Author Topic: How to measure peak current  (Read 5262 times)

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Offline amwales

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How to measure peak current
« on: March 03, 2014, 06:22:34 pm »
Hi Guys

This 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 available
Linear PSU with constant voltage - set to 6v.
Digital voltmeter Agilent U1241B
Good scope TDS 3054
Big ass 25W 1% 1ohm resistor


 

Offline michaelym

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Re: How to measure peak current
« Reply #1 on: March 03, 2014, 06:41:11 pm »
1) multimeter min max mode.
2) put current shunt in series and display the signal on the scope
 

Offline w2aew

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Re: How to measure peak current
« Reply #2 on: March 04, 2014, 02:49:40 am »
1) multimeter min max mode.
2) put current shunt in series and display the signal on the scope

The 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.
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Offline Wytnucls

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Re: How to measure peak current
« Reply #3 on: March 04, 2014, 03:12:15 am »
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.
 

Offline krish2487

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Re: How to measure peak current
« Reply #4 on: March 04, 2014, 03:32:47 am »
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.
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Offline Wytnucls

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Re: How to measure peak current
« Reply #5 on: March 04, 2014, 05:14:26 am »
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.

 

Offline amwales

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Re: How to measure peak current
« Reply #6 on: March 04, 2014, 11:40:22 am »
Thanks for all your input, I'll give the shunt and scope a try.
 

Offline qno

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Re: How to measure peak current
« Reply #7 on: March 04, 2014, 07:00:16 pm »
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.

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