In bipolar supply, L7815 regulator only passes ripple

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Please see the attached schematic of the power supply that I have physically tested at my bench. I measured 150 mV rms of ripple across each of the smoothing capacitors of the unregulated power supply, capacitors C2 & C3. At Vout of U2 (the negative voltage linear regulator) I measure 0 volts of ripple, that is AC voltage. At Vout of U3 (the positive voltage regulator) I measure 150 mV rms of ripple. In other words, the positive regulator is not eliminating the ripple at its input pin.

It seems odd to me that I'm measuring substantial ripple at one output, but not at the other. So I'm wondering what I'm doing wrong that the positive regulator does not severely reduce the ripple at the output of the unregulated DC power supply. I don't believe that this is a result of a faulty L7815 as I have tested the circuit with two different L7815 and get the same result. Also the linear regulators are located less than 3 inches distant from the smoothing capacitors C2 and C3.

I feel like someone is trying to make me loony! Your advice is appreciated.

Pete
 

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dendad

Joined Feb 20, 2016
3,514
Can you draw a diagram of what pin is numbered 1 2 and 3?
78xx.jpg
This is how I expect the numbers to be. Not the way you have them numbered.
Of course, the -ve reg has different connections, but the physical pins numbers are the same as the +ve regs.
7xxx regs.jpg
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Hi

Maybe the smoothing cap is faulty. Try swapping C2 and C3. See if the ripple transfers to the negative side.

eT
That was my thought also, but I discounted it because I was seeing the same amplitude of ripple at both positive and negative outputs of the unregulated supply on my scope..
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
There is ripple at the output of the positive regulator only because the L7815 has a dropout voltage about 1V greater than that of L7915.

To eliminate the need to attach heat sinks to the regulators, I was attempting to keep the output voltages of the unregulated supply just slightly above the minimum required head room. Unfortunately the circuit that I want to power requires about 100 mA . For the circuit of the schematic that I attached to my first post, the output of the unregulated supply is about +/- 16.5V.

The transformer that I initially wanted to use for the power supply has a secondary rated 36V / 350 mA. Trying that first in the power supply, that produced too much headroom without heat sinking the regulators. Is there anything I could do to reduce the output voltage of the transformer with the 36V (center tap) secondary?
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
The pin numbering follows the numbers given by National Semiconductor to the circuit of the regulators. These numbers have nothing to do with which pin of the TO220 package is what. The regulators in my circuit have been wired in correctly according to what is shown in the data sheets.

Thanks for your solution, @crutschow.
 
The transformer that I initially wanted to use for the power supply has a secondary rated 36V / 350 mA. Trying that first in the power supply, that produced too much headroom without heat sinking the regulators. Is there anything I could do to reduce the output voltage of the transformer with the 36V (center tap) secondary?
If you want efficiency the only real solution is to use switchmode power supplies. They are more complicated and expensive but an overall better solution. There are a few software tools to help design the power supplies. You should look at some of them and see if they suit your needs.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
If you want efficiency the only real solution is to use switchmode power supplies. They are more complicated and expensive but an overall better solution. There are a few software tools to help design the power supplies. You should look at some of them and see if they suit your needs.
My thought was that if these regulators are capable of delivering 1.5A, then a good design should make it possible to derive a power supply with current output of 100 mA or 1/15th of what the regulators at maximum can put out that does not require heat sinks for the regulators. Unfortunately I had already ordered the 36VCT/ 350 mA transformer without analyzing in detail what would work best for my design. Efficiency wasn't really my goal, just good design- after I had already received the transformer that I bought for my PS. .

My power supply is for a line level analog audio processing circuit. Would a switch mode power supply be suitable for that application?

Regards,
Pete
 

crutschow

Joined Mar 14, 2008
24,914
My power supply is for a line level analog audio processing circuit. Would a switch mode power supply be suitable for that application?
Stick with the linear supply.
Switch-mode supplies, especially inexpensive ones, can be quite noisy, and it will likely be audible in your audio output.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Stick with the linear supply.
Switch-mode supplies, especially inexpensive ones, can be quite noisy, and it will likely be audible in your audio output.
Yes, that is what I suspected from some of my previous reading, but I was not quite certain. Thanks for confirming my suspicion.

-Pete
 
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