Measuring Current Without Breaking Your Multimeter

Most good multimeters are able to accurately measure current in circuits.  However, many of them can only safely handle 10 Amps or maybe 20 Amps for higher-end models.  If you exceed these levels, you will blow a fuse at the very least.

This 0.5 ohm shunt allows me to measure higher currents with my inexpensive multimeter.

This 0.5 ohm shunt allows me to measure higher currents with my inexpensive multimeter.

There is another method for safely measuring current in a circuit without putting your multimeter at risk…The shunt resistor method.  By placing a resistor in your circuit with a known resistance, you can use the Ohm’s law formula to calculate current.  Ohm’s law is basically the relationship of voltage, resistance, and current.

The great thing about this is when two of the components of the formula are known, you can solve for the third.  For example, current can be calculated if you know voltage and resistance.  In the case of the shunt resistor, we use the “safe” measurements of voltage and resistance and calculate current.

Heat sinks don't compensate for bad circuits.  This power MOSFET was the victim of too much current. In addition, I was testing this H-Bridge by switching forwards and backwards without allowing enough time for the gate to close. Lesson learned. Although, the flame was quite spectacular.

Heat sinks don’t compensate for bad circuits. This power MOSFET was the victim of too much current. In addition, I was testing this H-Bridge by switching forwards and backwards without allowing enough time for the gate to close. Lesson learned. Although, the flame was quite spectacular.

Measuring using the shunt

1.  Place the resistor in series with the circuit.

2.  Measure the voltage from immediately in front of the resistor to immediately behind the resistor.  The red multimeter probe goes on the side closest to the positive battery terminal.  The black multimeter probe goes on the negative side.

3.  Calculate using the Ohm’s law formula.
Current = Voltage drop of resistor / Resistance in Ohms of resistor
For the example of a voltage drop of 1.4V and a shunt resistance of 1 ohm:
Current = 1.4V / 1 Ohm = 1.4 Amps

Measuring current is simple and safe using the shunt resistor method. The voltage drop of the resistor is measured and Ohm's Law is then used to solve for current.

Measuring current is simple and safe using the shunt resistor method. The voltage drop of the resistor is measured and Ohm’s Law is then used to solve for current.

Making the shunt

For my current shunt, I used two 5 watt, 1 ohm power (cement) resistors.  Even though calculations are easier when using 1 ohm as the base, combining two in parallel allows more current to pass through the shunt and less drag on the circuit.  I just so happened to have a a testing wire with alligator clips that bit the dust so I salvaged the clips.

The current shunt can be built from very inexpensive parts. Adding lead wires allows more flexibility for using for larger projects.

The current shunt can be built from very inexpensive parts. Adding lead wires allows more flexibility for using in larger projects.

1.  Twist ends of power resistors and lead wires together.  I chose to use 14 Ga stranded wire for my leads.  This is thicker than I use in most projects so the wire resistance won’t impact the current measurement.

2.  Solder to ensure a secure connection

3.  Use heat shrink to reduce the exposed conductive parts of the shunt.  Make sure to leave enough exposed to connect the multimeter.

4.  Add clips to the end for easy integration into the circuit.

There are probably many ways one could build a current shunt but this one should store nicely in a parts drawer.

There are probably many ways one could build a current shunt but this one should store nicely in a parts drawer.

Thanks, and happy building!

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