# Amps used - how to detect?

I want to set up a system to measure a set of motors which will be making their way into a robot.

I want to vary the voltage in, and then vary the load on the motor, and measure the amps used.

Voltage in control is easy. As is putting load onto the motor. Measuring the amps seems less easy.

I have found a couple of schematics for such a circut, but they are using shunts, with costs up to £45 a piece.

Am I looking at the wrong thing? Can someone point me in the right direction of a schematic that might help?

Thanks

Steve

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Question: Why do you want to send a variable voltage to your motors? Is it for speed control? If so, you may want to consider PWM as a better way to do it.

You can use a microprocessor or even just single IC circuit to do basic PWM.

http://www.robotroom.com/PWM.html

I too struggled with this quite a bit the first time I made an ammeter.  all ammeters are really volt meters reacting to very low voltages generated by the voltage drop across the shunt.

This exact scenario is where I finally learned to impliment ohms law. (Volts=Amps * Resistance).   You really need to get a good handle on ohms law to do this properly.

The concept I follow is this:  the more current passing through the shunt (0.1ohm resistor), the greater the voltage reading (voltage drop) across the shunt.   this happens in a linear proportionate way.   so if you put a volt meter across the shunt while current is flowing through it, you'll see a voltage drop.   so depending on what you want to do with the info, decides where you go from there....perhaps you want an LED to grow brighter with more amps ,etc.   or hook up a real ammeter.

from here, its all about mapping and scaling, and your exact application.

essentially your building a mini-ammeter.  using ohms law you can calculate max voltage drop of the max current you expect to pass through shunt.  then you use another resistor to scale the voltage drop to the "the full scale" of your mini-ammeter.

using pencil and paper, see if any of this makes sense   :-)

good luck!

This is just a guess, but would a RC Decay circut work for that? Actually, I think Krum said something like that, but I'm not aure at all if a RC Decay circut would work. My guess, if it is to measure Amps, probably not, but that is my guess.

You set up a very low resistance and measure the voltage drop across the shunt and the corresponding voltage drop is the amperage or something like that.

The motors I will be looking to measure will be ranging from 380 sized to 700 sized. The 380 sized motors will draw between 4.5v and 6v and maybe 1 amp. The 700 sized motors will be consuming voltages between 7.4v and 11.1v, and can draw up to 25 amps when close to stall - A very quick way of flattening a battery pack!

Accuracy is not crucial - I am setting up a rig to measure speed and current draw from a bunch of motors under different voltages and different loads. Will be a fully automated system with an audrino controlling it all. Wire up the motor, click the button, leave it 30 minutes, come back to a big set of data points. Ideally I would like to see 0.1amps or less, but I am not hugely fussy.

Thanks again

Steve

Tell us how much current you are measuring and how much accuracy is required. Yes you need a current shunt! A current shunt does not need to be expensive unless you are measuring very high currents with great accuracy. In most cases a 0.1ohm resistor will be fine. The very expensive shunts are made of a special alloy that maintains a fairly constant resistance over a wide temperature range.