Pulse discharge current measurement techniques - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 14, 2017, 06:43:36 pm
content: I want to measure the discharge current of a capacitor, the setup is pretty simple the capacitor is charged to peak mains voltage via a resistor and diode and an isolation transformer. After the capacitor I have a switch that connects to an inductor. When I switch it on the capacitor discharges into the inductor.I wanted to measure this current pulse.As of now I have a small piece of wire (10cm length) that measures the current I measure the voltage across this piece of wire I have calibrated the wire on a dc power supply.My current pulse would be no more that 20 to 30Amps, Currently when I measure across this wire with a scope I get a lot of high frequency noise as soon as I flip the switch. I have set my scope to single trigger mode so all I can see is that noise.Is there a better way I could make the measurement?

author: w2aew
author_rank: Super Contributor
timestamp: July 14, 2017, 08:28:14 pm
content: The ringing is probably normal. If you're taking a charged capacitor and "instantly" switching it into an inductor to discharge, you're basically ringing a resonant circuit. With little resistance in the circuit, you're going to get a high-Q resonant tank circuit formed between the capacitor and inductor. When excited by the transient, the tank will created a damped oscillation at the tank's resonant frequency.I use this ringing in this video to show how to measure an unknown inductor value. The process is described at 3:15 in the video, and is demonstrated beginning at 7:35:

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 15, 2017, 07:56:02 am
content: The the waveform that I see after flipping the switch is full of high frequency < 500nS period waves and they are not even damped. And I really do want to measure that exact ringing that you mention here but I get every thing else that that ringing. Do you think the bounce of the toggle switch is causing this, as I mentioned earlier I discharge the cap into the inductor with a toggle switch, while you seem to be doing it with a cleaner edge.

author: Circlotron
author_rank: Super Contributor
timestamp: July 16, 2017, 12:03:00 pm
content: Use an SCR instead of the switch. No contact bounce and it will latch on until the current runs out. Check the ITSMrating of the SCR. Maybe put a similarly rated reverse diode across the inductor to take the current once the capacitor voltage gets to zero so the cap won't recharge in the reverse polarity.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 16, 2017, 12:40:01 pm
content: Quote from: Circlotron on July 16, 2017, 12:03:00 pmI had considered using an SCR but was vary about some issues:1. di/dt rating of the SCR : in case the discharge current of the Capacitor exceeds this, I might get some false triggering or latching of the SCR.2. Since the LC will form a tank the SCR will only allow the first pluse thru and then turn off right? That way I will lose the remaining oscillations. Will I have to keep the gate current going for longer to get these damped oscillations in?3. How fast would the SCR turn on once I trigger it, would it be in the 10's of microsecond range?Lastly If I want to keep the circuit simple is there a bounceless mechanical switch that can discharge the cap into the inductor?

author: danadak
author_rank: Super Contributor
timestamp: July 16, 2017, 01:21:03 pm
content: Use a MOSFET and trigger its driver with the scope sweep gate output.Or a low Vcesat bipolar transistor.Regards, Dana.

author: Circlotron
author_rank: Super Contributor
timestamp: July 16, 2017, 01:36:50 pm
content: Quote from: ZeroResistance on July 16, 2017, 12:40:01 pmIf you are discharging the cap into any reasonable inductor then di/dt almost certainly won't be an issue.If you want oscillations then put a reverse polarity diode in parallel with the SCR and no diode across the inductor that I mentioned before. Keep the gate current on for as long as the oscillations take to die away.A BT152-600R that I have played with turns on in about 1uS.What are you playing with? A capacitor discharge ignition?

author: MrAl
author_rank: Super Contributor
timestamp: July 17, 2017, 03:39:59 pm
content: Quote from: ZeroResistance on July 14, 2017, 06:43:36 pmHi,Post some waveforms so we can see what is really going on.It's hard to say what is happening without seeing the test results from an experiment.If the scope input is set to high gain we often see noise even though the probe is connected to a very low resistance, even a current shunt. The way the noise appears will determine where it is coming from.