Computer SMPS 12v and 5V rails

[boiler95]

Hi Folks,

I'm using a computer SMPS for my project which requires a 12V and a 5V. SMPS have both a 12 v line and a 5 Volt line which i'm planning to use. The question is , Can the design share a common ground between 12V and 5V ? is it safe?

Please let me know.

Regards

 

[Tahmid]

Yes it's safe. There's no problem with sharing the ground for 12V and 5V. 12V isn't really a high dangerous voltage. So no problem.

 

[boiler95]

But one concern is, The '0' on the 12v rail and '0' on the 5v rail, are they same ? i mean, are they at same potential ? any idea ?

 

[nitishn5]

The two supplies are most likely isolated. So you can short their GND pins and get a 12V and 5V supply referred from the same GND. Other than any Ground Noise issue, I don't see any problem.

 

[Tahmid]

I don't know which PSU you have, but all the ones I've come across and/or used have the grounds ('0' for both 5V and 12V) internally shorted.

A simple way to check is to place the voltmeter -ve lead on the 5V ground and the +ve lead on the 12V +VE. Check the voltage. If you get a 12V reading, they must be sharing the same ground. You can try the other way round with the voltmeter -ve lead on the 12V ground and the +ve load on the 5V +VE.

Hope this helps.
Tahmid.

 

[tpetar]

Hi Folks,

I'm using a computer SMPS for my project which requires a 12V and a 5V. SMPS have both a 12 v line and a 5 Volt line which i'm planning to use. The question is , Can the design share a common ground between 12V and 5V ? is it safe?

Please let me know.

Regards

Gnd of 5V and 12V lines are connected together inside PSU. On connectors you have separate wires for 5V and 12V Gnds to avoid distributing current over one wire.

 

[boiler95]

Thanks guys. I'll check as suggested.

 

[kripacharya]

yes the gnd wire is shorted inside the smps.

in fact you can derive all kinds of voltages from smps by connecting different points.

most PC smps have +3.3v, +5v, +12v, -12v, 0v (gnd)

in addition you can get :
1.7v ( 5 -- 3.3 )
7.0v ( 12 -- 5 )
8.7v ( 12 -- 3.3 )
15.3v ( 3.3 -- -12)
17v ( 5 -- -12)
24v ( 12 -- -12)

Of course you have to be careful about current draw for each voltage.

 

[tpetar]

yes the gnd wire is shorted inside the smps.

in fact you can derive all kinds of voltages from smps by connecting different points.

most PC smps have +3.3v, +5v, +12v, -12v, 0v (gnd)

in addition you can get :
1.7v ( 5 -- 3.3 )
7.0v ( 12 -- 5 )
8.7v ( 12 -- 3.3 )
15.3v ( 3.3 -- -12)
17v ( 5 -- -12)
24v ( 12 -- -12)

Of course you have to be careful about current draw for each voltage.

I dont suggest combining voltage rails on suggested way, this can cause psu damage.

 

[kripacharya]

I dont suggest combining voltage rails on suggested way, this can cause psu damage.

why not ? please give a few reasons. i did not suggest shorting the voltages, or is that what you have understood by the word combined ?
and i did write that you have to be careful about the max current which gets limited by the lowest capacity rail. maybe lower.

of course you must also connect sufficient load between the rail and GND point. but this the experimenter must have done otherwise the PS would have malfunctioned already.

So here's an improved suggestion -

you can get those additional voltages provided you have connected a load on each rail internally to the PS. For e.g. connect a 5ohm/ 10w resistor between +5 & GND, 10ohm/20w between +12 and gnd, 5ohm/5w between 3.3 & gnd, etc etc.

 

[tpetar]

why not ? please give a few reasons.

On Internet you can find lots of people with damaged PSU, I saw several with my eyes, one my friend try to do this, and all works fine for some time with his big smile of success, tomorrow he told me that PSU is dead.

Generaly its not smart to combine power sources with different voltage potential, lower will always be as load, and potential will allways go to higher but depends from many things, also current draining on each rail will not be the same, we know that only 3,3V 5V and 12V have higher amperage, rest rails are weak, what will happen with current when you combine -12V with +12V where -12V rail have max 1A and +12V have 50A (for example).

i did not suggest shorting the voltages, or is that what you have understood by the word combined ?

I understand that you told that.

Can you then describe me what you mean how to get 7V ? What 12--5 means ?

in addition you can get :
1.7v ( 5 -- 3.3 )
7.0v ( 12 -- 5 )
8.7v ( 12 -- 3.3 )
15.3v ( 3.3 -- -12)
17v ( 5 -- -12)
24v ( 12 -- -12)

 

[kripacharya]

Can you then describe me what you mean how to get 7V ? What 12--5 means ?

it mean connect your load across +12v rail and +5v rail to get 7v.
same as writing 5v ( 5 -- GND ) means connect load across +5v rail and GND rail to get 5v

let me repeat one point -> you must also connect sufficient load between the rails and GND point. like 5ohm, 10ohm.
let me repeat another point -> be careful about the max current which gets limited by the lowest capacity rail. maybe lower.

this is a trick way to get other voltages, so the person doing it have to know a little how things work. surely no one shorts +5 and +12 ! of course ups will trip, or get damaged. Same reason no one shorts +5 and GnD. maybe your internet friends short +5 and Gnd to get 2.5volts ? or even direct connect +5 and +5 of different psu to get higher current ? everyone knows this is an incorrect way to do it. yes?