15V power supply output voltage help

[BuckarooBanzai]

Hi,

I'm very new to DIY electronics, but am learning a lot here.

I've assembled a simple 15V bipolar power supply using 7815 and 7915. I've double checked my wiring, but for some reason I get only +10.3V and -9.4V at the output.

I'm using a 15V transformer. Its AC output reads fine at 16.2V (no load), and after the rectifier it's 22.9V.

I've attached the schematic. Have I done something dumb?

 

[Mooly]

You really need a dual secondary transformer that has two 0-15 and 0-15 windings. Your circuit is fine, it is just the transformer that is incorrect.

 

[JonSnell Electronic]

Yes, you need a dual winding as there is no ground/centre point. 15 - 0 - 15 , (centre tap to ground/centre point) is the usual transformer secondary in that configuration otherwise there is no reference.

 

[theAnonymous1]

You can do it with a half-wave rectifier, but it's better to just get a proper transformer...

An externally hosted image should be here but it was not working when we last tested it.

 

[JMFahey]

Think about it: you are trying to get 30V end to end (from +15 to -15) out of 22V total.

 

[woody]

These regulators also may give real strange results if you measure the output without any load. Just load them with a resistance to pull about 5ma or more if you don't you may measure something crazy like 18v out.

 

[BuckarooBanzai]

Yup, I did something dumb XD. No dual windings!

Thank you very much everyone! I feel like a dumbass, but learning...

 

[AndrewT]

Yup, I did something dumb XD. No dual windings!

Thank you very much everyone! I feel like a dumbass, but learning...

Not dumb at all.

It's the technical language that needs to be translated into technical but understandable information.

A dual polarity power supply is TWO power supplies that are strung together in some manner.

To get a dual/two output one needs a dual/two inputs.
That is probably the bit that got overlooked.

If your loading is low, then post4 gives a good solution that saves having to buy another transformer.
for upto 250mAdc from each polarity, that would be perfect.
It could also supply upto 500mAdc with some loss of performance. I would not try to get 1Adc from the half wave rectifier. The maximum continuous DC current available from your 4Aac transformer if you want it to run cool, is about 1Adc.

If you draw 500mAdc from both polarities continuously, then the transformer sees that as a continuous current draw of 1A. That is already @ ~50% of the maximum continuous capability of the transformer.