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Hi Newby a little help please

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vlk

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Ok hope this is the correct area for this post if not please let me know as I am new to the group.

I need both a 12 and 5 volt power supply. No problem, have built these using regulators like the 78l12 and 05. But here space is very short so I want to use just one transformer.

Reading some theory it stated that it is good practice to keep the input to these regulators only 2-3 volts above what is need to help stop the heat build up. Sounds good so far.

Now I have 9-0 0-9 transformer I can use, which will give me 18 volts if I join the two windings. This I feel should be ok for the 12 volt, ok still some 6 volts too high but hopefully ok; but very high for the 5 volt supply as it will be 13 volts higher.

Now my thinking here is to use the 18 volt but also tap just one of the coils for 9 volt helping the heat build up in both circuits.

Can this be done or will I make problems as I have joined the two windings for the higher voltage?

Many thanks for any help you can offer.

vlk
 
Yes, you can put the two windings in series and connect a bridge rectifier to the output with a filter which will give about 24Vdc for the 12V supply (the filter capacitor charges to near the peak of the sinewave or 1.4*18V).

If you place a capacitor filter at the junction (tap) of the two windings you will also get about 12Vdc for the 5V supply. (It may not be apparent but two of the rectifiers in the bridge act like a full-wave rectifier for the tap, which gives half the voltage of the bridge output.)
 
If you test the transformer, bridge rectifier and filter capacitor then the output voltage will be higher with no load.
 
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