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Measuring current (up to 40A)?

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Measuring current (up to 40A)?

Postby skyflyer » Wed Sep 16, 2015 5:31 pm

Hi!

I've read that one can use a "shunt" resistor to measure current using an oscilloscope (using a regular probe). I don't have access to current probe (and I'm using Picoscope 2204A).

I'd like to measure current on my hobby quadcopter. It is powered by a 12V battery and current should not exceed 40A. I don't think wiring a small 1ohm resistor in series is a smart idea -- I don't have a resistor with a power rating of 360Watts (30A * 12V).

Is there some setup (perhaps resistor in parallel?) I could use to somehow measure the current?

Thanks, Miha.
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Re: Measuring current (up to 40A)?

Postby Martyn » Tue Sep 22, 2015 6:46 am

Shunt resistor or current probe are the options. If using a resistor put this in the ground return path and you can use one channel of the scope and scaling the reading.
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Re: Measuring current (up to 40A)?

Postby skyflyer » Tue Sep 22, 2015 7:02 am

Martyn wrote:Shunt resistor or current probe are the options. If using a resistor put this in the ground return path and you can use one channel of the scope and scaling the reading.


Thanks Martyn! Can you please suggest a concrete type of resistor? I don't know what kind of resistor to buy. So that it will be able to handle 12V and 30+A of current. I suspect this has to be quite big resistor. Searching for "Shunt resistor" on farnell returned no results. The current sense through hole resistors go up to 100W, which is not enought given my power calculation.

I then searched around the net, to find these: http://www.rc-electronics-usa.com/buy-c ... shunt.html

Given the pricing, I suspect I'm better of sourcing a used current probe from the internet?

Or am I misunderstanding something?

Thanks,
Miha.
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Re: Measuring current (up to 40A)?

Postby Martyn » Tue Sep 22, 2015 7:26 am

I would possibly use one of these, 60A Current Clamp, or similar.

For info the voltage across the resistor wouldn't be 12V at 30A, it would be much much lower otherwise the quadcopter wouldn't work.
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Re: Measuring current (up to 40A)?

Postby bennog » Tue Sep 22, 2015 1:37 pm

You should use something in the range of 0.01 ohm or something like that.
At 40A the 0.01 ohm still has 0.4V over it ! This is still a 3% voltage drop for your motor so your motor will perform 3% less !
Simple and dirty way to do this, use the wires from the battery to the servo as a shunt.

You only have to figure out a way to calculate the current from the measured voltage.
If your charger has a current meter you can use the charge cycle to calculate the exact factor from V to A

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Re: Measuring current (up to 40A)?

Postby skyflyer » Tue Sep 22, 2015 1:48 pm

bennog wrote:You should use something in the range of 0.01 ohm or something like that.
At 40A the 0.01 ohm still has 0.4V over it ! This is still a 3% voltage drop for your motor so your motor will perform 3% less !
Simple and dirty way to do this, use the wires from the battery to the servo as a shunt.

You only have to figure out a way to calculate the current from the measured voltage.
If your charger has a current meter you can use the charge cycle to calculate the exact factor from V to A

Benno


Hi Benno! Wouldn't have the cable too low resistance? I have a pretty good charger, so I'll be able to measure the the voltage drop over a constant current -- I'm just afraid I won't measure anything...
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Re: Measuring current (up to 40A)?

Postby bennog » Thu Sep 24, 2015 5:42 am

skyflyer wrote:Hi Benno! Wouldn't have the cable too low resistance? I have a pretty good charger, so I'll be able to measure the the voltage drop over a constant current -- I'm just afraid I won't measure anything...


I have used this method before, it was only 10A i needed to measure.
I had made some shunts of wire at the office and calibrated it to 0.01 ohm.
Problem with your measurement is the servo will probably do PWM to the motor.
So a wire of more than 0.5 meter can act as a coil and mess with your measurement.

The way i did this.
Start with a wire of 1.5 meter ans apply a current of 3 or 5 Amp.
measure the voltage over the wire and cut the wire down till the voltage is 0.03V (at 3 Amp) or 0.05V (at 5 Amp)
Be where of the power dissipated in the shunt 40A * 0.4V = 16W ! So you will be needing sufficient colling.

At saying this if you don't need 0.1 A precission then you can go for a 0.001 ohm shunt.
This will do 40A * 0.04V = 1.6W an your wire will be 1/10 of the lengt of 0.01 ohm.

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