Isolation transformer for differential measurements - Page 1
Forum section: electronics / projects

author: abhishek.2138
author_rank: Newbie
timestamp: December 31, 2016, 01:53:45 am
content: I am working on 1 Kw AC drive project. The single phase AC motor speed is controlled by driving it through IGBT full bridge. Now, for designing the circuit some measurements are needed by Digital storage oscilloscope. The ground of DSO is connected to earth. The earth & neutral are short at the circuit breaker. When I connect this DSO ground in my circuit the fuse gets blown.When I disconnect the earth connection to DSO the fuse doesn't blow, but the DSO probe has AC voltage shown by tester.What should be done to avoid AC voltage at DSO probe???Should I connect Isolation transformer at the DSO input (only phase & neutral of transformer secondary to DSO) & earth to be connected separately to DSO??Please help...

author: nctnico
author_rank: Super Contributor
timestamp: December 31, 2016, 01:56:08 am
content: Always use a differential probe. Don't even ask about using an isolation transformer!

author: Bryan
author_rank: Frequent Contributor
timestamp: December 31, 2016, 08:14:30 pm
content: ya, probably not a good idea to attach ground to the ear. "The ground of DSO is connected to ear"

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2016, 08:24:40 pm
content: What Nico said. ^^^An oscilloscope without the correct probe to give isolation from the DUT and safety for the operator is useless without differential probes of the OP's requirements.Yes, they are not cheap but they are necessary for these types of measurements.Unless you can find cheap SH units, a Pintek DP-25 is a good option to look at/for.

author: cdev
author_rank: Super Contributor
timestamp: January 02, 2017, 12:28:39 am
content: Or run the scope off of a battery supply. (And appropriately isolating it and operator ) might be easier

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2017, 12:37:33 am
content: Quote from: cdev on January 02, 2017, 12:28:39 amI wouldn't recommend this because the 'scope would float at a dangerous potential.You're better off using an isolation transformer to power the device under test.

author: cdev
author_rank: Super Contributor
timestamp: January 02, 2017, 01:55:19 am
content: Was it here that I saw somebody in some (some humor-rich-but-less-safety-friendly country) had connected their "earth ground" to a small styrofoam cup filled with dirt with a tiny plant in it?

author: tautech
author_rank: Super Contributor
timestamp: January 02, 2017, 02:10:06 am
content: Quote from: cdev on January 02, 2017, 01:55:19 am..............

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2017, 02:39:16 am
content: Quote from: cdev on January 02, 2017, 01:55:19 amProbably. IIRC it was a shower or wall mounted water heater with the ground wire running into a plant pot hung on the wall.If anyone's got a link please post it!@O.P. 1KW is a heavy load, but 1KVA isolation transformers are available, which could feed your drive and make it safe to ground the negative rail of its DC bus so you could take measurements with a grounded scope. However it would be unsafe to ground any parts of the drive except the negative rail (or possibly the positive rail if it doesn't use PFC) whereas a differential probe can safely measure between any two points within its voltage rating, which gives you far far more flexibility when testing circuits with fast-switching HV nodes like a mains powered H-Bridge.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2017, 04:04:56 am
content: You cannot check the gate drive performance of both high side and low side drivers, using an isolation transformer.Verify their performance with unit testing, outside of a full powered bridge. Perform switching and dV/dt testing with a test setup, just as gate drive datasheets do it.Tim

author: BravoV
author_rank: Super Contributor
timestamp: January 02, 2017, 04:35:04 am
content: Quote from: nctnico on December 31, 2016, 01:56:08 am+1 on differential probe, and even better, use a handheld fully isolated scope, not just ordinary ones.Beware though, not all "handheld" scopes are isolated.And yes, a truly decent isolated scope is quite expensive.Example what you can do with an isolated scope, watch the different hook points at both probes ground clips, you just can't do this with ordinary scope, even using an isolated transformer or floated scope/dut.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2017, 08:11:06 am
content: Note that you still get common mode errors, due to the fact that the probe cable has capacitance or impedance to space, and the scope has capacitance across its isolated input circuitry. (No amount of ferrite beads will save you from this.)If you're working with IGBT modules, they're slow enough to not really be a problem, but there are plenty of converters fast enough (smaller IGBTs, most anything MOSFET) that will induce measurement errors (or do worse things -- the isolation barrier on those scopes is typically specified with a voltage vs. frequency limit!).Tim

author: tautech
author_rank: Super Contributor
timestamp: January 02, 2017, 08:20:24 am
content: Quote from: T3sl4co1l on January 02, 2017, 08:11:06 amTim, if you're referring to the image Bravo posted, no that's incorrect.It's of a truly isolated input DSO where the Reference leads can be placed anywhere without fear of ground loops and furthermore there is no need to use a Maths mode differential setup to do high side measurements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2017, 08:35:16 am
content: Quote from: tautech on January 02, 2017, 08:20:24 amI know what I'm talking about. And I've performed the measurements, with the pictured instrument.It is physically, necessarily, impossible to achieve very high CMRR, with a leaded (partially unshielded) probe, and a nonzero length probe body and cable. You can get close, but you can't get perfect.Many real world, practical situations will not be on the "adequate" side of "perfect", hence this is a real issue. The range of cases where it goes from "okay" to "ugly" is what I laid out above.(A coaxial probe connector would obviously help a great deal to address this -- but the cable, connector and circuit simply won't be well enough constructed for this to be a truly general solution. Like, it might be okay for MOSFETs, but not GaN FETs.)A complete solution does exist: if you moved the entire scope input circuit, so that it is enclosed within an ideal Faraday cage, attached to the gate driver itself. The added bulk of the cage will increase the gate drive's loading capacitance on the inverter, but as long as the inverter doesn't mind (for most inverters, it'll be equivalent to some 10s of pF additional load -- no matter at all), the signal received by the scope input circuit will be true, with zero common mode error. This leaves the matter of coupling the received signal down to the rest of the scope, but that's for the scope manufacturer to solve.Which is actually kind of an interesting idea to think about... you know how Fluke makes a DMM with a detachable face? (And probably lots of others these days, I haven't checked.) Imagine that, but the probe, input coupling, trigger and ADC were detachable! RF or fiber optic would relay the data back to the base unit.Hmmm.........Tim

author: nctnico
author_rank: Super Contributor
timestamp: January 02, 2017, 12:22:16 pm
content: Quote from: tautech on January 02, 2017, 08:20:24 amIf you look at the specs there should be a capacitance mentioned between the channels and to ground. It is always there and usually in the tens of pf range so at 100MHz this provides a low impedance path (80 Ohms @100MHz / 20pf) between the two probes!

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 02, 2017, 01:08:20 pm
content: In BravoV pic the ground for the probes are connected to points that should have little AC component relative to ground so shouldn't this setup have little common mode issue.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2017, 01:52:00 pm
content: It's not clear whether OP wants/needs to measure high side gate drives, for which this is a problem. Low side is okay, as long as the bus isn't terrifically noisy.Tim