A 12V *AC* power supply

[Alon Barzilai]

I seconds that.
without this resistor the voltage at low currents will go sky high (~20V)
the 12V is  usually close to the maximum current.

Alon.

On 7/14/2010 11:46 AM, Erez D wrote:
> 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 
> <mailto: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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