Using Multimeter to test current draw

Hi,

I have a device consisting of step-up 5v converter, Arduino Micro, GPS and GSM module, and i wish to test the max current draw of the different states of the device.

I have with me a Sanwa Multimeter to be used for the testing.

This is my setup(omitting the gps for now):

Converter Vout >> Micro>>GSM module >>converter ground.

The problem is that no matter where i slot in the multimeter (connected in Series), the current does not flow through the multimeter.

For example: Converter Vout>>multimeter>>Micro>>ground

When i do this, the Micro does not even start up. The multimeter's own resistance test shows about 1ohm resistance, suggesting that the fuse has not been blown. Voltage and resistance measurements are working fine.

Am i connecting it wrongly? Thanks!

Do you have the meter probes in the correct meter position? For Amp measurement, the positive needs to be moved to the current input.

Is the fuse in the meter ok? Check the meter works on another load.

The meter should be in series with the power supply normally.

Weedpharma

Have you remembered to swap the +ve lead to the mA position on the multimeter? On most models that's necessary.

Russell Ha, beat me to it Weedpharma.

Hi

Thanks for the responses!

I am using this one

There aren't much options for me to swap leads or to choose ranges.

Yes it is in series.

I put the two probes together to test if the fuse is blown, but it gives me a result of 0.3~0.7ohms, so it should be okay.

Does the current usually flow through even if the digital multimeter is switched off/measuring voltage?

I am so confused. :confused: :confused:

The meter has to be on mA range for current to flow. Otherwise there is very high R between the probes.

Weedpharma

The problem is that no matter where i slot in the multimeter (connected in Series), the current does not flow through the multimeter.

What is the ma rating on the meter? The current you are trying to measure may be higher than the meter capability.

I put the two probes together to test if the fuse is blown, but it gives me a result of 0.3~0.7ohms, so it should be okay.

On a good meter, the fuse is only in the path for current measurements. If the fuse is blown you can still measure resistance & voltage.

Try measuring current for an LED & resistor. If the LED doesn’t light, the meter is “open”.

Blown current fuses are very common… All you have to do is accidentally switch the meter to current with the probes across a power supply or battery, and “poof”. Better meters usually have a separate banana jack for current measurement, so you have to move the leads to measure current. That way, you are less likely to make a mistake.

Does the current usually flow through even if the digital multimeter is switched off/measuring voltage?

When it’s off, who knows???

When measuring voltage the meter’s resistance is very high, meghoms. Yes. Current flows through the meter, but it’s a tiny amount of current (Ohm’s Law). The idea is that a high resistance in parallel doesn’t interfere with the circuit under test.

When measuring current the meter’s resistance is very low. This means that there is very little voltage drop across the meter and the low resistance in series doesn’t interfere with the circuit under test.

When measuring resistance, the meter puts-out a small voltage & current. As you probably know, you can’t measure resistance with the circuit-powered-up because that messes-up the resistance reading. In the old days, an analog meter could measure voltage & current without a battery, but if the battery was dead you couldn’t measure resistance.

If you put the (volt) meter in series with your circuit you are creating a voltage divider. In series with a “normal” load like the Arduino, most of the voltage will be dropped across the meter (the higher-resistance side of the voltage divider). You’ll measure the full power supply voltage but the Arduino won’t be powered-up because there is very little voltage across it.

In current mode, the meter has very-low resistance.

If you have some low-value resistors, you can use a voltmeter to measure current (Ohm’s Law). That’s how your meter works anyway… For example, put a 1-Ohm resistor in series and if you measure 200mV, you have 200mA.

The trick is finding a resistor that works for the current you’re trying to measure and the sensitivity/resolution of your meter… If the resistor value is too low (and/or the current is too low), the voltage drop will be too low to measure. If the resistance is too high (and/or the current is too high), you’ll get too much voltage drop across the resistor and your circuit won’t operate correctly.

With that meter, when it is Off, no current should flow. It is switching the leads as you turn the selector. My guess is that the leads are disconnected when it is off.