# Choke design

Discussion in 'Electronic Basics' started by bubbles123, Aug 2, 2007.

1. ### bubbles123Guest

I need a choke to block out AC signals of approximately 1 MHz. It
needs to work for 500mA for a +/- 5V supply. How do I choose a value
for the choke? What factors need to be taken into account?

2. ### Lord GarthGuest

Xl=2*PI*F*L Calculate for a high value of Xl

It's easier to find what values of L are available to you
at a reasonable cost unless you intend to wind your own.

3. ### ehsjrGuest

Factors: the choke current rating and DC resistance.
Get the highest value of inductance that will handle
the current for maximum attenuation of the ~ 1MHz signal.
Check that the DC resistance of the choke will not impact
circuit operation beyond specs.

For example, a 2.2 mH choke from Mouser (542-5900-222-RC)
will handle 500mA, but with 2 ohms DC resistance, you'll
get a 1 volt drop across it at 500 mA. Depending on where
that is in the circuit, it might be unacceptable.

Ed

4. ### Phil AllisonGuest

"bubbles123"

** The choke must have a high impedance value at 1 MHz and not affect the DC
power delivery.

This is not an easy double requirement to meet because all chokes have self
capacitance - the larger the value the more capacitance it has -
resulting in a self resonant frequency that may well be below 1 MHz and ruin
the wanted blocking effect.

Normal practice is to use a low value choke wired in series PLUS a
capacitor wired in * parallel * to filter DC power. It is easy to find
capacitors with very low impedances at and around 1 MHz - ie most
electros of over 100uF.

....... Phil