Power supplies combination to produce +15Vdc and -15Vdc ???

[i200yrs]

Hello All...we've got the original power supply with +/- 15Vdc output. But it blown already. I am thinking to combine two 15Vdc power supplies to replace the original as shown in the attached image. Any opinions or advice? thanks in advance.

 

[KlausST]

Hi,

should work.
Assuming they are properly isolated....

Klaus

 

[Akanimo]

I'm not sure this idea is going to work. The grounds of each of the power supply are on the negative. You don't want to be switching the ground.

 

[d123]

Hi,

It looks to me like the positive of power supply B will possibly short to ground through the negative of power supply A or cause another kind of issue I haven't read about. Also, What will come out of "Com": +15V, ~7.5V or 0V? I strongly doubt anything like 0V. Maybe it works but from what I know that looks like a possible blown supply #2 in the making.

 

[KlausST]

Hi,

an isolated power supply is like a battery.

Connect two batteries in series... then you have 3 nodes. Which one you name "GND" is up to you. GND usually is the reference for voltage measurement.

***
Your circuit:
Thus, if you name the center node "GND", then the other nodes are +15V and -15V respectively. No need to guess voltages.
No damage expectable.

Klaus

 

[wwfeldman]

do your power supplies have 2 connections, or 3?
your drawing shows 2 but some have 3: power, return, and ground

if return and ground are not connected, use those as the two connections, as in your drawing
if return and ground are connected with a jumper, remove the jumper and use power and return as above.

if there are only 2 connections, then use a cheater to disconnect the grounds at the wall socket.
that is, a three prong to two prong converter. which removes the ground connection

 

[i200yrs]

do your power supplies have 2 connections, or 3?
your drawing shows 2 but some have 3: power, return, and ground

if return and ground are not connected, use those as the two connections, as in your drawing
if return and ground are connected with a jumper, remove the jumper and use power and return as above.

if there are only 2 connections, then use a cheater to disconnect the grounds at the wall socket.
that is, a three prong to two prong converter. which removes the ground connection

Alright, so bottom line the ground (chassis or earth) should not be connected to common?

- - - Updated - - -

Hi,

an isolated power supply is like a battery.

Connect two batteries in series... then you have 3 nodes. Which one you name "GND" is up to you. GND usually is the reference for voltage measurement.

***
Your circuit:
Thus, if you name the center node "GND", then the other nodes are +15V and -15V respectively. No need to guess voltages.
No damage expectable.

Klaus

Thanks Boss...there should be no concerns if either of the power supplies turn-on first because they are in series right? thanks

 

[KlausST]

Hi,

Thanks Boss...there should be no concerns if either of the power supplies turn-on first because they are in series right? thanks

No.

But in detail it depends on the load circuit.
If inside the load the +15V in is only referenced to GND and the -15V in is only referenced to GND (no circuitry between +15V and -15V) then everything is safe.

Klaus

 

[asdf44]

Yes...this is really common stuff to do with standard isolated lab power supplies (isolated is the key word, but most are).

This is why they have 3 terminals. +, - and GND (chassis/earth). It lets you connect ground to +, - or neither depending on the application.

 

[dick_freebird]

I do this often (as shown, OP). The key is whether "minus"
is isolated from "ground" (chassis and power cord). Any
power supply worth having, is isolated to at least the max
output voltage. Most are far better, even the cheapos
on eBay (but they may not tell you how much).

 

[i200yrs]

thanks to all for sharing your knowledge...i would like to add one last question...what if I connect together different power supply voltage output like 15Vdc and 5Vdc. Should not be a problem right? thanks

 

[i200yrs]

Ok to support my question and become more clearer...in the attach image, that is actually we plan to do. Really appreciate any inputs from you all...thanks in advance.

 

[wwfeldman]

that should work, as long as the black triangle, i.e. power return
and dashed triangle, i.e. earth, are NOT connected

 

[i200yrs]

Wait a second...please check the video below, he is combining 2 power supplies and saying connect to building ground??? Any opinion guys?

https://www.youtube.com/watch?v=SHMwtO4QZcA

 

[wwfeldman]

I take the black triangles in your drawing to be connected to each other and nothing else.

I would not connect to building ground.

Setting the output voltage on an power supply sets the voltage between red and green.
The connections to other things reference the voltages to each other.

The earth ground and the power return should not be tied together either external or internal to the power supply
The outputs need to be floating or isolated from the earth ground.

The function of the earth ground is safety - to tie the metal case and metal chassis to earth in case of a fault.

Watch the automatic follow on videos.

 

[asdf44]

Generally you want your circuit earth grounded from one place.

If it’s already earth grounded then you probably don’t connect the earth at your supply. If it’s not earth grounded then you probably want to connect the earth connection.

Remember that earth can come from many places such as a USB connection or scope probe.