# Power Supply Voltages

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 2, 2006.

1. ### Abstract DissonanceGuest

I'm wondering why I can't get a multitude of voltage's out of just a single
transformer?

I want to be able to have several DC power supply levels out of my power
supply for miscellaneous uses... such as +Variable, -Variable, +5, -5,
+12, -12, etc.. (where each one is a distinct power output of the power
supply.

Why can't I just take different combinations off my center tapped
transformer's filter caps as

-----(+)------ A
|
---
---
|
GND B
|
---
---
|
-----(-)----- C

So I should be able to get A-B, A - C, C-B and C - A for use in my different
regulators(+ and - variables and fixed)?

i.e., lets say I have it as such

-----(+)------ 12V
|
---
---
|
GND 0
|
---
---
|
-----(-)----- -12V

then if I want 24 volts I just treat the (-) as ground into my + voltage
regulator, if I want -24 I treat the +24V into my + voltage regulator?

I mean, since potential different is relative then it shouldn't matter?

One more thing: lets say I have a power supply that has a gnd and +V volt
hook up... why can't I just treat the GND as -V and the +V and GND to get
a -V and GND hook up? (I have a feeling this has to do with the regulator
not being able to "work backwards" but I'm still not convinced why it won't
work?) After all, its just like you turned the external circuit "around"
(i.e., connected it backwards but everything should work "normal"?

Maybe to be more clear just incase I'm not understanding something, lets
suppose I have a circuit that requires -12V input w.r.t. its Ground(say its
a pic or something)... but I only have a power supply that supplies 12V...
why can't I use that same power supply to directly power the circuit by just
swaping the connections?

Thanks,
Jon

2. ### David L. JonesGuest

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

One transformer tap can power as many regulated outputs as you like
provided:
1) The total current out of all of them does not exceed the
transformer/rectifier maximum
2) You are happy with a single ground connection

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

3. ### Abstract DissonanceGuest

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

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

4. ### Abstract DissonanceGuest

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
|
|
24V in --- 12 V regulator -----| ---- 8Vout
| |
| |
-------- 8 V regulator -----
|
|------ 5V
regulator----....

???

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
whack?

Thanks,
Jon

5. ### Rich GriseGuest

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:
http://www.national.com/ds/LM/LM340.pdf
page 14, "Positive and negative regulator".

Cheers!
Rich

6. ### Jasen BettsGuest

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

7. ### David L. JonesGuest

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

8. ### Abstract DissonanceGuest

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

9. ### David L. JonesGuest

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

10. ### Abstract DissonanceGuest

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