Let's Make Robots!

How to measure Amp?

Could somebody explain in a very easy way (with a drawing of what goes to what) show how one measures how many amp a motor is taking?

Thanks!

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
So you simply do not know the answer?
THIS is insulting.

What's scarey is I am applying at Washington University to work on my PhD in computer science and to be a research assistant for a professor in robotics. That's right Ill be teaching the young adults of tomorrow how to measure amps with their tongues. Actually I really was reading this thread because I had the same question. I pull motors from broken electronic equipment that peopl eI work with donate. Most of them dont have any markings so a good way to come up with voltage and amps that the motor can take is helpful.

Sometimes I can't supress my inner child and I have t oleave stupid comments.

That is frightening. I used to be a "professor" (over here, "Professor" is a grade awarded to someone who's an very eperienced lecturer and researcher) of Computer Architecture. (Can't you tell?) It was back in the days when such posts were awarded on ability rather than on wether or not you had a PhD. I started my PhD: An Artifically Intelligent System for Closed Loop Control of a Refrigeration Plant. (Yawn.) My wife called it "A Clever Little Fridge." I packed it in after one year, because the money was better in the Aerospace Sector.

try what boa said, connect it in series with your motor and switch to amps, which is the A with the DC symbol (the line with 3 dots under it)

Soultion 1 (AKA BoA's answer)

It looks like your meter will only measure up to 2amps, so pray the motor's not drawing 4A.

Turn the dial to 2000 (3rd setting to the right of "off"). This means 2000mA (= 2 Amps).

Put the black lead to -ve of battery. Put the red lead to one pole of motor. Put the other pole of motor to the -ve of battery. This is "series"and you are now measuring current. It is displayed in milliamps.

(If that doesn't work, move the red lead to the rightmost socket andtry again, but I think on your meter you only need that for currents <200mA.)

Bear in mind that this is the "no torque" current. The motor will draw more current if you try to "stall" it (grab it with your fingers and see the current go up). You need to measure teh "stall current" (what current it will draw if some muppet stands in front of it blocking its path).

Solution 2 (AKA jip's answer)

Turn the knob to the other 2000 (6 clicks left of "off". This is 2000 ohms (Or 2K.). Put the red lead in the leftmost socket. Attach the end of the red lead to one pole of the motor. Attach the black lead to the other pole of the motor. Write it down (call it "R").

Write down your battery voltage. (Call it "V".)

Get a calculator. Perform the operation V / R. (V and R are the numbers you wrote down above.)

The result is your stall current in Amps. (Stall current is when you have put such a heavy load on the motor that it can't turn and it starts to gethot and very soon the blue smoke will come out.)

Solution 3 (AKA Frits' answer)

Tell me what toy the motor is from and if I have one, I'll measure it myself.

Solution 4 (AKA jklug80's answer)

Ask the question on www.letsmakerobots.com, so your so-called "friends" can send you useless, humiliating replies.

(Note how I put number 4 last? It the LAST thing you should try.)

I recommend you do both 1 AND 2 and compare the results. They might be quite different as the chances are the coil resistance is quite low.

actually the maximum on his measure thingey (if i can see well on the photo) is 200ma. the one you're talking about (3rd setting to the right of off is something with the "micro" next to it...so just go for the second solution (right boa?)

Bother. Yes. I only read the first thing on the scale rather than looking at all the options. Bad BoA. I assumed it was mA then A. It looks like it's uA then mA.

Fritsey, dude your DMM will only measure up to 200mA. Need more details on the motor before you even THINK about hooking it up. You risk blowing a fuse inside the DMM if you hook it up to a 2A motor.

Go with solution 2, like the man says. It involves a floating point division, but that should only take you a few cycles. And then go and get a DMM that measures up to 10A.

Yeah, my analog multimeter can only read up to 250 mA current too. That seems surprisingly low. Aren't there meters that can read several amps? That don't cost hundreds of dollars?

Dan

Now here's a thing. My $50 one and my $10 one BOTH have a 10A scale.

I hardly ever use them I just guessed they were for people who couldn't be arsed to put in a series resistor and measure the PD across it.