DC Power Supply questions

  • #1
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Main Question or Discussion Point

I've been looking ahead in my textbooks and reading some stuff online, and there's some things I'd like to know about DC power supplies.

The DC power supply we have at school has 3 independent outputs, 2 variable and one fixed. How would they be isolated from each other like that? Triple-secondary transformer? Seems pretty complicated, which might be why stuff like that is so expensive. Would it be possible to do it with only one secondary?

Second, the supply has a pair of push buttons in front of it that can hook the two variable supplies up in series or parallel, in a master-slave configuration; the voltage knob for the master controls both supplies. The one (naive) idea I had was that the two regulator's ground pins where hooked to the same potentiometer, but that would

I've done a little googling for schematics, but most of the schematics have been for basic supplies without these features.
 

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  • #2
Zryn
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I believe they are complicated versions of the simple Buck Converter (switch mode power supply) concept. AC in, rectified, smoothed, switched + controlled and DC output.

They are reasonably complicated and require quite a bit of design knowledge to get them to the robust 'student proof' versions you see in the labs.
 
  • #3
davenn
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Yep, pretty easy to produce 3 separate rails from a single transformer winding and a single bridge rectifier.

You only have to insure that the transformer and the bridge rectifier are rated ... current wise... for at least the maximum that would be expected as a total current drain if all 3 supplies were being used to their max at the same time.

cheers
Dave
 
  • #4
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I believe they are complicated versions of the simple Buck Converter (switch mode power supply) concept. AC in, rectified, smoothed, switched + controlled and DC output.

They are reasonably complicated and require quite a bit of design knowledge to get them to the robust 'student proof' versions you see in the labs.
A buck converter is just a switching regulator, right? (one kind, at least) I understand rectification and filtering, and the basic concepts of voltage regulation (although we aren't at that point in class, still on op amps), what I want to know is how to make the supplies independent. By my naive assumption, the regulators can't share a common ground, otherwise you wouldn't be able to hook a pair of them up in series to get the +/- voltages you need for something like an op amp, since that would short one of the outputs to ground.
Yep, pretty easy to produce 3 separate rails from a single transformer winding and a single bridge rectifier.

You only have to insure that the transformer and the bridge rectifier are rated ... current wise... for at least the maximum that would be expected as a total current drain if all 3 supplies were being used to their max at the same time.

cheers
Dave
No, there's quite a bit missing between "Single secondary and bridge rectifier" and "Multiple independent outputs". Basically, you left out everything important that I'm curious about: How those different outputs are made, and kept separate from each other.

If either of you knows of a schematic for something like this so that I could study/ask my teacher about it/use as a reference for more questions here, that'd be a lot more helpful.
 
  • #5
davenn
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No, there's quite a bit missing between "Single secondary and bridge rectifier" and "Multiple independent outputs". Basically, you left out everything important that I'm curious about: How those different outputs are made, and kept separate from each other.
nope, I didnt leave anything out :) I was answering this Q of your's ....

Would it be possible to do it with only one secondary?
you can easily produce all 3 rails off a single DC line out of the regulator :)

When I get home and have time (instead of the bosses time at present) I will draw a basic layout for you

cheers
Dave
 
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  • #6
davenn
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ok here's a 3 rail PSU, just the basic layout ... 2 variable and 1 fixed
I havent gone into the series /parallel current limiting etc, there's plenty of info on the net to tell you how to add those features :)
Also is an image of the 3 rail supply I use that can be series/parallelled or used independantly
a really nice unit

Dave
 

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  • #7
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ok here's a 3 rail PSU, just the basic layout ... 2 variable and 1 fixed
I havent gone into the series /parallel current limiting etc, there's plenty of info on the net to tell you how to add those features :)
Also is an image of the 3 rail supply I use that can be series/parallelled or used independantly
a really nice unit

Dave
We have something like that power supply at school, that's why I was curious about how it worked. Though it only has analog meters to measure the outputs, not fancy digital ones.

A diagram like that is what I originally came up with when thinking about it, but with the regulators all having a common ground, if you connected two of the outputs in series wouldn't one of the outputs be shorted to ground?
 

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  • #8
davenn
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but with the regulators all having a common ground, if you connected two of the outputs in series wouldn't one of the outputs be shorted to ground?
ok I see what you are getting at :)

I have tried to get a cct for my PSU been unable to.
but the thought struck me that if you wanted to series 2 of them like you showed
then throwing a series/ normal switch that would connect the appropriate shown + and - terminals and disconnects the gnd on one of them when in series mode.
I mite have to quickly wire somethig up and see if it works :)

Dave
 
  • #9
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ok I see what you are getting at :)

I have tried to get a cct for my PSU been unable to.
but the thought struck me that if you wanted to series 2 of them like you showed
then throwing a series/ normal switch that would connect the appropriate shown + and - terminals and disconnects the gnd on one of them when in series mode.
I mite have to quickly wire somethig up and see if it works :)

Dave
If you disconnect one of the grounds, then there's no return path and that regulator circuit will be open, and won't work.

And if you connect the ground of one of the regulators to the output of the other, than again, that regulator shouldn't work since its input and ground are both being fed by the same output from the bride rectifier.

Quite a conundrum. :confused:

Edit: Just had a thought, what about multiple bridges in parallel off the transformer? Each bridge would have it's own ground, and could (if my thinking is right) be tied together however you wanted. I'd have to do some thought/simulation see if that works.

