well, its been long i have not around. hello everybody (as if somebody remember me :P). and something confusing me, is the question like the subject.
so, i.e. there are two psu, one for 5 and 12, the other for 3.3V. both psu's ground is connected. is it safe doing this, any consequence or what?

thanks
Posted on 2006-07-19 01:30:41 by dion
Yeah, that's fine.
Posted on 2006-07-19 01:31:51 by Homer
if you don't mind, could you give more details, why that would be fine?

should there be somewhat voltage reference conflict?
does it mean it safe too, to connect while they're live?

thanks
Posted on 2006-07-19 03:28:43 by dion
In general, as long as the ground is "grounded", there shouldn't be any voltage conflict/issues.

Specifically speaking for generic DC circuits that have a positive/negative(ground), you are drawing power from one common source, so think of multiple PSUs on the same DC ground, as multiple vacuum hoses pulling X amount of "water" out of a common "swimming pool".

The same applies for AC, in theory... just don't confuse my DC example of positive/negative(ground) with AC hot/neutral/ground.

I wouldn't recommend connecting any electrical while it is "live" (hot). Power-off, connect, turn-on... then watch it fry when you are at a safe distance :P
Posted on 2006-07-19 03:40:38 by SpooK
  Hey Dion  :P I remember you darling. As far as the electrical it sounds good to me but I'm gonna stick my neck out and look real stupid here and ask what is a PSU? I would say that anywhere the different voltage come together through a load you probably what to have some resistance to allow for the voltage drop.
Posted on 2006-07-19 13:01:54 by mrgone

(as if somebody remember me :P)


I remember you dion, welcome back !
Posted on 2006-07-19 13:42:27 by donkey
PSU = Power Supply Unit
Posted on 2006-07-19 18:18:03 by drhowarddrfine
dion,

    You should be OK as long as you keep the circuit branches that each power supply (PS) feeds isolated from each other.  If you decide to connect PS in series to boost the voltage, make sure you isolate the switching PS.  This is usually done with a external isolating transformer if the PS circuitry does not contain one.  And of course it goes without saying, do not connect multiple voltage source in parallel.  Ratch
Posted on 2006-07-19 22:50:38 by Ratch
connect PS in series to boost the voltage
You can do that?!  I've been out of hardware design for 10 years but...are you sure?!
Posted on 2006-07-20 09:20:16 by drhowarddrfine
drhowarddrfine,

    Yes, an isolation transformer provides complete electrical isolation between PS.  Expensive and heavy however.  Ratch
Posted on 2006-07-20 12:26:47 by Ratch
Do NOT try to run switchedmode powersupplies in series.
As for the power and earth connections... in both analog and digital electronics we often refer to 'the rails'... many circuits are designed around the concept of 'power and earth rails'. Draw two horizontal lines, one above the other. Let's call the top one 'the power rail', the the bottom one 'the earth rail'.
We can design a circuit that connects to both rails, obviously, since any circuit requires power and earth... less obviously, we can hook up any number of circuits to the SAME rails in parallel to each other (assuming that they are designed to operate at the same voltage).
We can share the Power rail, we can share the Earth rail.
Each circuit we add increases the amount of current being drawn from the power rail, since it acts as a parallel resistor, and if we learned anything about analog electronics, we learned that parallel resistors REDUCE the total resistance and INCREASE the current.
So long as we don't draw 'too much' current from whatever is feeding our power rail, we are ok.
Did that help at all? :)
Posted on 2006-07-21 06:44:27 by Homer
thanks pals for the welcome back greet :D

Ratch... you mean using isolated transformer, i can do series connection??? like, 12+12=24VDC?

the problem why i came to this, was it so often that one of supply voltage needed by the machine didn't enough.
i.e. the 5V current not enough or the 12V one. so, i add another one that have big ampere spec to feed them.

but yet another confusing related problem is a two linked machine. someone ever played daytona usa (sega) or sega rally championship or the similar simulator machine? that what i mean a linked.

the thing is, older design by other vendor, is not using fiber optic for linking two same independent machine. most of them using somekind of audio cable. so, both ground is tied together. meanwhile, it was ok for a while, it turns out that the psu unit was broken so often, in addition of wierd interference in rgb signal (known by look at the picture at crt).

there is another problem that seems to proof that grounding supply unit together can be a bad results. anyone know vending machine?
its like there is somewhat a claw robot that we control to grab, i.e. a doll or any other gifts inside a glass box?
it using 5, 12, 24 and 48VDC. i know one design that grouding all those voltage together and after only couple day operation, the psu was burned. the 48V itself was used to supply the motor via relays.

i mean... i want to believe that tied ground together is safe in one case, but another case, i can not understand it.
Posted on 2006-07-21 21:14:39 by dion
dion,

Ratch... you mean using isolated transformer, i can do series connection??? like, 12+12=24VDC?


