Choke design

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?

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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.

Reply to
Lord Garth

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.


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** 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

Reply to
Phil Allison

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