Measuring 1mA to 2mA on a 200V 2 millisecond signal - Page 1
Forum section: electronics / projects

author: biomurph
author_rank: Contributor
timestamp: December 18, 2022, 04:48:11 pm
content: Hi,I am repairing a custom signal generator that is designed to produce a 200V, 1-2 millisecond pulse of current in the 1-100mA range, with minimum current output down to 1mA.The device receives 2 control signals: 5V pulse to trigger the 200V output pulse, and a variable voltage to determine the current (0-5V).I feel pretty confident about measuring the 200V output signal, but I am wondering about measuring the current.I have one of the signal generators that 'works', and I want to measure it's functionality and then use to calibrate other devices after repair.My lab has a Siglent SHS820 portable scope, and a Siglent SDS1202X-E.I do have a uCurrent Gold, but I believe the 200V signal is out of spec for using it?Do I need to build my own current sense circuit? Is there a simple way using my test equipment to get an accurate current measurement?Or is there a current probe that that would measure in the range of mA that can be recommended?Cheers!

author: bdunham7
author_rank: Super Contributor
timestamp: December 18, 2022, 04:51:24 pm
content: Does the device have a defined load that it delivers this pulse into?

author: biomurph
author_rank: Contributor
timestamp: December 18, 2022, 05:17:11 pm
content: Thanks, great question.It is designed to deliver electric stimulation to a human subject, in research lab studying recovery from nerve injury.The system includes EMG sensors to verify the signal is delivered to the subject.The maximum current output of the device is controllable as stated in the original post.I imagine that I will need some sort of load to stand-in during testing.An appropriately rated load resistor could work.470R with rating of 100W would be beefy enough to range up to about 400mA which is more than I need.

author: bdunham7
author_rank: Super Contributor
timestamp: December 18, 2022, 05:24:52 pm
content: Quote from: biomurph on December 18, 2022, 05:17:11 pmSo simply include a precision resistor in the low side of that load and use it as a current shunt. The larger the resistor, the better the result.

author: biomurph
author_rank: Contributor
timestamp: December 18, 2022, 05:30:25 pm
content: @bdunham7Yes, that is certainly a way to do it. However is that the least invasive method?I have designed current shunt resistor method for lower voltage applications. Are there any traps for designing around high-voltage millisecond pulses?(understanding, of course, that high-voltage usually means kV... 200V is high to me).Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: December 18, 2022, 05:57:34 pm
content: Quote from: biomurph on December 18, 2022, 05:30:25 pmWell typically the current shunt is 'invasive' so you want to use as low a resistance as possible. But if you incorporate it into a test load that you need anyway, then there really is no net effect, right?You typically might want a non-inductive WW or film type resistor for this, but really for your application you would have a hard time picking a totally inappropriate resistor. Milliseconds are pretty slow and I doubt a device like this has fast rise times to worry about.If you decide to use an entirely resistive load model, then you don't even need to measure current, you can just calculate it with the MATH function on your scope.

author: biomurph
author_rank: Contributor
timestamp: December 18, 2022, 06:37:00 pm
content: I can get a power rated non-inductive film resistor relatively cheaply.And, I do like the idea of using a relatively large resistor for the shunt, easier to read and better resolution.

author: biomurph
author_rank: Contributor
timestamp: December 18, 2022, 06:44:23 pm
content: I could use this resistor from vishay for the bulk of the loadhttps://www.mouser.com/ProductDetail/Vishay/LTO100H470R0JTE3?qs=sGAEpiMZZMtlubZbdhIBIFITUtFUppA2kN98QxHOo0A%3D470 ohm rated for 100W.Then use a 10 ohm (properly rated) in series as a low-side shunt for current measurement.