# Measuring DC motor resistance

#### mikeoz

Joined Sep 25, 2017
86
I've attached a drawing of my circuit below. Would I connect the DMM like I have it in the drawing?
Also, my DMM only goes up to 10A. Would you think I'd need to use a resistor?

Thanks

Michael

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Joined Jul 18, 2013
28,770
You have a 15a fuse in circuit, also does your meter for protection if you happen to exceed the rating.
Max.

#### mikeoz

Joined Sep 25, 2017
86
I think the standard blade fuse in the circuit is rated for 15A continous current and it will blow at roughly double that. My DMM does have a replaceable fuse I think. Would you test it like i have it in the drawing I attached then? What would you do to measure this if it does exceed the 10A of the DMM?

Thanks

Michael

Joined Jul 18, 2013
28,770
I would measure it in the battery lead, as a last resort use the couple of ohm resistor in series and measure voltage.
Max.

#### mikeoz

Joined Sep 25, 2017
86
I see, so put the DMM between the battery and driver. Does it mater which side it goes - positive or negative?

Thanks
Michael

Joined Jul 18, 2013
28,770
No the meter usually read bi-bidirectionally.
Max.

#### mikeoz

Joined Sep 25, 2017
86
Great, thanks. The reason for testing this is someone suggested maybe a safer way of protecting the motors in the event of a stall might be to use lower current motor drivers (say for example 20A) which still have protection features such as overcurrent. That way the drivers will trip out before the motors see the 44A (we calculated) at stall. The drivers i'm using at the moment are rated for 43A. Do you think this sounds like a good idea?

Michael

Joined Jul 18, 2013
28,770
You can always try it!
Max.

#### mikeoz

Joined Sep 25, 2017
86
Haha! But does it sound to you like a good idea for protecting the motors - as it'll mean buying new drivers?

Michael

#### mikeoz

Joined Sep 25, 2017
86
Just a question on measuring the running current:
If the fuse blows in the DMM, what value resistor would you use to measure the voltage?

Thanks

Michael

Joined Jul 18, 2013
28,770
It should ideally be as low as possible, a fraction of an ohm if possible.
Max.

#### mikeoz

Joined Sep 25, 2017
86
Thanks Max, I'll see what I can find.
Does it need to be a particular wattage?

Michael