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Bipolar Power supply problems - AC present after rectifiers

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Muchomass

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Hello,

I'm building this circuit to create +/- 9 supply using a 12VAC wall-wart.

https://www.generalguitargadgets.com/pdf/ggg_bipolar_ps.pdf

My VDC measurements [with a load] at the output of the regulators are both fine, however, when I flip my multimeter to measure VAC it reads between 17 to 18vac between the positive regulator output and ground, and 0VAC between the negative regulator output and ground.

Additional measurements are:
37.9VAC and 17.5VDC on the cathode of D1
0VAC and 17.5VDC on the anode of D2

I've tried adding larger additional filter caps, as well as diodes across the in/out of the regulators, none of which has had any effect.

I've rebuilt this several times on a breadboard, always with the same effect, so I don't believe I'm omitting anything or misreading the schematic.
It powers the circuit I intended to use it with, but leaves a very noticeable AC hum/oscillation on the output.

Any ideas on what I'm missing?
Thanks in advance!
 
Firstly the vast majority of meters don't block DC on the AC ranges, so your meter readings is probably completely bogus.

Where are you measuring from?, it should be the 0V point on the right of the diagram, and should read +ve on D1, and negative on D2.

Note this is only a VERY low current supply, half wave rectifiers and small capacitors make it so.
 
Attached is an image defining what Nigel was getting at. Both meters are connected to a 24 VDC supply. The meter on the left is an inexpensive generic DMM and the meter on the right a high quality more expensive DMM. The meter on the left is reading 52.2 VAC while the quality meter reads a few milli-volts. The meter on the left is an average responding RMS indicating junk meter not using the AC coupling Nigel mentions. The DC component is not blocked. What you are seeing is just bogus numbers as was mentioned.

Ron
 

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I suspected that the numbers I was getting were bogus, but without a scope it was difficult to make sure. Thanks for the clarification.

Unfortunately that does not seem to be the end of my power supply woes:

when I use this power supply to power a simple spring reverb circuit using a tl047 op amp I get a harsh sounding low frequency oscillation (concurrent with the intended signal). However, when powering it with two 9V batteries it works perfectly.

Could this be caused by the fact that my +/- supplies are not precisely matched, causing the op amp to oscillate?
Might a series resistor on the higher of the supplies be a good fix in this case, or is there something else I should be looking out for?

Thanks,
Nick
 
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As was stated above, this power supply can only provide a low current. I suspect what is happening is that the load on the supply is too great and is pulling the regulator input voltage down below the minimum needed for it to function properly. If the reg cuts out the input voltage can recover. This process repeats cyclically, so your intended output probably has some awful triangular-ish variation superimposed on it.
 
Yes, probably too much current - first off replace the transformer with a centre-tapped one, and the two diodes with four (to give full wave rectification), and fit bigger capacitors (4700uF would be good).
 
Last edited:
What Nigel said and
If you can get a second 12VAC wall-warts.....use 4 diodes on each. "wave rectification"
 
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