A 12V *AC* power supply

[Erez D]

The transformer gives 12V DC RMS, The peak voltage however is higher (but
with lower current)

I guess the reason for the resistor is to reduce the peak voltage and keep
it closer to 12V when there is little or no load, or when connecting the
load.


cheers,
erez.

2010/7/14 Shachar Shemesh <shachar at shemesh.biz>

>  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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