Current sensing

Making an E-bike. I want to find out how much current the motor is sucking up. Does anyone know of any good induction sensors?

I'd recommend a Hall effect sensor like these:

http://www.allegromicro.com/en/Products/Categories/Sensors/currentsensor.asp

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The Quick Shield: breakout all 28 pins to quick-connect terminals

Thanks for that link. I like those, but I don't quite understand the Measurement Range on them. If it says ±5 to 30, are we saying it can handle plus or minus 5 to 30 amps? So below 5 amps I'd not really be able to measure anything?

And another thing I was thinking as I looked at a really beefy (90 amp) one on SparkFun called the AttoPilot that says it measures in increments of 36.60mV per amp. I have been busy and don't have much time to test, does anyone know how precisely the duemilanove can read a voltage?

So below 5 amps I'd not really be able to measure anything?

Yes that's right, the signal will be so low it will be lost in the noise.

how precisely the duemilanove can read a voltage

With a 5V reference voltage each step is 5/ 1023 = 4.88 mV per step.
If you switch to the internal 1v1 reference this becomes:- 1.1 / 1023 = 1.075mV per step.

Bear in mind that you can increase the maximum range of these sensors by using a shunt. Of course this will be at the expense of resolution. They're also pretty jumpy when sensing current consumption of a motor. This can be solved in software or by increasing the size of the filter cap.

you might consider using one of these.

http://www.diodes.com.tw/zetex/_pdfs/3.0/pdf/ZXCT1009.pdf

Bear in mind that you can increase the maximum range of these sensors by using a shunt. Of course this will be at the expense of resolution. They're also pretty jumpy when sensing current consumption of a motor. This can be solved in software or by increasing the size of the filter cap.

Good call, that may help me. Not sure yet how much current my motor will draw, but I'll bet a shunt may come in handy later on. Thanks.

EDIT: Wait hold on - I've been looking around because honestly I didn't know what you meant by a shunt. I found some stuff, but need to make sure I know what I'm talking about. Are we saying that I can throw something in series with the device that senses current so as to draw some of the current and not exceed the max current rating of the device?

I read on Wikipedia about throwing a precision resistor in series with the load and measuring voltage off of that and do some math to get my current, but I don't think I could find a resistor that could handle 30+ amps...

I read on Wikipedia about throwing a precision resistor in series with the load and measuring voltage off of that and do some math to get my current, but I don't think I could find a resistor that could handle 30+ amps...

Just a matter of searching around. They can sometimes be found in surplus outfits or at good prices on E-bay.

Lefty

I read on Wikipedia about throwing a precision resistor in series with the load and measuring voltage off of that and do some math to get my current

A resistor used in that way is called a shunt.

Gotcha. But I think I'm confused with what daveg360 said:

Bear in mind that you can increase the maximum range of these sensors by using a shunt.

I'd like to emphasize the "of these sensors" part. I can see the logic behind measuring the voltage drop of the resistor, but what's daveg talking about? wire in a shunt and the sensor somehow together....?

He is talking about the sensors mentioned in reply #1.

You can get it to read higher currents if you bypass some of the current in a shunt (or series resistor). However I thought your problem was you wanted to increase the sensitivity not reduce it.