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60 cycle hum in a power supply

I recently scavenged a 5 VDC phone charger to use as a power supply for my small FM transmitter. I have the transmitter plugged into the computer so I can listen to my favorite internet radio station on the stereo in the living room, rather than only in the office. The whole thing works really well, except that during quiet songs or in between songs, I can hear that 60 cycle hum coming from the radio. Does anybody have any suggestions (cheap, free, or re-purposed parts are a plus) on how to minimize this? I don't want to spend money on this, and simple is my favorite adjective with electronics projects. Can I put a capacitor across the + and - wires to smooth out the power? If so, what size would be good? Thanks for your help! ~PC

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tech-king8 years ago
if its white noise from the power supply, a capacitor or zener diode could fix this. otherwise, youd need some sort of filtering device
lemonie8 years ago
Do you only get the hum from the transmitter, or is it a general artifact of the radio? A capacitor isn't going to fix this, a low frequency filter might, but you'd lose the low frequency end of the spectrum. L
Poppa Chubby (author)  lemonie8 years ago
It's from the transmitter, and it goes away if I use batteries instead of the power supply. If the power supply gave perfect 5 vdc, there would be no hum, but there is some ac or pulsating dc component in the power supply to the transmitter. It's one of those cheap gizmos you plug into an mp3 or cd player so you can listen in the car, I've simply re-purposed it for use in the home.

Thanks! ~PC
Searching "notch filter" will find you stuff for the audio signal, these can filter a narrow band. But since it's in the 5V supply, maybe just adding a large capacitor across the output would smooth it enough. If that's what you originally meant (+ and - wires on the 5V supply), you were thinking of the right thing. I reckon anything over 1000uF would do it.

L
Poppa Chubby (author)  lemonie8 years ago
That's exactly what I meant, and thanks for your help. Now to the junk pile to hunt for a capacitor to scavenge. Peace! ~PC
. If the cap doesn't kill the hum, I'd get a new supply. A notch filter would just be throwing good money after bad.
(if you get stuck I can mail you one) L