Power supply for FM transmitter

[Electrojosh86]

I am guessing that this is due to the 60hz mains hum or maybe ripple but everytime I use a 12v 1a adapter I had laying around and connect it to a small FM transmitter I made I hear a very loud "Hum" coming from the receiver and it becomes hard to tune and unstable. With a battery all this goes away and the FM transmitter works great. How do I filter this out? Large Elecrolytics? Around 10,000uF? Would like this to be as simple as possible though.

 

[tpetar]

There is many reasons why you have this situation. Radio and TV interference from solar equipment and many DC operated items, especially fluorescent lights and solar charge controls is a growing problem. Always use a voltage regulator. No matter how well the "raw" DC is filtered, total ripple elimination does not become a reality until the circuit has the ability to set the output voltage at least 2,5-3 volts lower than the lowest dip in raw DC input to the regulator. Bypass your regulated outputs both with small electrolytic caps and also with ceramic disc capacitors, or similar types with low resistance at high frequencies (the electrolytics alone are ineffective at very high frequencies).

You mention some adapter, are you using SMPS psu for receiver?


Best regards,
Peter

 

[Electrojosh86]

What values would be best for the bypass caps? And I am guessing they would go before the regulator I am not going to build into the circuit. I am thinking of droping down to 5v with a L7805 that way I can use a 9v battery as well as 12v power adapter. As for the adapter, it is just a run of the mill adapter that you see everywhere. I just have some extra ones laying around. And the receiver that I am using is just a normal stereo AM/FM receiver. I am just experimenting so I don't need anything crazy. Also, getting a RFI snap on ferrit would work? No?

- - - Updated - - -

Nevermind, just reread. Bypass with caps on the regulated end.

 

[throwaway18]

>I am guessing that this is due to the 60hz mains hum or maybe ripple

There is often a 120Hz component as well because of the rectification.

>What values would be best for the bypass caps?

Exact values are not critical.
For a small transmitter drawing under 0.1amp I'd use 470uF electrolytic and 10nF ceramic on input and output of a regulator, or whatever capacitors I had handy near those values.

>getting a RFI snap on ferrit would work? No?

A snap on ferrite only causes a medium impedance at high frequencies, at least 10MHz upward, maybe 100MHz depending on material. It will not reduce 60Hz or 120Hz ripple from a power supply.