need to stabilize a wallwart adaptor, how to ?

[andre tchmil]

Hi, I want to replace the batteries in my fm transmitter with a DC adaptor.
batteries were 3X AA, 1,5 volt.
So I bought me a DC adaptor with selectable voltages, swithed to 4.5 volt, and soldered the leads to the battery springs.
All seemed fine , but it produces a loud hum, so I guess it is not stabilezed enough.
What components to use and how to hook the up to get a perfect DC voltage?
Basic description please, thank you
:thumb:

BTW, I have plenty of space inside the battery holder and I can switch the DC voltage on the adaptor to 6-9-12 volt (300mA)

 

[gyraf]

A 7805 regulator, and a 4001 diode in series with the + from this to drop the voltage 0.6V further from 5V to 4.4V...

Jakob E.

 

[andre tchmil]

thanks Jakob.
I just did it the way you desribed, but the hum, (50 hz cycle) is still there.
I then placed a 1000uf cap over the + and -, no luck

I then placed a 100uf cap on the input, before the regulator. same :sad:

I reinstalled my batteries , and the transmitter is dead clean again.

 

[Guest]

I tried to do a similar thing with some cheep wall warts. I ended up needing so much filtering to get rid of the wart noise that I could have purchased a decent wall wart in the first place. If you can look at the output of the wall wart you will see how much noise it supplies. Try a good one and you will see a marked difference. You will still need to further filter it though. Jakob's method works best I think.

 

[JLM Audio]

You will need to turn up the DC adaptor up to about 8 to 12v if it is a switchable one for the 7805 to regulate 5v properly. And the diode after will remove 0.6v to give you 4.4v which will work perfectly. If you don't turn up the volts the 7805 will not regulate so it will not respond to bigger filtering caps etc.

Joe

www.jlmaudio.com

 

[Family Hoof]

Figure out current draw (ammeter in series with one of the power leads while device in turned on) and add an appropriate RC low pass filter. That means the resistor is in series with the +supply voltage and the capacitor follows, connected from +voltage to ground with the negative side at ground. DC is equivalent to 0Hz and ideally this is the only frequency you want your filter to pass through.

Choose the R to permit enough current (ohms law says 4.5V divided by current draw + %15 to be safe = resistor value) and then the cap should be as big as you can fit, or as big as necessary to bring the cutoff frequency way below 60Hz. For example, < or = 1Hz is great. That way it's already down 18dB by 64Hz if using a single layer filter = 3dB per octave. Add a second layer to this filter and it's now a 6dB/octave slope.

cutoff frequency = 6.2832 x ohms x farads

I suggest the use of a resistor because it is hard to get the cutoff frequency low enough with just a capacitor alone.

 

[andre tchmil]

I tried it all, resistor, different caps , different 7805, raised supply voltage, moved the 7805 and caps further away from the transmitter, used another wallwart.

No luck, the battery setup wins hands down. :?