scopes, spec is 500 V (DC + peak AC) to 1.3 Mhzhttps://w140.com/tekwiki/images/0/06/070-6432-03.pdfThe "voltage rating" has a max allowable V ( damage or insulation threshold) and a max voltage for rated accuracy ( high resistance parts have a voltage coefficient of resistance).Bon courageJonPS: Manual is a great reference

author: HighVoltage
author_rank: Super Contributor
timestamp: October 28, 2022, 09:48:09 am
content: Testec makes a very good and reliable 100X probe that goes up to 2.5 kVAnd the price is reasonable.I have several of them and they have had no problems so far.

author: rogerggbr
author_rank: Contributor
timestamp: March 15, 2024, 01:07:43 pm
content: Testec also (I think) manufacture a 100:1 4KV rated probe. I found the data sheet herehttps://warwickts.com/_data/site/261/product/1141/TT-HV200instructionsheet.pdfI have tried to buy one but could only find it in stock or available (with some delay) in EU land, not in UK yet. It doesn't appear on Testec website so may be a new product. Model TT-HV-400