Measuring between mosfet drain and source with oscilloscope - Page 1
Forum section: electronics / beginners

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 02, 2021, 06:16:22 am
content: Is it possible to measure signals between a mosfet drain and source without using a differential probe? I do not want to destroy my oscilloscope and or be electrocuted.I'm using a Hantek DSO5102P oscilloscope with the probes that follows.If not... Can i use Micsig DP10013?

author: bob91343
author_rank: Super Contributor
timestamp: March 02, 2021, 06:39:41 am
content: It depends on how precise you want to be. Most scopes have a selection of A - B. Use the same type of probe on both channels as well as the same V / DIV. Or invert one channel and use A + B.

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2021, 06:46:21 am
content: It very much depends on the circuit. Any sort of line-powered SMPS circuit, is likely to have excessive dV/dt at the drain of the switching transistor(s) and is probably capable of blowing up your scope even though a x10 probe! Probing the upper transistors in high voltage all N-MOS H-bridge circuits is also very problematic for similar reasons.Generally, if the peak voltages involved are sufficiently less than your scope can handle with a x1 probe, the worst that will happen,(provided you don't clip probe ground to a non-ground node),is a burnt x10 probe but if you were hoping the probe attenuation would extend your scope's input voltage range, it can get real $expen$ive$ real fast!Always look at your probe's spec. sheet for voltage derating curve vs frequency, then consider the expected risetime of the signal, and the resulting high frequency component: freq=0.35/rise_time to see if fast high voltage edges are likely to be risky.Edit: See 'srb1954's comments below andEEVblog #279 - How NOT To Blow Up Your Oscilloscope!

author: srb1954
author_rank: Super Contributor
timestamp: March 02, 2021, 12:00:34 pm
content: Quote from: ElectricPower on March 02, 2021, 06:16:22 amDepends on the circuit application. If the MOSFET is in a SMPS running off mains then you can't connect the probe ground to the MOSFET source as this will likely have full mains potential on it. You either have to use a HV differential probe or an isolating transformer to isolate the circuit under test from the mains.You should also be aware of the voltage vs frequency derating required with most probes. A standard x10 probe might be be good for 500V peak at low frequencies but that voltage rating typically has to be reduced as the frequency increases to avoid damaging the probe and/or the scope input. Check your probe specifications.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 02, 2021, 04:37:59 pm
content: Thanks for answersI did not understand much of the answers here, but i think i will be safe with an differential probe. Is the probe i mentioned in the first post o good shoice?

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 02, 2021, 05:35:02 pm
content: The answers you get are only as good as the information you provide.In your original post, you asked if you can measure between drain and source of a mosfet without a differential probe. The proper answer is maybe, it depends. It depends on the voltage levels and the source of the voltage. A 12v battery, no worries. 230v mains, that’s usually a problem for a mains powered scope. Have you seen Dave’s video on how to not blow up your scope?Now you’re asking which probe is okay to use but don’t know what voltage levels you’re working with. That’s a very important part of the discussion. You forced the responders to make assumptions about what you wanted to measure. If the answers don’t make sense, perhaps the question was incomplete or incorrect?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 02, 2021, 06:19:08 pm
content: I`m sorry. My english is not that good.But we talking a mosfet that get power from an ac/dc adapter with grounded AC plug. The adapter gived my Mosfet (n-channel logic level mosfet) 12v DC that the mosfet will switch on/off.I want to measure/read signal from my Arduino to the gate pin and also in same time measure source voltage.

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2021, 08:40:12 pm
content: For under 50V (greater of supply voltage or peak of any inductive ringing), either ground referenced or actually fully floating, you'll be OK with a two channel scope in Y1-Y2 mode (may be Y1 + Y2 & Invert Y2) and two identical x10 probes. Make sure both probes are properly calibrated using the scope's test signal, then, with both probes on the test signal put it in Y1-Y2 mode and tweak one of the channels variable Y attenuator for best cancellation. You *may* also need to tweak the calibration trimmer of one of the probes.Once you've done that you can connect your probes to the points you want to measure the differential voltage between. It helps reduce noise if you can connect the ground clips to circuit 0V but *ONLY* do so if *either* the circuit is fully floating *or* 0V is already ground, via the same power-strip as your scope is plugged into.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 02, 2021, 09:12:32 pm
content: Thanks alotI will try that