All Tutorials / Steve G / Amps, Volts, Power Converters and Power Supplies.

Amps, Volts, Power Converters and Power Supplies.

Step 7. Multimeters.

A Volt Meter.

Measures the potential difference, in Volts, between the two points being measured. The voltmeter gets connected in "parallel". It has a high resistance in order to divert the majority of current through the main circuit.


Measure the current of the circuit in Amps. It essentially counts the electrons as they pass through the ammeter. The ammeter is connected in "series". It has a very low resistance so as not to disturb the functioning of the circuit too much.


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A device that has the options to measure both voltage, and amperage. Basically, the two meters mentioned above, in one unit. They can also measure resistance.

This step will explain how to measure voltage and amperage with a multi-meter. The thing to know before you start, is that there are usually 3 functions which will read DC (direct current), AC (alternating current), and A (amps). For reading voltage, the multi-meter used in this example has three different symbols for AC, DC and Amperage, which is seen in the picture below...

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Some multi-meters are auto ranging, but this one is done manually, so it has different voltage ranges for DC on the left of the meter, and AC on the right...

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The three terminals on the bottom of the multi-meter are as follows...

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10 ADC Can measure up to 10 amps before the fuse between the "10ADC" and "COM" terminals blows if the 10 amp limit is exceeded.

COM Is the common terminal for the black probe.

V mA Can measure up to 200 milliamps before the fuse between the "V mA" and "COM" terminals blows before the 200 milliamp limit is exceeded.

Measuring "DC" Voltage.

First, we will start by measuring the voltage of a 1.5v battery (DC power supply). This is essentially a "parallel" circuit we are creating... a power source powering one device directly.

1.) Connect the black probe jack in to the "COM" terminal, and the red probe jack in to the "V mA" terminal.

2.) Move the multi-meters selector to the DC side of the meter, and select "2V DC" so we set the meters voltage reading to the next highest DC reading (we know the battery is 1.5v, as it says so on the battery).

3.) Touch the black probe to the negative "-" terminal, then the red probe to the positive "+" terminal of the battery. You should now get a reading from the battery...

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We get a reading of a little over 1.4 volts. What happens though, if you turn the selector to a higher voltage reading? Well have a look at the following images...

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All that is happening here is the decimal place is decreasing, (1.470v, 1.48v, 1.5v, 1v), which as you can see, will also mean that the readings will lose accuracy.

Measuring the voltage of a DC wall adapter is no different. Set the voltage reading dial on the meter to the next highest setting than what the voltage output is from the adapter. Place the red probe in to the centre of the adapter jack, and touch the black probe to the outside of the jack...

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you see a reading of 14.69v in the image above, that is reading the adapters output, which is 15V.

Measuring "AC" Voltage.

Now we will measure the voltage from a mains wall outlet (AC power supply).

This is just as easy as testing DC voltage, but much more care should be taken, as we are dealing with the higher power AC output. This example will be measuring a direct 240v mains output from a UK wall outlet.

1.) Turn off the wall outlet power.

2.) Move the meters dial to the AC side, and select the 600V AC option (we know we are measuring 240V, so we go to the next highest AC meter read setting... in this case, 600V AC).

3.) Insert the probes in to the wall outlet (or cable in this example) and... "Wait a minute, are you going to say turn the power on? what way do the red and black probes go?" Well, it doesn't matter. AC by definition, is alternating current, so it is not polarized. So with that being said... turn on the power.

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4.) In the image above, you will see a reading of 250V AC, not 240V AC as this is because AC is not regulated, so this reading is indeed, correct.

Measuring Amperage.

This is done differently to measuring voltage where we create a "parallel" circuit, as we will now create a "serial" circuit... one power source powering two devices chained together.

Here, we will measure the amp draw on an LED strip powered off two 1.5V batteries in series (=3 volts).

1.) On the multi-meter, plug the black probe jack in to the "COM" terminal, and the red probe in to the "10ADC" terminal.

2.) Move the dial to the "10A" option.

3.) On our LED circuit, we need to make a break in the positive wire so the meter can be chained to it (parallel). Cut the wire, strip the two ends, and wrap the striped wire coming from the battery around the red probe. The wrap the stripped wire going to the LED around the black probe (if this is reversed, you will still get a reading, but you will see a minus (-) symbol telling you that it is reversed.

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4.) Once power is applied, we see that we get a reading of 0.03. As the dial is set to read 10A maximum, the 0.03 equals to 30mA (30 milliamps).

Reading current draw this way is great for deciding what fuse amperage to go for to protect your circuit, and your robot.

If you are unsure of what the voltage or amperage range of the device you are measuring is, start from the highest reading setting on the meter, then turn the dial to reduce the maximum reading. If it is set too low, you will more than likely get an "error" reading, or worse case, blow the fuse in the meter to protect it.

So with the basics covered with how to use a multi-meter to read amps and voltage, this will guide you away from supplying too little or not enough voltage and current to your motors, servos, sensors, and of course, LED lights.

NOTE: When measuring power with a meter, it must be remembered that you are measuring potentially dangerous power. For example, you could be worried about getting an electric shock from measuring an AC voltage or current from a wall outlet. The thing is, that is what these devices are made to do, but never the less, great care should be taken. If you are not sure or nervous about using a multi-meter, or indeed working on any electrical system or device...PLEASE CONSULT AN EXPERT OR QUALIFIED ELECTRICIAN.

With that said, let's move on to power regulation options, in the next step.