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Electronics => Electronics => Topic started by: Redcap on February 17, 2011, 09:05:28 PM

Title: [Solved] How do you tell amps drawn by motor?
Post by: Redcap on February 17, 2011, 09:05:28 PM

Hey all.

I've been doing a little research into what I need to control DC motors from an Arduino.
At this stage I'm looking at a SN754410 (http://www.futurlec.com/Others/SN754410.shtml) MD chip (instead of buying a motor shield).

I'm planning to run two hobby motors off it, I'm not sure of the motor specs (not even sure when or where I got them).
As such, how can you tell how many amps a motor is drawing?

(I know the SN754410 can handle 1amp per driver.)

Thanks in advance,
Redcap

Title: Re: How do you tell amps drawn by motor?
Post by: waltr on February 17, 2011, 09:15:06 PM

Well the Stall Current is close to what is calculated with Ohm's Law and a measurement of the DC resistance of the motor winding. This would be the maximum current the motor will draw and a good rule of thumb for sizing the driver.

The no load current can be measured by putting a low value resistor is series with the motor and battery. Measure the voltage drop across the resistor and calculate the current with Ohm's law. Chose a resistor value to get around 0.1 to 0.3 Volt drop.

Title: Re: How do you tell amps drawn by motor?
Post by: Redcap on February 17, 2011, 09:41:00 PM

Ah, much thanks Waltr :)
I knew it had to do with Ohm's law, but wasn't sure how to implement it as such :)

Cheers :)