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Using AC/AC wall adapter and converting to DC

jgauthier

Mar 22, 2013
63
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Mar 22, 2013
Messages
63
I have a "12VAC" wall adapter. The RMS voltage is really about 9.8V. It's rated at 1000mA. I have a small rectifying circuit to get DC +/- voltage for an op-amp.

The negative side of the DC voltage is -13V. The positive side is about 10V. The downstreamdevice seems to pull about 630mA (at the wall)

On the negative side, there isn't a load at all. Just the opamp signal. On the positive side, that's is where the draw is. My suspicion is that the power supply isn't the highest quality and the DC drop is due to load. Without a load, the AC voltage measures 10.6V.

All this to ask: Is it possible that the discrepancy between + and - is due to the power supply quality?

1683481153940.png

(The resistors pictures were intended to reduce inrush, but they are actually not there)
 

crutschow

May 7, 2021
833
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May 7, 2021
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833
Can you measure the plus and minus currents from the supply?
 

Bluejets

Oct 5, 2014
6,854
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Oct 5, 2014
Messages
6,854
The displayed voltage of the plug pack, if shown as 12v is the RMS value.
 

AnalogKid

Jun 10, 2015
2,860
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Jun 10, 2015
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2,860
All this to ask: Is it possible that the discrepancy between + and - is due to the power supply quality?

Maybe quality is a contributing factor, but a minor one. All transformer outputs decrease with load. Separate from that, and even without R25 and R26 in the circuit, the two output voltages are completely unregulated. This means that the AC ripple voltage component of the output voltage is directly proportional to the load current. The greater the load, the greater the AC ripple on top of the average DC output. This probably is confusing your meter. The only way to know the values of the peak and trough voltages on the output is with a scope.

Now, about those resistors - what are they for. They will cause a significant drop in the output voltage as the load current increases. 58.3 is not a standard E-96 resistor value, so what are they and why are they in the circuit?

ak
 
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