ESP project 5

I have built the ESP project 5 preamp powersuply and connected it to an 2x 9v trafo. When i measured the output of the trafo the actual power was 20 V AC. So I connected it to the PSU and measured the output. It was 35 V....

I want to use this PSU for the ESP P88 with two OPA2134 opamps, which have an maximum Vi rating of 36V DC.
So I guess my freshly built PSU is no good at all for this preamp. Does anyone know why the output voltage can be this high? I used one 7815 voltage regulator and a 7915 (negative) voltage regulator. I looked at the datasheets of these regulators and the maximum input voltage can be much higher for these devices.

Can anybody help?
This is confusing for a newbie.

2x9VAC is 18 volt at rated load so the open load voltage will probely be 21-23 volts _AC_ and when you rectify you will get the peak voltage of sinus wave, minus losses in the diodes. Multiply the AC voltage with 1.2-1.3 (in theory (ACin x 1.41)-1 )

23 x 1.3 = 30 Volts

Check the voltage across the smoothing caps, should be max 35 V

Check the voltage between GND and output, should be 15 volts +- 5 %.

The regulators can take max 35 volts between Vin and GND.

If the transformer gives a little bit too much you can burn some voltage in large resistors.

Check my super power supply QSXPS at my homepage, maybe you can get some ideas.


2001-06-01 4:53 pm
You should see your 35 Volts between the + side of C1 and the - side of C2. This should show up as +/-17.5 Volts if you measure relative to GND. This is what the regulators will see as input. Their outputs should be +/- 15V, again measured relative to GND. If not, check your wiring.
You may actually not have enough input voltage; these regulators may require 3 volts drop or so to regulate properly. But under no load conditions, the voltages should measure properly.
Thanks for your tips so far.
I am a newbie indeed, but I think I'm a fast learner. It's just that I want to check things with you guys and get tips from the more experienced DIY'ers.

I shall measure the output voltages relative to ground, which I haven't done so far. I will also measure the input voltages of the regulators. Maybe that wil give me a clue.

I don't understand this part:

2x9VAC is 18 volt at rated load so the open load voltage will probely be 21-23 volts _AC_ (I get this part, good to know :))

and when you rectify you will get the peak voltage of sinus wave, minus losses in the diodes. Multiply the AC voltage with 1.2-1.3 (in theory (ACin x 1.41)-1 )

23 x 1.3 = 30 Volts (so the regulators see 30 V on the input???)

The input voltage is high enough, Rod Elliot advised a 16V AC transformer for this PSU. So my transformer gives too much actually.
Paul, the supply he is referencing he is referencing is a full-wave voltage doubler.


Okay. What we call 18VAC, is 18V RMS. The waveform itself is a sine wave, with a magnitude of 18*1.414, or 2*18*1.414 peak-peak voltage. Under no load, acapacitors will charge to this, 25.45V, minus any losses in the diodes.

But, the supply you referenced is a voltage doubler. A virtual ground is created between the two supply caps, so the voltage across both capacitors should be closer to 50V, with about 25V from each cap to virtual ground. Plenty for your application.

Make sense?

Now, if you start drawing too much current, the xform voltage falls, the capacitors can't recharge up to the peak of the sine wave, and the voltage your reg sees falls. If you are powering a few opamps, you won't notice this effect.

Now, as far as what is (or is not) working, try taking your reg and apply your postive supply to Vin, attach GND to the midpoint between the caps, and hook a meter up to Vout. It should be 15V. If not, you've probably toasted your reg.

That done, repeat for the negative half of the supply. Note that Vin, GND, and Vout are not the same pins on a negative regulator as a positive.

Assuming you've got a working set of regs, reassemble the regulator part of the circuit carefully and all should be well. Don't attempt to power up your opamps until you are certain the supply is working.
I just finished my own P05 from ESP. I remember reading through some of the info from Elliot and he said that sometimes the voltage won't be correct until it has a load. Try hooking 1K resistors to ground from each output. This will result in tens of milliamps of current. Then check the voltage. If this works, then it will be able to power your preamp with no problem. I am curious if this will work. Let me know what your result is.
Oops, that was a little terse of me. I got interrupted and sent it before finishing.
As I was saying, it's two half-wave rectifiers, which is sometimes termed a half-wave doubler. Each half of the output should see about 25V, as tiroth stated. The input to each regulator should be somewhere around this value. Should be okay unless you pull a lot of current out of the regulators (in which case they'll overheat and shut down).
My earlier voltage numbers were wrong, sorry for the confusion.
Many useful replies, thanks for that.

Ok, now I'm measuring with an 1k load on the negative and positive outputs.

On the positive side the resistor is working properly, the voltage is 15.15 V. Yes, within the 5% tolerance, and good enough for me. On the negative output the trick doesn't seem to work. voltage is still 17.9V. Well what can be wrong? The negative voltage regulator 7915, or my solder skills. Have to check both.

If someone has more ideas, please let me know.

Phishead for what project did you mak the P05 for?
Ahhh, Progress.
I'm glad that at least something worked!
Now, for the negative voltage: perhaps it needs more current draw? Try a 750 or 500 Ohm resistor. At 17 Volts that will still be 20 to 30 milliamps. You can keep try different resistors and see if you can get the negative to stabalize near 15. Obviously, don't use too small of a resistor. You don't want to blow anything. I really don't have much experience with this stuff, but I don't mind throwing you ideas. You can always email Rod Elliot, he was very responsive to me.
I will use my P05 to power an electronic crossover, P09 and preamplifier, P88. I've successfully measured the power supply and electronic crossover. Both work wonderfully. Now, I just need to see if the preamp works. Then just throw them in a box and call it done!
Good luck with your project!