connecting two switching power supplies

[Captain Kilowatt]

I'll explain this way.
DC circuits when running feeders: The total length of all current carrying conductors must be calculated.
Ex: This power supply hook-up
A: Power supply #1... has #4 ga wire 10 feet long to load (thus 20ft true length) @50 amps
Power supply #2 has #4 ga wire 3 feet (6 feet true length) @50 amps.
Thus, you have a Parrell DC circuit resistance.
You now have an imbalance of voltage drop across each load.
PS#1 equates to 5.1% voltage drop (12v in @load 11.4v)
PS#2 equates to 1.5% voltage drop (12v in @load 11.8v)
This is calculated at roughly room temperature (77 degs F) as the wire heats up due to current surges or usage, the resistance increases, thus the imbalance will increase.
This illustrates possible issues, if one was to use different size conductors on each supply.
EX : PS #1 #4 ga ... PS#2 #8 ga
Those voltage drops across each load becomes even more dramatic.
Having delt with these types of AC or DC circuits with Phasing AC Transformers / DC loads etc. for almost 30 years.
I call tell you it must be considered when building a system.
Even values this small, maintaining the equal balance across each load must be considered. IMHO
Consider this, 2 "phased" power supplies:
What happens to the current draw from each supply when you have a difference in voltage drop across the feeders?
One supply will have a HIGHER current draw than the other when suppling the load.
My worthless 2 cents
All the Best
Gary

That is true Gary. When I said small length differences didn't matter I was thinking the way It would do things. A few inches different won't be an issue yet it is not EXACTLY the same length as was suggested It should be. What I am saying and failed to be clear about above was that you do not need a micrometer to measure the cable lengths. A regular tape measure will do.

 

[Handy Andy]

I just have to ask why I have seen two people now say that wire length connecting multiple power supplies would matter? It's DC and I have never seen that in any industrial applications that were high amperage DC. Maybe the shortest possible length to reduce cost and resistance but never for any other reason. Is there a reason? I asked an electrical engineer in the past year and they just laughed at me and said, it's DC and emphasized the "direct" part of the name. I said that's what I thought. Is there something I am missing?

Ok, let's do the "devil's advocate" ....

You buy 1 radio at $300 + 250W Amp for another $250.

But to run both at home on a power supply? You'll need more than 50Amps

Ever price a 50 Amp power supply? Let alone switching?

Linear versus Switching - both will cost you LOTS of $$$

Now you ask about "parallel" two of those power supplies to offset the current draw both will use when running?

You've already spent $550 - how much more do you need to spend before you realize the question becomes the costliest mistake you ever made - when it's ignored?

It's your money and the bill you'll have already paid - on top of more bills to pay for the mistake of running too much amperage draw with not enough power supply to back it up.

By the way, you might want to add in the cost of a Fire Extinguisher...


It's been known to happen...

 

[Allen B]

Ok, let's do the "devil's advocate" ....

You buy 1 radio at $300 + 250W Amp for another $250.

But to run both at home on a power supply? You'll need more than 50Amps

Ever price a 50 Amp power supply? Let alone switching?

Linear versus Switching - both will cost you LOTS of $$$

Now you ask about "parallel" two of those power supplies to offset the current draw both will use when running?

You've already spent $550 - how much more do you need to spend before you realize the question becomes the costliest mistake you ever made - when it's ignored?

It's your money and the bill you'll have already paid - on top of more bills to pay for the mistake of running too much amperage draw with not enough power supply to back it up.

By the way, you might want to add in the cost of a Fire Extinguisher...
View attachment 57371

It's been known to happen...

I hate to break it to you but I probably know a whole lot more about electricity than you think. And your message is not answering my question at all. Just more like a vague threat that I might burn my house down by using a very ridiculous hypothetical situation. My question was why do some people painstakingly make the wires the "exact" same length when paralleling DC supplies? This is NOT done in high amperage industrial applications. You just run the wire from the supply to the bus. This is NOT done in high amperage AC power systems when paralleling transformers or generators. So why when it's a radio power supply? The load balancing example is a good one. But a few inches here and there if the wire is the correct gauge, does not matter.

If you draw too much current from a power supply that's not a cheap POS it is not a fire threat. Even most cheap switchers have overload protection. Have you ever heard of a fuse or a breaker? Over loading a power supply usually results in voltage sag and popped fuses, not a fire. I mean seriously how long do you sit there beating your power supplies to death with undersized wires and no fuses inside your home?

 

[Greg T]

I used 2 Megawatt 36A paralleled together for long periods with no issues .... so I don't know what you're talking about.

Unless you have the proper circuit, I guess you got lucky. One psu is the bull and the other makes up the deficit.

 

[Handy Andy]

Threat?

Hmmm... Well, I see I got thru to you.

Thank you for being attentive.

Then do what you wish, I'll leave it at that...

 

[BJ radionut]

This is NOT done in high amperage AC power systems when paralleling transformers or generators.

 

[Allen B]

Threat?

Hmmm... Well, I see I got thru to you.

Thank you for being attentive.

Then do what you wish, I'll leave it at that...

I'm not taking it as you being rude or anything like that. It's all good.

 

[Allen B]

Unless you have the proper circuit, I guess you got lucky. One psu is the bull and the other makes up the deficit.

Set the voltage as close to each other as possible within 1/100th of a volt, then use resistors or diodes for load leveling. That's what is done inside a linear supply with multiple pass transistors to ensure an even load on the transistors. Put an amp meter on each supply and load it down. If one is pulling more than the other adjust the voltage until it's the same amperage draw on both supplies.

Or..... some of these newer switchers can be hooked up as a master/slave configuration. Then you only have one regulator regulating multiple supplies and it is even better.

 

[Greg T]

Set the voltage as close to each other as possible within 1/100th of a volt, then use resistors or diodes for load leveling. That's what is done inside a linear supply with multiple pass transistors to ensure an even load on the transistors. Put an amp meter on each supply and load it down. If one is pulling more than the other adjust the voltage until it's the same amperage draw on both supplies.

Or..... some of these newer switchers can be hooked up as a master/slave configuration. Then you only have one regulator regulating multiple supplies and it is even better.

Yes, of course. I believe that is what I said. OR, you can gang as many 700-12s as you like because they are built to be ganged with no external equalization needed.