# Measuring Current Using A Resistor?

Sometimes in videos made by the YouTube creator ElectroBOOM the creator says he is using a 1 ohm series resistor to measure current. I looked it up and found some links to forums where people are saying that you can use a resistor rather than a DMM to measure current, but I can't find anything saying how to do this. Are they usimg the voltage drop across the resistor from a voltmeter or are they doing something else?

• Yes, you measure the voltage drop across the resistor to determine the current. – brhans Jun 26 '18 at 22:05

Yes, you can use a resistor and voltage range on a multimeter (to measure the voltage across the resistor) and calculate I = V/R. That can be useful if you need to measure current outside of the range of your meter.

For example, if you need to measure 100A you could use an inexpensive 100A shunt and a multimeter on voltage setting. The meter might read 75mV for 100A (assuming a 75mV shunt), so on the 199.9mV range you can read with a resolution of 130mA up to +/-100A. In the case of shunts, the resistance (0.75m ohm) is not usually given, rather the voltage at the measuring terminals at full rated current is given.

In most cases you can assume the meter impedance does not affect the measurement (shunt) resistor, but not always.

For example if you need to measure 1nA and you just have an inexpensive handheld multimeter with 10M ohm input impedance on the +/-199.9mV range you can just use the meter on voltage range as a current meter. It will read 10.0mV for 1.00nA, giving you a resolution of 10pA.

I = V/R. If you know the value of R you can calculate the current by measuring the voltage.

This is the method used in a lot of automated current monitoring solutions, though the resistors are usually smaller (1 - 100 mOhms), and the voltage is amplified before being sampled by an ADC.

Many DMMs cannot measure current directly. They all measure voltage, though. The idea is you measure the voltage across the resistor and use Ohm's law to calculate the current (I=V/R).

Yes. The method is very effective in many cases, I use it all the time to get higher frequency current measurements with an oscilloscope rather than a meter. All you do is probe the potential across a known resistance and calculate the current with Ohms Law (V = I * R, or in this case I = V / R) You will still need a volt meter of some sort, and you will need to know the resistance between the two points you are probing. If you are probing a PCB for instance, you will need to be sure that if there are any resistances in parallel you take R for the equivalent resistance of that network. If you are just inserting a resistor as a current shunt after the fact you are fine!

• That makes sense, but what is the advantage of using this mothod over using the current measuring function that most DMMs come with? – user180969 Jun 26 '18 at 22:28
• Sometimes its easier to fit, sometimes its a cheaper option to get current measurements that change very quickly (thousands of times per second or more). its all a question of how precise you measurement needs to be from a certain perspective. Most DMM's use the same exact method to measure current anyway. – Luke Gary Jun 26 '18 at 22:55
• Standard probes with DMMs require they be inserted in series with the circuit during current measurements. This can be inconvenient if the circuit is already soldered together. – user159625 Jun 26 '18 at 23:52

If you have only one DMM, and you want to measure several different things, then it's very inconvenient to measure current with it.

The method is to switch off the circuit, break the connection, insert the meter, switch it to amps, power up the circuit, and reverse all that again to get the meter out, ready to measure voltage elsewhere. Oh, did you remember to switch it back to volts before measuring the power supply voltage? Ooops! Just blown the fuse.

The alternative is to have a current shunt permanently wired into each conductor that you want to measure current. Leave the DMM on volts, and measure the voltage across the shunt resistor. Sure, there are some sums to do, which is why people tend to use 'easy' shunt values like 1ohm, or 100mohm. And the meter is still ready to measure volts elsewhere in the circuit.