Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Yes. You will need to connect an R-C network to the secondary.

Posted by Al Sekela (A) on April 27, 2007 at 22:05:03

In Reply to: I'm thinking I might need to rolll my own posted by Christopher Witmer on April 27, 2007 at 19:51:46:

A big transformer can provide the same ability to absorb RF noise as a choke, but the secondary winding(s) may resonate if not properly loaded.

My approach is to determine the resistor that will be transformed to 120 ohms at the primary, then put it in series with a capacitor that will limit the power dissipation in the resistor. This is because the characteristic impedance of Romex-type plastic wire, as used in the US, is about 120 ohms.

Transformers have a turns ratio approximately equal to the ratio of output to input voltage. Thus, a transformer that has a 120-volt primary and a 12-volt secondary has a turns ratio approximately 10:1.

The impedance transforms as the square of the turns ratio, so in this case it will take a 1.2-ohm resistor on the secondary to look like 120 ohms at the primary.

Most of the secondary voltage will be dropped across the series capacitor, so a good approximation is to calculate the current in the resistor that will keep its power dissipation under 1/2 of its rated value: P = I*I*R, then calculate the capacitance needed to limit the secondary current to this value with all the secondary voltage across the capacitor.

Capacitor impedance is given by Z = 1/(j*omega*C), where C is in farads. The capacitor current with the secondary voltage across it is given by Ohm's Law, I = V/Z. You will need to do the math.

This works better with a higher secondary voltage, as the capacitor can be a lot smaller.

When shopping for used transformers, be careful of units that look like they have been out in the rain. Rusty frames or spotted insulation mean they are risky for line exposure. Even ones that look new may have a loud mechanical hum, so plug them in before buying if possible.

Finally, use a circuit breaker or fuse in series with the transformer filter, in case the capacitor fails.

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