Home >> Science & Nature >> Science

How to Calculate DC Filter Choke

Chokes are another name for coils, which are also known as inductors. The term "choke" is usually used for coils with iron cores and very high inductance. They look like small transformers, except they have only two wires. Chokes are used in power supplies to filter the rippling DC that comes out of the rectifier. Filter chokes are more common in high voltage power supplies, such as those associated with equipment that uses vacuum tubes. There are four important parameters that must be calculated when choosing a DC filter choke.

Things You'll Need

Ohmmeter

Instructions

- 1
  Calculate the data you will need to choose a DC filter choke by finding four parameters: DC current, resistance, inductance and DC voltage rating. Get the DC current -- the current that will be running through the choke -- by dividing the output of the power supply by 100. For example, if you have a 50 watt amplifier, you will need a 500 millihenry -- or 1/2 henry -- DC filter choke.
- 2
  Measure the DC resistance with an ohmmeter. The lower the resistance, the better the choke will work, but the more expensive it is. Typical DC filter chokes have 200 to 250 ohms.
- 3
  Give the meter some time to settle down before you take a reading. Coils resist the change in current flow, so when the meter starts to measure the resistance, the coil will resist the buildup of current. After a few seconds the meter will settle down and give an accurate reading.
- 4
  Compute the inductance using the formula Inductance = Reactance (2 Pi frequency). The reactance is the voltage divided by the current and the frequency is 60 in the U.S. and 50 in Europe. Typical values for a guitar amplifier or stereo equipment is 5 to 20 henrys. Coils outside of the power supply are usually measured in millihenrys.
- 5
  Find the final one of the important parameters for choosing a filter choke by multiplying 1.5 times the output voltage of the power supply. Theoretically, the DC voltage rating of the choke must be higher than the power supply voltage. The 1.5 is in the calculation as a safety factor. The cost does not increase dramatically for the higher voltage ratings.

Science