Damn, half of my brain is saying "No, there's no way that should work!" and the other half saying "No it makes perfect sense!"
 
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  • #10
berkeman
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I've been looking ahead in my textbooks and reading some stuff online, and there's some things I'd like to know about DC power supplies.

The DC power supply we have at school has 3 independent outputs, 2 variable and one fixed. How would they be isolated from each other like that? Triple-secondary transformer? Seems pretty complicated, which might be why stuff like that is so expensive. Would it be possible to do it with only one secondary?

Second, the supply has a pair of push buttons in front of it that can hook the two variable supplies up in series or parallel, in a master-slave configuration; the voltage knob for the master controls both supplies. The one (naive) idea I had was that the two regulator's ground pins where hooked to the same potentiometer, but that would

I've done a little googling for schematics, but most of the schematics have been for basic supplies without these features.
Yes, for independent outputs, you would use separate secondaries.
 
  • #11
davenn
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If you disconnect one of the grounds, then there's no return path and that regulator circuit will be open, and won't work.
And if you connect the ground of one of the regulators to the output of the other, than again, that regulator shouldn't work since its input and ground are both being fed by the same output from the bride rectifier.

Quite a conundrum. :confused:

Edit: Just had a thought, what about multiple bridges in parallel off the transformer? Each bridge would have it's own ground, and could (if my thinking is right) be tied together however you wanted. I'd have to do some thought/simulation see if that works.
Damn, half of my brain is saying "No, there's no way that should work!" and the other half saying "No it makes perfect sense!"
OK then, for a series supply the only way you can do it is to have them on their own separate transformer windings and bridge rectifiers. Have floating grounds for each, ie, not connected to the PSU chassis :)

I suspect multiple bridges off a single winding would interact badly with each other

Dave
 
  • #12
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Yes, for independent outputs, you would use separate secondaries.
Is that really the only way? :frown: That seems so mundane...
OK then, for a series supply the only way you can do it is to have them on their own separate transformer windings and bridge rectifiers. Have floating grounds for each, ie, not connected to the PSU chassis.

I suspect multiple bridges off a single winding would interact badly with each other

Dave
Why would they interact badly with each other? I mocked up my idea in MultiSim and it seems to work fine, at least until I connected them in series, then MultiSim choked on it.

I know having a pair of diodes in parallel with each other is usually a bad idea, but a bride is significantly different than a single diode.
 
  • #13
berkeman
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Is that really the only way? :frown: That seems so mundane...

Why would they interact badly with each other? I mocked up my idea in MultiSim and it seems to work fine, at least until I connected them in series, then MultiSim choked on it.

I know having a pair of diodes in parallel with each other is usually a bad idea, but a bride is significantly different than a single diode.
And there's a reason the simulation choked -- the two circuits are galvanically connected (diodes do not provide galvanic isolation).

The only other way you could get galvanic isolation so that you can stack the supply outputs, would be if you followed a single secondary with one isolated DC-DC per output. So you would make one overall internal intermediate DC supply that the multiple output stages would draw from to generate the final outputs. The DC-DC converters would need their own switching transformers in order to provide the galvanic isolation. You could not use buck topology DC-DCs, for example, because their output is not isolated from their input.
 
  • #14
davenn
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I know having a pair of diodes in parallel with each other is usually a bad idea, but a bridge is significantly different than a single diode.
its still going to result in pairs of diodes in parallel personally I wouldnt even consider it
dual secondary transformers are easily obtainable :)

D

PS thanks for responses berkeman.... you are hopping in there whilst Im still typing haha

BTW is 1.2GHz as hi as you go on the ham bands ?
 
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  • #15
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And there's a reason the simulation choked -- the two circuits are galvanically connected (diodes do not provide galvanic isolation).

The only other way you could get galvanic isolation so that you can stack the supply outputs, would be if you followed a single secondary with one isolated DC-DC per output. So you would make one overall internal intermediate DC supply that the multiple output stages would draw from to generate the final outputs. The DC-DC converters would need their own switching transformers in order to provide the galvanic isolation. You could not use buck topology DC-DCs, for example, because their output is not isolated from their input.
Yeah, I figured out something like that after a bit of thinking. First time I've heard "galvanic isolation" though.

You lost me after "isolated DC-DC" though. I'll probably have to wait until we get to voltage regs in class to understand that.

Just to be clear though, a 3 output independent power supply like the one at school and in Dave's link would require 3 secondaries?
its still going to result in pairs of diodes in parallel personally I wouldnt even consider it
dual secondary transformers are easily obtainable :)

D
As long as the two bridge outputs aren't hooked in parallel, each of the diode pairs would be in series with their own load. That sort of thing happens all the time with LEDs without a problem.

So how would the master-slave thing be set up? With galvanic isolation being necessary, they can't be tied to the same potentiometer. Would it just be that two potentiometers are mechanically hooked to the master knob, and the slave is switched between its own potentiometer and the second master one depending on configuration?
 
  • #16
davenn
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As long as the two bridge outputs aren't hooked in parallel, each of the diode pairs would be in series with their own load. That sort of thing happens all the time with LEDs without a problem.
you forgot about the input pairs of diodes of the bridge :) they would be parallelling up across the transformer secondary

D
 
  • #17
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you forgot about the input pairs of diodes of the bridge :) they would be parallelling up across the transformer secondary

D
Taking out the reverse biased diodes in each alternation, wouldn't it look like the attachment here? LEDs are hooked up like that all the time with no problem.
 

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