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Two separate 28VDC power sources powering 2 different diodes on same bar and ground return

brosh

Sep 21, 2017
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I have a question about a system I have with a 115VAC source, and two 28 VDC power sources (A and B) The 28 VDC source (Source A) provides initial power to a diode on a bus bar.

The 115 VAC source powers a component that outputs 28 VDC to energize a relays to provide power to other components in the system. Through these relays, a second 28 VDC source (Source B) powers a different diode on the same bus bar with the diode that Source A powers.

This is the main potential issue with the system, the grounds for the diodes powered by the relays through the 115 VAC power source run back as a return to Source A. After the system is energized through the 115 VAC source, will turning off Source A pose a problem to the system as the grounds return back into it? Let me know if this anything needs clarification or if I posted in the wrong subforum. Thanks for your help
 

kellys_eye

Jun 25, 2010
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Let me know if this anything needs clarification
.... all of it???

It sounds as if the two 28V supplies are used in parallel as backup (if one fails, the other keeps the supply going - the diodes acting as a simple 'or' gate and preventing one supply feeding back into the other).

Can you provide a schematic of what you're describing or at least indicate what the system(s) is/are?
 

brosh

Sep 21, 2017
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I am unable to provide the schematic due to confidentiality constraints, basically my question is the components that are powered by the 115 VAC source output a 28 VDC that powers a control panel.

Source A is needed to power the initial diode on the bus bar, but another diode on the same bus bar is powered via the 115 VAC powering components that then output 28VDC. The issue is that the ground output for the 28VDC from the components powered via the 115 VAC return back into Source A. Would turning off Source A pose any issues with the grounding for the rest of the system returned to it?
 

dorke

Jun 20, 2015
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What do you mean by "initial diode"?

A schematic is needed if you want help.
You can draw one yourself,doesn't have to be the original one.
 

brosh

Sep 21, 2017
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Excuse the crudeness but basically after diode 2 is powered via the 28VDC outputs from the components initially powered by the 115 VAC source.

The 28 VDC Source A is not needed, would turning off this power source be an issue as the grounds for the rest of the system are fed into the return for it?
 

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dorke

Jun 20, 2015
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Things are not clear yet.
Are points C,D connected?
Can you please show the grounds of "the parts" in more details.
20170921_131337.jpg
 

brosh

Sep 21, 2017
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Yes sorry, C and D are connected

Basically the grounds from the 115 VAC to 28 VDC loop to power the second diode are fed back into the return for Source A 28VDC, is that a problem?
 

kellys_eye

Jun 25, 2010
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Source A is a battery of 28V ???

Seems like this is a simple case of AC supply (with 28V DC output) plus a battery (backup) supply of 28V both feeding one system.

If the AC derived 28V is slightly higher than the battery 28V (by a factor of 0.6V or more) then the AC source 28V will power the system. If the AC fails then the battery supplying 28V takes over - zero transition time therefore no-interrupt. The diodes, as per my first post, are 'steering' diodes and also prevent the 28V AC derived supply from back-charging the battery or the battery trying to drive the 28V from the AC supply.

The drawing is a bit dubious but the fuctionality is there.
 

dorke

Jun 20, 2015
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Not a problem.
Ground wires should be same thickness.
Preferably a "star ground" scheme as shown.
if there are other blocks connected in the system,they should be connected to the star ground.
20170921_131337.jpg
 

brosh

Sep 21, 2017
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Not a problem.
Ground wires should be same thickness.
Preferably a "star ground" scheme as shown.
if there are other blocks connected in the system,they should be connected to the star ground.
View attachment 36291

Thank you, so the crude drawing rendition of the system would suffice w/o the "star ground" scheme

was asked to verify this system and was a little concerned that putting the ground into the return of source A when source A was turned off after diode 2 was powered could cause a slight issue
 
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