Can I share a single transformer for 2 power supply circuits

[ArJunaBug]

I am just getting back into electronics. It has been about 30 years since I worked in electronics. I spent the last thirty years as a computer engineer. Previous to that I worked as an electronics technician at a research lab and also in the US Navy. So I am a bit rusty with this stuff. I appreciate any help I can get.

I am building a power supply and the transformer I have is a simple 28VCT. I want to provide a variable output from about ±2-13 volts and a separate ±5 volt output.

I built the circuit last night and it seems to work fine.

What I want to know is does anyone see a problem with doing this as illustrated in the schematic? See the section highlighted in red.



Is it necessary to have a separate transformer or a separate secondary output from the transformer?

Thanks

 

[erikl]

Re: Can I share a single transformer for 2 power supply circ

Is it necessary to have a separate transformer or a separate secondary output from the transformer?

No, in principle this will work fine, as you share a common COMMON.
But there are 2 problems:
- the 2 elkos between the ADJ terminals of the 7805/7905 regulators and COMMON should probably be short circuits to COMMON - I guess this wasn't intended so

- the rectified DC voltage (before the regulators) is too high for the regulators, at least if you account for an overvoltage of 10 .. 15%. And in any case for the 7805 : it's max. input voltage for 5V output is only 35V .

 

[ArJunaBug]

Re: Can I share a single transformer for 2 power supply circ

No, in principle this will work fine, as you share a common COMMON.
But there are 2 problems:
- the 2 elkos between the ADJ terminals of the 7805/7905 regulators and COMMON should probably be short circuits to COMMON - I guess this wasn't intended so

You are correct, I drew the schematic to illustrate the dual circuits off the transformer, I just basically copied the adjustable circuit above. In any case the schematic is not complete.

- the rectified DC voltage (before the regulators) is too high for the regulators, at least if you account for an overvoltage of 10 .. 15%. And in any case for the 7805 : it's max. input voltage for 5V output is only 35V .

The 7805/7905 regulators are seeing only plus or minus 18VDC respectively on their inputs. Is this too high you think? What would you recommend?
Thanks

 

[IanP]

Re: Can I share a single transformer for 2 power supply circ

The 7805/7905 regulators are seeing only plus or minus 18VDC respectively on their inputs. Is this too high you think? What would you recommend?

Voltage up to 35Vdc is not a problem, but all depends on how much current do you intend to draw from 7805 and 7905 ..
In any case, consider a decent size heatsink for both of them and keep in mind that in the case of the 7905 the tab is not the common (as in the 7805), so you will need an insulation washer (and bush) if you want to mount it on to the same heatsink ..

Rgds,
IanP

 

[ArJunaBug]

Re: Can I share a single transformer for 2 power supply circ

In any case, consider a decent size heatsink for both of them and keep in mind that in the case of the 7905 the tab is not the common (as in the 7805), so you will need an insulation washer (and bush) if you want to mount it on to the same heatsink ..

Rgds,
IanP

Thanks for that, I would have missed that tab thing. I have a huge heat sink ready to go.

 

[erikl]

Re: Can I share a single transformer for 2 power supply circ

The 7805/7905 regulators are seeing only plus or minus 18VDC respectively on their inputs. Is this too high you think?

18VDC wouldn't be too high, of course, but in your schematic all your regulators receive the rectified 28V AC voltage, which results in ≈ 38 .. 40VDC, at least at no-load condition. If you consider a possible mains overvoltage of 10 .. 15%, this could result in a no-load primary DC voltage of up to ≈45V, and this is clearly too high for all your regulators. The 7805/7905 regulators are limited to max. 35VDC input voltage for +5V resp. -5V output.

 

[ArJunaBug]

Re: Can I share a single transformer for 2 power supply circ

The 7805/7905 regulators are seeing only plus or minus 18VDC respectively on their inputs. Is this too high you think?

18VDC wouldn't be too high, of course, but in your schematic all your regulators receive the rectified 28V AC voltage, which results in ≈ 38 .. 40VDC, at least at no-load condition. If you consider a possible mains overvoltage of 10 .. 15%, this could result in a no-load primary DC voltage of up to ≈45V, and this is clearly too high for all your regulators. The 7805/7905 regulators are limited to max. 35VDC input voltage for +5V resp. -5V output.

My regulators are receiving only half of the rectified voltage as they all share the center tap for ground. In my final circuit I will have some protection diodes in a few places. Under no load conditions they are seeing 19-20VDC at the most as I measured it.

 

[erikl]

Re: Can I share a single transformer for 2 power supply circ

My regulators are receiving only half of the rectified voltage as they all share the center tap for ground. In my final circuit I will have some protection diodes in a few places. Under no load conditions they are seeing 19-20VDC at the most as I measured it.

You're right! I've simply overlooked this, sorry for the trouble! :-(

 

[ArJunaBug]

I have a new problem with my power supply design.

I have a new problem with my power supply design.

I have decided to go higher than ±15VDC output and opted for a 60VCT transformer instead of the 28VCT transformer.

I am seeing a voltage on each input side of this circuit of ±40 volts when the transformer by itself outputs only 30 volts on either side of the center tap. What is the best way to take this voltage down when the circuit is not under load so I don't exceed the input limits of the 317 and 337?

Thanks

BTW, I exploded a 2200µf 50v capacitor while testing. It was pretty spectacular and stinky.

 

[IanP]

Re: Can I share a single transformer for 2 power supply circ

You can consider Zener diodes, shunts and all that stuff, but a workable solution will be a pre-regulator circuit for both rails based on a principle of Emitter-Follower-Regulator .. see attached picture ..
This circuit is self-explanatory; version with NPN transistor works on the positive rail, similar (mirror) circuit with PNP transistor and Zener connected up-side-down works on the negative rail ..

Rgds,
IanP

Ps. All capacitors should be rated for ≈1.5 * Vmax ..