How to Test Resistors in Circuits

Written by j.t. barett
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How to Test Resistors in Circuits
On rare occasions, resistors may go bad in a circuit. (electronic component board 4 image by PeteG from Fotolia.com)

While resistors are one of the most durable and reliable electronic components, they are known to occasionally fail. A voltage surge may put too much current through the resistor, burning a resistor out and interrupting the circuit. In some do-it-yourself (DIY) projects, the resistor may have the wrong value, causing the circuit to malfunction. Lastly, a resistor may simply be defective and need replacing. The first step in fixing a resistor problem is to look for burnt parts. Then, with the help of a schematic, take the resistors out of the circuit one by one and test them with a meter.

Skill level:
Challenging

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Things you need

  • Multimeter
  • 15-30 watt soldering iron
  • Electronic solder
  • Desoldering pump or wick
  • Replacement resistors
  • Resistor colour code chart
  • Circuit schematic
  • Marker pen

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Instructions

  1. 1

    Turn the power off to the circuit and allow a few minutes for capacitors to discharge.

  2. 2

    Inspect the circuit board's component side. Look for resistors that appear burnt or cracked. Replace any that look damaged.

  3. 3

    Examine the circuit's schematic, noting the resistor values. Check that the resistors on the circuit board match the values on the schematic. Incorrect values are not likely, but simple to check. Replace incorrect resistors with ones that match the schematic, unless the schematic or service manual indicates otherwise.

  4. 4

    Desolder one lead of a resistor. Heat the solder pad with the soldering iron and draw off the liquid solder with a desoldering pump. Pull that leg of the resistor free of the board. This takes the resistor out of the circuit so it can be measured accurately. Test the resistor with the multimeter set to read resistance. If the resistor checks bad, replace it. If it reads OK, resolder the lead into the board using a small amount of fresh solder. Repeat this step for all suspected resistors. When you finish checking a resistor, mark it with a felt-tip pen to avoid rechecking the same one.

Tips and warnings

  • You can substitute a resistor with a higher wattage rating, but never a lower one. For example, if you're replacing a 560-ohm, one-watt resistor, a two-watt is perfectly acceptable, as long as it fits on the board.
  • When circuits are exposed, always be aware of any hazardous voltages. Do not attempt repairs while the power is on.

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