A 12V *AC* power supply

[Shachar Shemesh]

geoffrey mendelson wrote:
>
>
> I just took a DC adaptor and removed the components that coverted the 
> AC to DC. I did that for some modems that needed AC but came without 
> adaptors.
>
>
That's what I ended up doing. Now I'm having second thoughts whether I 
did that correctly.

The DC adapter had the voltage selector, connected to a diode bridge, 
connected to a capacitor and a resistor in parallel to the connection to 
the polarity selector. I understand what the diode bridge and capacitor 
were doing. The diode bridge was keeping the positive end of the AC sine 
wave, and the capacitor was leveling off the signal. Making it more like 
a DC line. What I'm not sure is what the resistor was doing.

This was a 460Ohm resistor, which at 12V would mean about 0.4 Watt. 
Seems like a lot of wasted energy for a 12Watt power supply.

I checked the output signal after removing all of the above (resistor 
included), and a volt meter and an oscilloscope  both shows exactly what 
I was hoping to see - an AC line giving the correct voltage. The only 
problem is that I'm not sure whether it was correct to remove the resistor.

Thanks,
Shachar

-- 
Shachar Shemesh
Lingnu Open Source Consulting Ltd.
http://www.lingnu.com

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