Power Supply Voltages

Used one at a time - yes. Used together - no, as you only have one common ground point whcih most circuits will need.

Yes, you can do this.

It's all about common grounds.

Yes you can, that will work on it's own.

One transformer tap can power as many regulated outputs as you like provided:

Lab power supplies are *much* more versatile when they are fully isolated from each other, i.e. they each have an isolated transformer tap (or a seperate transformer). Then you can combine each one any way you like to get all sorts of combinations.

If you want to create a big versatile lab power supply with many outputs then use multiple mains transformers and have all your outputs isolated with their own rectifer, filter and regulator.

You can just "swap the connections" to get a "negative" supply, but you usually want a positive supply too, relative to a common ground connection. If you had two floating +5V supplies you can connect the GND of one to the +5V of the other and this will give you a +/-5V supply with a common ground.

If you had a >+/-15V centre tapped transformer and rectifier as in your diagram above then you could use that to produce say a variable +/-15V supply, fixed +/-12V rails, and fixed +/-5V rails. That would be a handy power supply, but it will all use a common ground.

Your thinking is correct, but you just have to be careful that you don't do anything silly with the "grounds" and accidently short out one of the rails.

Dave :)

heh, yeah... I won't do it intentionally but I can't guarrantee that I won't do it by accident ;)

I have several transformers at different ratings and such but I will probably just run them off my 25VCT @ 2A for now since I just need to get a decent regulated supply that can give me +5 and +12 for now(but I will add a variable channel from 1 to 25V incase I need something different)... just making sure I can get -5 and -12 out of it so I don't have to worry about adding those.

I don't mind having different grounds for the different levels though(is this a bad thing? Can it cause ground loops and oscillations and such?)

Thanks, Jon

oh, 1 more thing...

can I run voltage regulators in cacade without to many ill effects to reduce the power dissipation in subsequent regulators?

i.e. -------- 12Vout | |

???

I know that my current rating its somewhat trashed(maybe I can each of them to run a transistor to handle the current if my regulators cannot handle them individually?) in that I have to add all the current leaving every Voltage out I use and that it does overload any regulator but shouldn't be to much of a problem(since I don't plan on using to much current)?

i.e. I was going to do it with each regulator in series but as you can see I'd have to dissipate 19 volts(ofcourse * whatever current I draw) or so in the 5V regulator instead of 3 in the cascaded way, right? I'd also benefit from increase regulation through step? (so the 5V should have pretty good regulation)? Although maybe loading each step would throw everything out of whack?

Thanks, Jon

This will work fine, just remember that _all_ of the current - the total of the 5V, 8V, and 12V load currents - will be flowing through your 12V regulator, so size it and its heatsink appropriately.

FYI, you don't dissipate volts. You drop volts, and this dissipates power. Get your terminology right! ;-)

And, about using a power supply "upside down" to get a negative rail, that's quite routine, as long as the supply is isolated. For example:

Cheers! Rich

As long as it is relative it'll work. But for it to work you need to connect the terminal labeld GND above to the centre tap, and not to protective ground (the ground pin on the plug) also be sure to use a double-insulated transformer. (most new ones are)

If the transforrmer's output is not connected to protective ground you can.

usually you can.

Bye. Jasen

That will work and it is common practice. Just make sure you size the heatsinks appropriately. All of the combined current will now flow through the 12V regulator. In theory you'd get better regulation at each step, but these regulators are pretty good anyway, so you probably won't see the difference in practice.

Dave :)

Why is this? Your saying that I cannot earth ground the secondary side? Is this what is ment by floating?

Is there any inherent reason that is bad to "float"(if thats what its called)? the secondary?

Whats a double insulated transformer? I just brought some and I don't know if they are or not... whats the reason for this?

protective ground = earth from the 3 prong wall outlet? The reason to do not to connect it is why?

heh, yeah, but the other 3% is what worries me ;)

Thanks, Jon

You can earth the secondary GND terminal if you want, but usually you leave it floating from mains earth. Most good lab supplies will have a seperate mains earth connection on the front panel so you can choose to connect it to the floating GND output if you want to. It depends on the circuit you are powering, in most cases you will want to use a floating supply output. A floating supply is good for when you are probing around with an earthed measurement device like an oscilloscope. If your circuit is floating relative to mains earth then you can probe anywhere you want and not cause any damage. However, if your power supply output is tied to mains earth then it's possible to short out your power supply or circuit with the earthed ground lead of the oscilloscope = nasty.

Usually not. Noise pickup in your circuit is one of the major reasons.

Yes, it is the mains earth on the wall plug. One of the reasons why is stated above. Earth loops is another.

Don't worry, once you blow a few things up you'll understand! ;-)

BTW, don't take any chances with main voltages, it can kill you.

Dave :)

ok.

yeah... thats the best way to learn too ;)

ofcourse but I can't keep on being scared of it. I guess I'd rather it kill me than be afraid of it... I feel it deserves respect instead of fear ;) (I mean, I used to be so afraid of it I wouldn't ever do anything cause I was afraid it would kill me... I'm now trying to get over that so I can actually do some projects).

Thanks, Jon