# Measuring Current

1. Feb 19, 2005

### Haftred

I wasn't sure where to post this, so feel free to move this thread if necessary. I want to measure the current of a circuit using a multimeter, a power supply, and 3 resistors. How do I insert the "rods" on the multimeter to measure the current in the circuit? I know I have to measure it in series, but where do I disconnect the wires/resistors in the circuit, and how do I insert the "rods" in the multimeter to measure the current. Also, how do I make sure I don't blow a fuse? Thanks!

2. Feb 19, 2005

### pervect

Staff Emeritus
The multimeter will in general have a current setting - for a direct measurement of current, you set the multimeter on a current scale, put the meter in series with the circuit, and measure the current.

There is a potential risk to the multimeter if the current is very high (well beyond the scale setting on the multimeter) and you measure the current in this manner. This can happen unintentionally if the circuit you are measuring is shorted, for instance. There's no risk if you set the multimeter to the proper current setting. Some multimeters have a special set of terminals for high current readings, you have to look at the multimeter to see if yours has such arrangement.

You can also put a resistor of known value in series with the circuit (but a resistor that's also small enough not to seriously disturb the current flowing through the circuit) and then measure the voltage across the resistor with the multimeter. This is a less direct measurement, but it has the advantage that if something goes wrong, you only fry the resistor you put in series, not the multimeter. You still have some chance of bending the needle if it's an analog meter and the voltage range setting is wrong.

3. Feb 19, 2005

### Haftred

this may be a stupid question, but how do I connect the multimeter in series? I want to measure the different currents in a parallel setup, but I don't know what to disconnect and where to disconnect it; also there are two different colours on my multimeter (black and red) and I don't know what to put where. Thanks.

4. Feb 20, 2005

### pervect

Staff Emeritus
If the original circuit is

-----load------------------

then either

----load----multimeter-----

or

---multimeter----load------

is a series connection

So you have to break or disconnect a wire where you want to measure the current, and insert the multimeter in series, with the correct range setting. The current has to flow into the positive terminal of the multimeter and out of the negative one if it's an analog meter, because an analog meter can't measure a negative current properly. If the needle tries to move the wrong way, then you put it in backwards. If it's a digital meter, it'll probably read read a negative current.

Be careful, sometimes meters have different terminals on the front for measuring currents - when in doubt read the directions or manual for your meter.

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