Power Supply

[ronjodu]

I'm breadboarding a power supply for a PGA2320 volume control chip I'm using with an amplifier I built.
This is the schematic of the power supply.
https://www.aaroncake.net/circuits/supply3.asp
Output voltage is +15/0/-15
I'm also using a PIC 16f876 to control everything.
My question is can I use the same transformer (2 X 18 volt AC)to power another rectifier and LM317 for the micro power?

I tried using one leg and the center tap (18 volt) and the transformer got very hot.

Any comments are welcome.

 

[ljcox]

The only reason I can think of as to why the transformer became very hot would be if the 5 Volt regulator (or micro) was consuming excessive current.

What current does the micro need? Did you measure the current?

I would use a 5 Volt regulator such as a 7805 (1 Amp max) or 78L05 (100 mA max) rather than a LM317 as you need less components.

I would also have used a 7815 (15 Volt 1 Amp max) and a 7915 (-15 Volt, 1 Amp max) for the +/- 15 Volt regulators. These are internally set to the voltage so you don't need resistors to set the voltage.

The LM317 is excellent for non standard voltages, but for the standard voltages, ie. 5, 6, 8, 9, 12, 15 & 24 Volt, the 78 or 79 series are better.

Note that there are also LM equivalents, eg. LM340T5 is equivalent to the 7805, etc.

 

[mvs sarma]

Hi Ronjodu,
as you already have +15V output try to regulate it to 5V by 7805. don't again go to transformer secondary -one and and centre tap? If you take like that and apply a bridge rectifier and try to common the negative to common ground it iwll short circuit the transformer. make a schematic and trace it and it will be a good exercise.
If at all you want to tap from secondary, try using halfwave rectifier--NEVER A BRIDGE RECTIFIER.

 

[ronjodu]

Thanks

Thanks for the replies. I will try a 7805 as I do have one on hand. The micro needs very little power as I only have the PGA2320 connected to it.

I will also try the suggestion as to connect the regulator to the existing rectifier rather than add an additional.

Learn something new every day.

Thanks again, much appreciated.........

 

[ljcox]

Hi Ronjodu,
as you already have +15V output try to regulate it to 5V by 7805. don't again go to transformer secondary -one and and centre tap? If you take like that and apply a bridge rectifier and try to common the negative to common ground it iwll short circuit the transformer. make a schematic and trace it and it will be a good exercise.
If at all you want to tap from secondary, try using halfwave rectifier--NEVER A BRIDGE RECTIFIER.

He does not need another rectifier bridge or a diode since the existing rectifier bridge provides unregulated + & - voltages. Just connect the input of the 7805 to the unreg +.

If you take it from the +12 V regulated line, you have to be sure that the 12 Regulator can handle the extra current.

 

[ronjodu]

Works great.

Finally had time to try your ideas. Although I'm still using the LM337/317's for the +15/-15 volt I went to the 7805 fed from the existing regulator for the 5 volt. All working very well. Volume control is up and running under PIC control with Gain output to Hyperterminal. Very Cool...

Thanks again for all the help.