Rf Interference being fed back into my bench power supply. Solutions?

[diydidi]

Hi.
I have some chinese digital readout variable power supplies. Recently I built a little RF transmitter. It operates at around 100Mhz. When I set my power supply for 9V output, and power the little transmitter from it, the digital Led readout goes up to 13V, and the current Led display to 60mA.
If I connect my fluke to the power supply output, it still shows 9V.
So the transmitter is feeding RFI back into my power supply. How do I fix this??
I know it has something to do with chokes, but I don't know much about these.

[yozas]

First, you need to use a proper load on the output, and tune the output correctly so that it does not go where it is not supposed to go. Chokes on the cables that are going to the power supply would also help, but you need to tune the output properly either way.

[T3sl4co1l]

  *Linear* power supplies that don't pass basic EMC.  On the plus side, when it burns out a few months later, you'll be older and wiser, and shopping for a unit that's actually worth the money you'll pay for it.

Conversely;

I don't know what your transmitter is doing, but it's doing it wrong.  There should be no common mode current coming back from the antenna.  "Duh" cases where this will happen: 1/4 wave 'whip' with no ground plane, any balanced antenna type (e.g., dipole) driven unbalanced.

If what you built is the dumb old two transistor oscillator circuit, you can try putting a tapped coil on the output, instead of a single one; and connecting equal length antennas to both sides, so they get complementary voltages.  This is a basic dipole + balun.

Tim

[Davy]

You could try ferrite clamps on all the leads, including the AC input cord, trying grounding the case might also help..... another way in is direct pick up.

If ferrite clamps don't work try some Hi mu ferrite rings and loop the wires through a couple of times, the output voltage is usually monitored by the regulator circuit and all you need is RF modulation on the output lead.

One question is is it a plastic case?
If so, there's kitchen foil or a squirt of graphite coating on the inside and grounded by a 10n high voltage ceramic cap might need be called for.

Dave

[diydidi]

Thanx! Will give all suggestions a try. It is a steel case.

[Richard Head]

If you open the PSU and place a 100pF ceramic bypass cap across the error amplifier and two from the terminal posts to chassis you will probably solve it. They have to have very short leads to be really effective thoug. If it's stubborn you may also require a small common mode choke between the terminal posts and the PSU output. Hopefully the PSU has a metal case.