How to measure peak current - Page 1
Forum section: electronics / beginners

author: amwales
author_rank: Regular Contributor
timestamp: March 03, 2014, 06:22:34 pm
content: Hi GuysThis may be a really straight forward question.I have a circuit driving a large number of servos, 18.I am looking for a way to measure the peak current draw.The servos have a 20ms period with a 0.5ms min pulse and 2.5ms max pulse at 6v.I have a display on the PSU that I am using but the values I see look pretty low,maybe its an average being displayed.The equipment I have availableLinear PSU with constant voltage - set to 6v.Digital voltmeter Agilent U1241BGood scope TDS 3054Big ass 25W 1% 1ohm resistor

author: michaelym
author_rank: Regular Contributor
timestamp: March 03, 2014, 06:41:11 pm
content: 1) multimeter min max mode.2) put current shunt in series and display the signal on the scope

author: w2aew
author_rank: Super Contributor
timestamp: March 04, 2014, 02:49:40 am
content: Quote from: michaelym on March 03, 2014, 06:41:11 pmThe DMM might not sample fast enough to reliably sample when the transient peaks occur.A better choice would be to use the scope to probe across the current shunt resistor. Of course, since the scope is not a floating measurement instrument like the DMM, you've got a couple of choices to deal with this.1) Insert the shunt in series between the circuit and low side of the power supply, and probe across it with the scope.2) Insert the shunt in series with the positive supply lead to the circuit under test. Probe either side of the resistor with the scope using two channels, one channel on each side of the shunt. Use the scope's math function to display the difference between the two channels.Or, obtain/borrow a current probe for the scope.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 04, 2014, 03:12:15 am
content: If you don't have a current shunt, you could use the DMM current shunt for the same procedure. The amp shunt and fuse resistance is usually published, so you can work out the voltage-current relationship. Measure voltage across it with your scope in single trigger mode.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 04, 2014, 03:32:47 am
content: Google for a rogowski coil.If you cannot buy a pedigreed one then make one yourself using bis ass ferrite coils or even a coaxial cable. You can then use your shunt to standardize the coil for a confidence of measurement.It will give a fairly close estimate of the peak current draw.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 04, 2014, 05:14:26 am
content: Seems to work with the DMM procedure to find the peak current of a small 6V motor.I used the mA range which has a resistance of 5 Ohms.

author: amwales
author_rank: Regular Contributor
timestamp: March 04, 2014, 11:40:22 am
content: Thanks for all your input, I'll give the shunt and scope a try.

author: qno
author_rank: Frequent Contributor
timestamp: March 04, 2014, 07:00:16 pm
content: It is not going to e the peak current.Motors have a start current of up to 10 times the nominal current.If you put a 1 ohm resistor in series with the motor you are seriously limiting the startup peak.Use a resistor as low as possible 0.1 ohms or 0.01 ohms.As long as your scope displays a relative noise free signal.Put some capacitors on the output of the power supply to prevent current limit or voltage control dips.Then you will be closer to the real peak current drawn by the motor.