好bsp; 好bsp; 好bsp;Yes, but you might not find that practical.好bsp; Isolation transformers are expensive and heavy.好bsp; They decouple the connection between one circuit and another.好bsp; Also they help block circuit noise coming from the power mains caused by motors and other power supplies.好bsp; Look at the link below, and Google for "isolation transformers" to receive theory and other information.好bsp;

http://www.action-electronics.com/phciso.htm

好bsp; 好bsp; 好bsp;If you want to increase the voltage, connect in series with both power supplies capable of handling the required current.好bsp; To increase current capacity, connect in parallel, with both supplies capable of maintaining the same voltage at their rated current.好bsp; In series, the idea is not to let more current exist than any power supply is capable of passing.好bsp; In parallel, don't let the voltage on any transformer drop below the voltage of the other transformer好bsp;so that a reverse current passes through it.好bsp; Consider using a diode to prevent this.好bsp; Make sure a balance exists insofar as no power supply is exceeding its limits.好bsp; Ratch
Posted on 2006-07-22 00:38:32 by Ratch
Ratch quote:
keep the circuit branches that each power supply (PS) feeds isolated from each other


sorry if i'm slow in thinking the answer :)
do you mean i must use isolation transformer, if i want to connect both psu's ground?
does it mean each psu need one iso-transformer?
can i just use one iso-transformer?

thanks
Posted on 2006-07-23 20:45:21 by dion
dion,

    If you isolate one power supply, that should be sufficient.  You should really look at at the power supply schematic to be really sure what you want to do is OK.  Ratch
Posted on 2006-07-24 00:03:50 by Ratch
Cough - What a bunch of crap.
You can't just run switchedmode powersupplies in series.
They operate on a duty cycle, and the frequency of that duty cycle is controlled by some kind of timing circuit (possibly RC network), and may also be dynamically variable to cope with varying loads.
First, you can't guarantee that the two supplies have their duty cycle synchronized, so you can't guarantee the output voltage - if they are totally out of phase, you get NOTHING.

Switched mode powersupplies are MORE COMPLEX than you imply.
You are describing a simple DC power supply, NOT a switchedmode supply (PC power supplies are typically switchedmode because they are more efficient and generate less heat).



Posted on 2006-07-24 11:32:28 by Homer
Homer,

Cough - What a bunch of crap.
You can't just run switchedmode powersupplies in series.


    Why not, with the proper percautions and isolation?  Read on.

   
They operate on a duty cycle, and the frequency of that duty cycle is controlled by some kind of timing circuit (possibly RC network), and may also be dynamically variable to cope with varying loads.


    That is quite true, but the switching and duty cycle pertains to the part of the circuit that boots the voltage.  The final output after the voltage is rectified, filtered and regulated is DC. 

First, you can't guarantee that the two supplies have their duty cycle synchronized, so you can't guarantee the output voltage - if they are totally out of phase, you get NOTHING.


    Two independent switching power supplies don't work together, they work separately.  It should not matter what the switching frequency or duty cycle or phase of either PS is.  As long as they both put out their rated DC, they can be treated as two DC sources.

Switched mode powersupplies are MORE COMPLEX than you imply.


    Yes, indeed.  They resemble a oscillator/power amplifier whose output is transformed to DC. 

You are describing a simple DC power supply, NOT a switchedmode supply (PC power supplies are typically switchedmode because they are more efficient and generate less heat).


    No, I am talking about a switch-mode power supply.  Their higher frequency compared to line frequency enables them to use smaller components and their variable frequency/duty cycle permits them to dispence with the pass transistor need on linear power supplies.  Ratch

Posted on 2006-07-24 17:17:41 by Ratch
Well, my best guess is..
You can't put switchmode PSUs in parallel because the tiny difference in output voltages will cause huge current surges as the slightly higher voltage supply tries to drag the slightly lower supply up.
But, you can usually fix that with a SMALL series resistor in series with each supply output.

For putting then in series it depends on the setup.
If they are isolated then there is no problem.
(Homer:Duty cycle really doesn't come into it, there is no duty cycle on the DC output, it's DC. Duty cycle is an internal thing which each supply will control to maintain its own output voltage.)

If they are not isolated, and I suspect most of them are not, then connecting the ground of one supply to the Vout of the other will short the other to it's own ground and won't work, but it might cause a few sparks.


But, in reply to the original question, can 2 switchmode supplies share a common ground, the answer is YES, always. And most of them do anyway even if you don't intend them too.

Paul.
Posted on 2006-07-24 20:16:23 by pdixon

Well, my best guess is..
You can't put switchmode PSUs in parallel because the tiny difference in output voltages will cause huge current surges as the slightly higher voltage supply tries to drag the slightly lower supply up.
But, you can usually fix that with a SMALL series resistor in series with each supply output.


Well, that takes care of the internal feedback regulation voltage conflict which would cause instability through-out all the power supplies, but I would still stay away from *any* deviation to the power supply circuit itself.

As for the "series" debate, save yourself the headache and avoid any trouble altogether... unless you *really* want to see what the remains of a charred inductor looks like... because it *will* happen... eventually.

Either get a power supply that handles all your needs (3.3V/5V/12V) *OR* perform some post-PSU power transformation for the needed 3.3V (it is a small step-down from 5V, I doubt the amperage spike would be harmful whatsoever).
Posted on 2006-07-24 21:12:16 by SpooK
pdixon,

You can't put switchmode PSUs in parallel because the tiny difference in output voltages will cause huge current surges as the slightly higher voltage supply tries to drag the slightly lower supply up.
 

    It makes a difference if there is a load for both power supplies to work into.  If there is a slight imbalance in voltage output between the two supplies, then the slightly higher voltage supply usually won't be able to sustain its higher current load and the voltage will drop to whatever the other PS can help to support.

But, you can usually fix that with a SMALL series resistor in series with each supply output.


    Yes, that is a common technique to balance two power supplies in parallel.  And there is also the internal output resistance of the PS.  I believe I suggested a diode earlier, which supplies both resistance and reverse voltage protection.

If they are not isolated, and I suspect most of them are not, then connecting the ground of one supply to the Vout of the other will short the other to it's own ground and won't work, but it might cause a few sparks.


    That is for damn sure.  If the DC common of both supplies are connected to the same point like the computer chassis, and no isolation is present, then you will short out the PS if you connect the DC+ to the DC common of the other PS.  Ratch
Posted on 2006-07-24 21:20:40 by Ratch