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Dual power supply

gregfox

Mar 25, 2013
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HI,

I’m trying to make a dual +/- 15 volt 100mA power supply. I using 7815 and 7915 regulators, with a center tapped transformer. The transformer has two 20V outputs rated as 100mV each, and a center 10V output that I am using as ground. The circuit uses a full wave rectifier and 2200uF electrolytic caps on the input and .1uF on the outputs of the two regulators.

When the circuit is turned on I get the expected +/- 15 volts, the strange thing is if I put even a small load of 5-10 mA, the voltage of the regulator sag by a volt, and if I increase the load to over 20mA they go completely out of regulation.

I’m scratching my head over this, can someone suggest an explanation or a fix?
 

davenn

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Sep 5, 2009
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I’m scratching my head over this, can someone suggest an explanation or a fix?


a helpful start would be to show us your circuit ... schematic and construction
it's probable you are erred somewhere but we don't know that without the info ;)


Dave
 

gregfox

Mar 25, 2013
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a helpful start would be to show us your circuit ... schematic and construction
it's probable you are erred somewhere but we don't know that without the info ;)


Dave
OK, I'll draw one up.
 

duke37

Jan 9, 2011
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If the transformer is 10-0-10V then the best you can get on C1 is 14.1V less the diode volt drop. You want at least 18V on C1 to get 15V regulated output.
Some voltage measurements are required.
A small capacitor close to the input of the 7815 is recommended.
 

davenn

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The transformer has two 20V outputs rated as 100mV each, and a center 10V output that I am using as ground


If the transformer is 10-0-10V then the best you can get on C1 is 14.1V less the diode volt drop. You want at least 18V on C1 to get 15V regulated output.


yes, exactly ... therein lies the problem


A small capacitor close to the input of the 7815 is recommended.


and output 0.1uF or 0.01uF are the common values ..... VERY close to the input and output pins to GND/0V rail


Dave
 

gregfox

Mar 25, 2013
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I've ordered a 20-10-20 at 1.5 amp, and some smaller caps.
I have in stock the attached picture of a transformer (DPC-40-250) with the following specifications
Voltage - Secondary (Full Load) Parallel 20V, Series 40V
Current - Output (Max) Parallel 500mA, Series 250mA.
Question;
1. Would it be possible to connect pins 6 & 7 and use that as a center tap?
2. If so would I connect 1 to 3 and 2 to 4?
Sorry I get really stupid when it comes to AC.
 

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davenn

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I've ordered a 20-10-20 at 1.5 amp,

I will assume that was a typo and you really meant 20-0-20 ?


1. Would it be possible to connect pins 6 & 7 and use that as a center tap?


yes ... so if each winding is 20V then you will have a 20 - 0 - 20 secondary

But you really should have just got a 15-0-15 or a 18-0-18 as Duke37 suggested
20-0-20 will be starting to get a bit too high once rectified and smoothed and you will have a 15V regulator running quite hot
as it tries to bring the smoothed DC voltage down to 15VDC


2. If so would I connect 1 to 3 and 2 to 4?

no, you would connect 2 and 3 together IF you region uses 230/240VAC
 

gregfox

Mar 25, 2013
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Our region uses 120V, but I believe you're right 18-018 would bring down the heat.
 

(*steve*)

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You also have to remember that for a DC current of 100mA, the transformer needs to be rated for 142mA. 15VAC when rectified will give you around 20VDC and that is ideal to be regulated down to 15V.

I would recommend a transformer that had a secondary described as either 15-0-15 or 0-15-30 and rated at 200mA or higher.
 

duke37

Jan 9, 2011
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If you get a dual 15V transformer, then the two windings need to be connected in series to give 30V at the ends and the connection will be used for 0V.
 
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