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How to Measure Filter Chokes

Filter chokes are large coils or inductors with metal cores that are used to filter out the ripple in power supplies. After a power supply changes AC to DC, there is still a lot of fluctuation in the DC. Filter chokes look like small transformers, but they have only two wires. A combination of filter chokes and capacitors are the last stage of the power supply. Because of their specialized use, filter chokes are not just measured by their inductance. They are measured by their maximum DC current, DC resistance, inductance, and a voltage rating.

Things You'll Need

Ohmmeter

Instructions

- 1
  Choose a choke with a maximum DC current rating that insures that it will not be damaged by your application. The maximum current that the choke is exposed to is actually higher than the maximum current that comes out of the power supply, because the current before filtering has peaks that are the square root of two times the filtered output. If you choose a rating that 1.5 times your expected current, you will have a slight safety margin.
- 2
  Measure the resistance between he two wires of the filter choke. Ideally, this should be zero but it never is. Typical values will be 200 ohms to 250 ohms. Anything higher than this means that the choke will have little value in reducing ripple. The lower the resistance, the more expensive the choke will be and the better it will work.
- 3
  Use the highest inductance choke you can afford. Inductance ̵1; measured in henrys ̵1; is how much the choke resists the changes in current. Typical filter choke ratings are 5 to 20 henrys. Coils elsewhere in the circuit will probably be measured in millihenrys. The higher the inductance, the better the filtering, and the more expensive the choke will be.
- 4
  Insure that the voltage rating is higher than the output of the power supply, Otherwise, the insulation on the winding wire may burn off and short out the coils.

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