noise-free 6V DC power supply

Hey Guys, I have a set of motorola FRS radios that contain a 6V rechargeable battery. As with most batteries, after 6 months the battery is no longer functioning.

The radio is stationary, therefore I do not need the wireless benefits of having a battery,and it can be plugged in for the power supply.

I built a 6 volt power supply to bypass the battery using an LM317 circuit which I found.

There is no connection for external power on the radio, so I removed the battery and hooked up the wires from the 6Volt power supply I built, directly to the battery terminals.

I installed a 1000uF cap to filter the power on the input and a 10uF cap on the output (6Volt) connection.

I have tried other wall adapters(some noisier than others), however I am getting a great deal of noise when using the radio, so much that it is unuseable.

I am just wondering if someone knows of a power supply that can be built easily and cheaply to do this task noise free? Or maybe a mod that I can do for my existing circuit.

I also tried an L78M06 regulator, still lots of noise.

I was using a power supply for my XM radio that is exactly 6Volts DC with no noise at all, it works well. This is a 12Volt lighter plug I use in my car to power the XM radio. I don't want to purchase one of these power supplies if I don't have to.

Any help/suggestions on how I can eliminate the noise will be appreciated.

What is the output voltage of your wall wart, with the radio connected?

Perhapps the radio is the source of the noise, have you measured it off load?

and what is the curent needed by the radio and the current of your transformer?

6 Volts supply

Try to run your LM317 or 7806 regulated supply from a 12 Volts car battery with no engine running.
You have a clean dc source and see if the noise still persists.

if still noisy you have a problem with the radio and not your regulator.

Check the load too!
You may draw too much current from the 317 or 7806,
If more than 1 amp your voltage will drop and proper regulation may be lost while the IC will go in over current mode and reduces the output voltage, and possible extra noise is created while the IC is trying to protect itself.

Ron H said:

What is the output voltage of your wall wart, with the radio connected?

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output is 13.24VDC (I measured it) 300ma.

Hero999 said:

Perhapps the radio is the source of the noise, have you measured it off load?

Click to expand...

Radio works perfectly (noise free) when connected to a battery.

mister_e said:

and what is the curent needed by the radio and the current of your transformer?

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i don't know, it does not specify voltage requirements on radio. Originally it uses 4 AAA nimh, but will run on 4 AAA Alkaline or equivalent.

RODALCO said:

Try to run your LM317 or 7806 regulated supply from a 12 Volts car battery with no engine running.
You have a clean dc source and see if the noise still persists.

if still noisy you have a problem with the radio and not your regulator.

Click to expand...

good idea, I will try it and post results. If anyone has a schematic or information I can download please let me know. I would like to learn how to fix this myself.

Cheers

What sort of noise is it? Is it white noise or mains hum?

The smoothing capacitor on the mains adaptor probably isn't big enough to filter out the ripple, try adding a 4700uF capacitor to the input of the regulator.

I had a similar problem with a FM radio. Originaly used 4NiCD but i tried 4Xalkaline an was OK. When tryng a 6.3V PS (high tolerance resistors ) I've heard a loud hum. The PS was filtered with a 2200uF capacitor. When I modified the PS (built around a 317) to 4.8V the radio was working OK.
Twilight zone?

Tarsil said:

(built around a 317)
Twilight zone?

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The LM317 isn't in the twilight zone.
Look at its datasheet, it doesn't regulate if its input voltage isn't at least 2V or 2.5V more than its output voltage. When it says the dropout voltage, it is the voltage that it has already dropped out!

audioguru said:

The LM317 isn't in the twilight zone.
Look at its datasheet, it doesn't regulate if its input voltage isn't at least 2V or 2.5V more than its output voltage. When it says the dropout voltage, it is the voltage that it has already dropped out!

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The 317 was droping the output from a adapter giving more then 13V without a load.
So......Twilight zone?

Tarsil said:

So......Twilight zone?

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No.
With a supply of 6.3V the radio oscillated and made a loud hum. When the supply was reduced to 4.8V then the radio worked fine, because then it didn't oscillate.
It also didn't oscillate with a battery because the internal resistance of a battery is much higher than the output impedance of a regulated supply.

audioguru said:

No.
With a supply of 6.3V the radio oscillated and made a loud hum. When the supply was reduced to 4.8V then the radio worked fine, because then it didn't oscillate.
It also didn't oscillate with a battery because the internal resistance of a battery is much higher than the output impedance of a regulated supply.

Click to expand...

Haven't tought of that. ....I gues that solves the mistery.

Ok I tried using a 12V car battery as a power supply running through my L78M06 voltage regulator and its crystal clear, noise free.

So I guess what I need is some noise filtering capacitors or something better.

I am using a Phonemate DC wal wart with an output of 12.83 VDC.
On the case it says OUTPUT: 9VDC, 100ma.
For use with Telephone.
I tried other wal warts but same thing, NOISE!

Any help is greatly appreciated!

Sometimes the bridge rectifier in a wall-wart makes radio interference buzzing as it switches at the mains frequency and at double the mains frequency. Some radios have a 0.01uF ceramic disk capacitor across each diode to stop the buzzing.

The problem probably is that the RF from the radio is getting back into the power supply and being rectified by the diodes. Try some 0.01 ceramic caps across each diode on the bridge rectifier.
If that don't work then another solution is to put an external antenna on the radio at least 20ft away from the powersupply.

Rf can do all kinds of strange things if it gets into a circuit it shouldn't be in.

I wouldn't use such a big capacitor at the input. If the output of your wall wart is a DC voltage, you don't need it. It probably has a high ESL and resonance can occur. Are you using long wires?
The LM317 has a good ripple rejection for low frequency noise/ripple.
I think that a 10 microF cap is better. Also put a 10 nF ceramic cap in parallel.

That's the problem, often these crappy adaptors don't have a big enough smoothing capacitor. To have a ripple of 1V you need at least 1000uF per 100mA, but one of the 300mA units I dismantled only had a 470uF capacitor.

I would add a >3300uF capacitor, then maybe 1uF and 100nF on the output to improve the transient response.