# How to measure current with multimeter

I want to measure both voltage and current from an output expansion port, of the BeagleBoard-xM. I managed to measure voltage by putting measuring lead (red) to the expansion port and reference lead (black) to ground.

How can I measure current of the expansion port, without shorting it? Should I transfer current to a breadboard and taking the measurement somehow from there?

• What are you trying to measure the current of. – Andy aka Nov 10 '13 at 14:34
• Via this expansion port, I will send signals to an H-bridhe ic component. I need to know signal's voltage and amperage, as H-bridge receives specific values of voltage and current. – dempap Nov 10 '13 at 14:42
• if you wanted to measure water flow rate down a pipeyou insert a flow meter in series with the pipe. It's exactly the same with current. – Andy aka Nov 10 '13 at 16:02

The real problem is not that you just want to measure current. If so, you'd put the multimeter in current mode and connect the leads. You want to measure maximum current a source can safely deliver. That's very different.

The correct answer is you can't measure this, you have to get it from the datasheet. That is because at best you can only determine what your single sample does, which does not tell you what restrictions you have to design to to guarantee safe operation over the variation of possible units. You also can't always know what "safe" is for the device just from the outside.

However, for something with a normal digital output, we can make a few assumptions. Again, these are assumptions only and without a proper spec we can't know whether we are damaging the device. Let's say that we assume that dragging the voltage down to 90% of its open-circuit level will not hurt the digital output. You didn't say what voltage this port puts out nor provide any link, so we can only use hypothetical values. If this is a 5 V output, for example, then that means we assume we can load it in the high state without damage so that the voltage goes down to 4.5 V.

To measure the current the port is sourcing in this case, you first have to find the load that drags the output voltage to 4.5 V. Hook up the current meter in series with a variable resistor to the port when it is driving high. Use a separate meter to monitor the port voltage. Keep increasing the load (decreasing the resistance) until the voltage goes down to 4.5 V. The current meter is then showing you the current that the port can source at what we assumed was the safe voltage of 4.5 V.

If you don't have a suitable variable resistor, you can use a succession of fixed resistors to get close enough. Most likely the port will be able to source a mA or a few mA. A 4.5 kΩ resistor would draw 1 mA at 4.5 V. Start with 10 kΩ and go down from there carefully. You will probably end up with a resistor in the 500 Ω to 5 kΩ range.

If you use fixed known resistors, you can dispense with the current meter altogether. Start with 10 kΩ and keep decreasing the resistance connected to the port until its voltage gets to 4.5 V. Since you know the voltage and the resistance, the current the port is sourcing is simply that voltage divided by that resistance.

You measure current by putting the multimeter leads in series with your circuit. Do not short the port by putting the multimeter across it in current mode. That could damage the port and will not give you the information you want. Be aware that the port may not be able to supply the required current at the required voltage. Also, you say the H-bridge needs specific values of voltage and current. Normally, a load specifies the voltage and the maximum current. To specify both implies that the load is a fixed resistor which is probably not the case here. You should try to find the specifications for the port which would give the maximum voltage and current that can be supplied. You could try putting various resistive loads on the port (I would start with 1 kilohm) and measure Slowly decrease the load until the port is supplying the current specified by the H-bridge. However, if the output voltage starts to drop (say by 10%), then you have probably reached the load limit of the port and you should stop. If the current is not large enough, consider adding a transistor current booster stage.