# Amp measuring

Discussion in 'General Electronics Discussion' started by Chassis, Aug 11, 2015.

1. ### Chassis

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2
Apr 21, 2015
I got a 400A Clamp on meter from a old man today,but I don't understand how the Amp function works. In the picture I got 230V supplied through a lead to a small old tv .What I got on the meter was 00.25,does that mean 25A or 25ma? When I do the same on a car battery with a 12v 55w halogan bulb it just shows 00.00.can anyone help.

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2. ### Gryd3

4,098
875
Jun 25, 2014
That meter may only be capable of measuring 'AC' and not 'DC' which is why you read 0 on the halogen light.
These things work by picking up the changing magnetic field around the wire. The more current there is, the stronger the magnetic field. With AC, this field changes constantly... with DC, this field only changes twice... when the power turns on, or when the power turns off... So the meter can't read it.

As far as the scale is concerned, it's really difficult to read the image. 25A seems incredibly high, most house breakers trip at 15!
That... and it seems like a pretty small number for a TV if it's a cathode ray tube, but perhaps not for an LCD.

3. ### Chassis

34
2
Apr 21, 2015
Thanks a lot for the speedy response buddy.I think I'm just going to use it for measuring Volts,Ohms and Continuity rather.Thanks again Gryd

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4. ### Gryd3

4,098
875
Jun 25, 2014
Well. It will still be incredibly handy for measuring current, even if it's just for AC.
Are you familiar with Ohm's Law?

There are some tricks you can use to measure current in a DC circuit
'Some' clamp on meter's support measuring DC, but are not as accurate because of the whole changing field vs a non-changing field... the meter could accidentally measure surrounding magnetic forces.
Additionally, other meters can measure current, but actually need to be used like a 'jumper' wire, so that all the current goes through the meter. These meters usually have a pretty low limit to the current they can measure.
Additionally, you can google around and learn how a 'shunt' resistor works.
Simple way of putting it, is that if you know the resistance of a wire or resistor, you can measure the voltage from one end to the other and know the current!
So.. with that halogen light...
You disconnect the + wire and connect it to a 1/4Ω resistor. The other end of the resistor goes to the halogen light. The light should now turn on. If you measure 0.625 Volts from one side of the resistor to the other... you can do this:
Current = Volts / Resistance
Current = 0.625V / 0.25Ω = 2.5Amps!

Only draw back.. is that this is an invasive test... you actually need to alter the circuit to do this.

5. ### Bluejets

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1,100
Oct 5, 2014
Meter appears to be "auto-ranging" AC tong meter.
Reading shown would be 1/4A (00.25A)
not 25A and not 25mA.

6. ### Colin Mitchell

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314
Aug 31, 2014
You have to connect the meter around only one wire.

That's why it is best to take it to the meter-box where you will find individual wires.
That's what electricians do.