measuring max value pulsed dc current without instruments  - Page 1
Forum section: electronics / beginners

author: elektronJack
author_rank: Contributor
timestamp: March 05, 2020, 01:15:40 am
content: Is there an uncomplicated way to measure the maximum value of a rapidly pulsing dc current without using an oscilloscope or other instrument? The maximum value I wish to measure I expect would be up to 15 amps but I could be wrong. I want to measure the current going into and out of a FisherPaykel washing machine motor ,which is a large bldc motor.6 marWhat if a dmm can be used? Is there a way to measure ?7 marI have no more interest in this topic. electronJack.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2020, 01:27:29 am
content: Quote from: elektronJack on March 05, 2020, 01:15:40 amThat makes it rather difficult.

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2020, 01:40:08 am
content: Quote from: Brumby on March 05, 2020, 01:27:29 amSure does, that is a 3ph PWM speed controlled motor running from a single phase supply.Only the mains input can be measured with only a basic tool set.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 05, 2020, 04:07:39 am
content: Withoutanyinstruments?You could put the current through your tongue, and estimate stuff based on the Ow! factor and the burn marks, I guessKidding aside: a current sensing resistor with a big electrolytic capacitor in parallel?Turn on the motor, connect the cap in parallel to the sense resistor for a short moment (so it charges to the voltage of the sense resistor), then disconnect, and measure the voltage potential across the capacitor, via a much bigger resistor (to limit the cap discharge rate), with a multimeter?Essentially, a sampling capacitor across a current sense resistor, poor man's MacGyver style.The sense resistor would have to be really low, sub-ohm range... and wiring the cap using the wrong polarity probably leads to BANG! ... so I'm not sure how feasible this is. Probably quite dangerous, too. I probably should have not posted this.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2020, 06:41:51 am
content: I had mused about powering through a series of fast acting fuses of different values, finding where the change of 'blowing' and 'not blowing' occurs.The only trouble with that is fuses are a thermal device, so you would need to know the duty cycle and do some entertaining math to work out the current. That sort of takes it out of the realm of "uncomplicated".

author: Simon
author_rank: Unknown
timestamp: March 06, 2020, 08:03:48 am
content: Quote from: elektronJack on March 05, 2020, 01:15:40 amA DMM that may cost as much as a scope set to max mode

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: March 06, 2020, 08:34:22 am
content: you could add a fast blow fuse in series with the BLDC motor and check if it blows up. If it does not, keep reducing the fuse rating till it blows off.Not an accurate method, but can provide a decent estimate of the peak current.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: March 06, 2020, 08:35:47 am
content: Quote from: Brumby on March 05, 2020, 06:41:51 amI just posted the same without reading this reply...Your method is probably the only one without using any measurement equipment.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 06, 2020, 05:30:45 pm
content: At this point the fuse becomes the measuring equipment, giving a simple yes/no answer...

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2020, 06:57:38 pm
content: Unfortunately, unless you use expensive 'FF' semiconductor protection fuses, I suspect that the time-constant of the fuse element is likely to give something closer to an RMS current response (as in Thermal RMS voltmeter of course) for its failure current.Ideally something like a current sense resistor in series with the supply, with an ideal diode and capacitor to capture the peak voltage drop across it would give the peak current. Unfortunately, in practice, the diode wouldn't be ideal and a higher than ideal sense resistor value would be needed to compensate the diode voltage drop.It might just about work though with a Schottky or Germanium diode if you could tolerate the voltage drop on the supply. You would need to add the diode forward voltage drop to the measured voltage, when calculating the current.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: March 07, 2020, 02:55:07 am
content: Or may be add a small series resistor and digitise the voltage across it using the sound card from a PC. Most PCs will have a line in or MIC input. You may get something which is very close to the peak.

author: Simon
author_rank: Unknown
timestamp: March 07, 2020, 10:23:54 am
content: True, but remember that the ground of the sound card is probably mains earth.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 07, 2020, 12:11:20 pm
content: Use a laptop PC..?EDIT: Just noticed the OP seems to be asking for the thread to be closed.

author: Simon
author_rank: Unknown
timestamp: March 07, 2020, 12:13:46 pm
content: running on batteries and be careful about the potential any chassis item may float to.