How to measure milliamps in a circuit

Written by ed wagner
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How to measure milliamps in a circuit
A digital multimeter can measure AC and DC current. (Polka Dot Images/Polka Dot/Getty Images)

Several methods can be used to find the current flowing in a circuit. Some are direct measurement using a multimeter attached to the circuit. Others require calculation. To measure current, opening the circuit for a test point is necessary. There are several ways to perform this task.

Skill level:

Things you need

  • Multimeter
  • Test leads
  • Soldering iron
  • Solder
  • Break out cable
  • One ohm resistor
  • Oscilloscope

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  1. 1

    Measure the current in a circuit by opening it and inserting a multimeter. If this is a one-time measurement that will not be repeated, simply use a soldering iron to lift one end of a series-connected component. Set the multimeter to measure amperes and attach the multimeter leads in series with the component and the board. Most multimeters are capable of reading up to 10 amps and the inputs have fuses for over-current protection.

  2. 2

    Use a breakout box to provide test points for measurements. These range from simple to elaborate. A simple box might be connected in line with a power cord to measure input current. On a 12-volt DC power cord, for instance, one conductor would be split and its ends connected to a pair of female banana jacks. The jacks would be in a small box. The multimeter leads plug into the jacks for current measurements. A more elaborate version consists of a circuit board with connectors on both ends and test points on every conductor. A computer card plugs into one end and the breakout card plugs into the computer mother board.

  3. 3

    Use a breakout box with a high-wattage 1 ohm resistor to monitor for intermittents. Connect a 1-ohm resistor in series and attach the oscilloscope leads across it. Do not use a wire-wound resistor for AC measurements. Oscilloscopes cannot measure current directly. The scope measures the voltage drop and Ohm's Law can be used to calculate the current. Current in amperes equals voltage divided by resistance, so if the voltage drop is 0.75 volts, the current is 0.75 amps or 750 milliamps. The oscilloscope display reveals any changes as you apply vibration or chill spray while troubleshooting.

    How to measure milliamps in a circuit
    A complex waveform on an oscilloscope. (Stockbyte/Stockbyte/Getty Images)
  4. 4

    Check the bench power supply for current. Most bench supplies can be switched between voltage and current, and some have an adjustable over-current limiter that switches the power off if the current is too great. Some power supplies can measure current down in the milliamp range, but not all do so.

Tips and warnings

  • Get in the habit of testing the power inputs of any unit with an ohm meter before connecting it to a voltage source. Some inputs are protected with diacs that short when exposed to high-level transient voltages. A shorted diac will pop the fuse on a power supply or cause it to switch off when it detects an over-current.

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