WastefulMiser
Senior Member
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Is it advised to put blocking diodes on both the postive and negative in lieu just on just the postive for power supplies (24VDC)?
100926-1830 EST
WastefulMiser:
Please provide a clear description of your circuit and what these diodes are supposed to do.
Parallaling (4) 500W, 48-24VDC, ungrounded power supplies.
You have a 24 VDC supply. Is either + or minus - defined as common and which if either one is connected to the metal enclosure of the equipment, and/or earth (ground)?
Neither the + or - are grounded. I do not know if the manufacture uses "conventional" electron flow: + to -; or if they use actual electron flow: - to +. I hate to assume, but I believe most products use conventional electron flow theory as that's what is taught.
What does the power supply supply?
PLC inputs and outputs.
Is the output of the power supply switched with a switch or relay contacts?
I imagine relay contacts -- the PLC is probably controlling VFD's for some process. The PLC is not in my scope actually nor is the redundant power supplies, I am just curious. Mayhap you could clarify on the difference between switch and relay contacts.
Is the load inductive?
Is there a concern with induction for DC? Nevertheless, I would say no.
Are you trying to prevent or reduce transient noise?
I do not know the actual intent the end-user has for the blocking diodes. The spec isn't in front of me, but it says blocking diodes on the secondary of the power supplies. I guess they are worried about back-feeding the 48VDC circuit. I assume most power supplies have blocking diodes on the secondary internally.
Are you trying to protect the power supply in some way?
Yes. I think that is the intent.
Where are you picking that up at? I still have figured out what the OP has, he has not clarified.I'm reading this as 48VDC in, 24VDC out PSUs, such as you would find in a telecom facility where 48VDC is pervasive.
Where are you picking that up at? I still have figured out what the OP has, he has not clarified.
For floating outputs, although diodes in both +ve and -ve wont do much harm (other than dissipating 15W of extra heat over just one diode), they shouldn't be necessary, as shorts to ground are not common power supply failure conditions.Diodes are needed on both positive and negative for reliable isolation from a faulted power supply.
I'm reading this as 48VDC in, 24VDC out PSUs, such as you would find in a telecom facility where 48VDC is pervasive.
Assuming that the outputs of the power supplies are truly floating, then just a single diode per PSU will do fine.
Please do not assume that the presence of steering diods will make these power supplies loadshare, giving you 2KW (or n+1 1.5KW). The combined safe rating of the four units is still going to be 500W, but you will have a lot of resilience (n+3). The reason they won't loadshare is that the PSUs will all have slightly different output voltages, and thus one steering diode will conduct, and the other three will be reverse biased and thus not conduct.
This is a common application for critical equipment like plant control systems that require reliable power or for UPS logic power supplies.
Two 24V power supples, each rated for 100%+ of the load are fed from separate sources. Diodes on the output of each PS isolate the common DC load bus from the power supply. The power supplies might or might not share the load, but when one fails or has an internal short, the other one can still supply control power.
Diodes are needed on both positive and negative for reliable isolation from a faulted power supply.
As mentioned in a below post, how do you compensate for the 1.4VDC voltage drop across the diodes?
Diodes are needed on both positive and negative for reliable isolation from a faulted power supply.
Making the negative bus common is only an issue if one power supply has an internal ground fault and another ground occurs on the system. Isolation diodes on just the positive outputs will protect against 98% of the problems. We do both + & - because the cost is low compared to the $1M + cost of our control systems. Most systems don't need that level of reliability.
I didn't think adding steering diodes meant load sharing. I was just wondering if selecting a power supply with load sharing communication interconnect between the power supplies was applicable.
This is because of the properties of a diode. A diode will only conduct when it is "forward biassed", which means that the anode is more positive than the cathode, and that voltage differential exceeds (silicon) 0.7v.I don't understand why/how slight different voltages of the power supplies would impact the additive property of the power supplies when paralleled. I would agree there could be an imbalance in loading of the power supplies if load sharing is not used.