Selecting power supply with outputs connected

Thread Starter

toweroak

Joined Oct 31, 2018
2
Hi, I need some advice. I have 2 24V switch mode power supplies. I have a selector switch to select the power supply that I want to run, this is in case one power supply breaks, I can select the other one. I have connected the output of the power supplies together; positive to positive and negative to negative (see attachment 1). I think I have read that there might be back flow of current into the unselected power supply, so I want to confirm if this is an issue. Solution to this I believe is to add diodes (see attachment 2). Just want to check with you guys if I am correct.
 

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ScottWang

Joined Aug 23, 2012
7,245
I have connected the output of the power supplies together; positive to positive and negative to negative (see attachment 1). I think I have read that there might be back flow of current into the unselected power supply
That's true, the unselected power supply will be become a load for the selected power supply.

Solution to this I believe is to add diodes (see attachment 2). Just want to check with you guys if I am correct.
That's correct.
 

Danko

Joined Nov 22, 2017
1,372
Solution to this I believe is to add diodes (see attachment 2). Just want to check with you guys if I am correct.
You do not need diodes D3 and D4 . Connect minuses together, and leave diodes D1 and D2.
EDIT:
And YES, one or both PS may be fried without diodes, because practically two PS can not have ideally equal voltages and current always will flow between them.
 
Last edited:

ScottWang

Joined Aug 23, 2012
7,245
I see thanks. Just wondering, what would happen if there was no diode? The unselected power supply would burn?
Maybe not, it depends on the different design method, some of outputs has internal output diode then it will be ok, if not then maybe it has a safety concern.
 

ScottWang

Joined Aug 23, 2012
7,245
You do not need diodes D3 and D4 . Connect minuses together, and leave diodes D1 and D2.
EDIT:
And YES, one or both PS may be fried without diodes, because practically two PS can not have ideally equal voltages and current always will flow between them.
When two power supplies all fine then just use two diodes is ok, but if considering when any one of the power is damage or something wrong then four diodes isolation is better than two diodes when they are 24V not 5V.
 

Danko

Joined Nov 22, 2017
1,372
When two power supplies all fine then just use two diodes is ok, but if considering when any one of the power is damage or something wrong then four diodes isolation is better than two diodes when they are 24V not 5V.
If unselected PS is damaged, then maximum what can happens - common minuses will connected with N wire of line - it is safe.
In addition, diodes D3, D4 do not protect from negative half cycle of line voltage (if exist).
 

ScottWang

Joined Aug 23, 2012
7,245
If unselected PS is damaged, then maximum what can happens - common minuses will connected with N wire of line - it is safe.
In addition, diodes D3, D4 do not protect from negative half cycle of line voltage (if exist).
Are you confusing with the AC and DC(24Vdc)?
 

ScottWang

Joined Aug 23, 2012
7,245
In addition, diodes D3, D4 do not protect from negative half cycle of line voltage (if exist).
Positive halves of high voltage, Negative halves of high voltage.
The TS mentioned this -- I have 2 24V switch mode power supplies.
So it should be two 24 Vdc power supplies, what I felt quite strange was that you described were using with AC to DC, do you have any links to showing that describe with just DC voltage?
 

ScottWang

Joined Aug 23, 2012
7,245
Yes, I do.
Look at TS's image below.
You can see letters "L" and "N" (Line and Neutral) on every power supply.
"L" and "N" belong to AC line.
View attachment 162918
What the TS shown the block were not only the bridge rectifier diodes and your description was used on the DC Vout side (since you discussed the D3, D4), so no one will use negative half cycle to describe it.
 

Danko

Joined Nov 22, 2017
1,372
I have a selector switch to select the power supply that I want to run, this is in case one power supply breaks, I can select the other one.
@toweroak, if selected PS will breaks by shorting between AC and DC sides inside PS (for example, damaged switch mode power transformer), then on DC terminals will appears AC line voltage.
For safety you can connect minus 24V DC terminals together (without diodes), and ground them.
 
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