Voltage Across Common Point When 5V, 10V, 15V Sources Connected in Parallel

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#1 21666391 04 Feb 2013 17:00

numan Hasan numan Hasan

Anonymous

Post #1
21666391 04 Feb 2013 17:00

if three different voltage sources of 5V, 10V and 15V are connected in parallel, then what will be the voltage across the common point of these sources???
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#2 21666392 04 Feb 2013 17:30

stephen Van Buskirk stephen Van Buskirk

Anonymous

Post #2
21666392 04 Feb 2013 17:30

power supplies are designed to output their set voltage at their current limit or whatever current they are capable of supplying . Connecting supplies of different voltages is very much like shorting them to ground. How they respond depends on their configuration, They may have overcurrent protection, in which case they may shutdown. If they are fused it is likely the fuses will blow. They may short out and produce odd noises and smoke, or if you're lucky, flames. Supplies are generally protected from a short much better than they are protected from overrvlotage, so it may be that the lower voltage supplies will be destroyed.
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#3 21666393 04 Feb 2013 17:49

stephen Van Buskirk stephen Van Buskirk

Anonymous

Post #3
21666393 04 Feb 2013 17:49

It's possible that (considering only two supplies for simplicity) The 5v supply would just shut off as the voltage was at or above 5v. Then the 15v supply would put out 15v and everyone would be happy. However the 5v supply output parts may not be rated for much more than 5v, and probably not for three times the voltage. The capacitors would swell and bubble or possibly explode. I've seem tantalum capacitors that made a noise like a firecracker and left little black strings in the air. The output over voltage protection (designed to filter short spikes) would be seen as a short to the 15v supply. The spike protection would probably fail, possibly spectacularly, but if the 15v supply has a low current limit it might just pull the 15v supply down. Who knows?
#4 21666394 04 Feb 2013 18:26

Steve Lawson Steve Lawson

Anonymous

Post #4
21666394 04 Feb 2013 18:26

There's no way to tell, from the information you have given. It will be a struggle between supplies that will involve their internal impedances, any current limiting and/or over-voltage protection and other such factors.

But, what the heck, try it and see that happens. But stand back and have a fire extinguisher ready, and if you burn the place down, well, you've been warned ;)

Also, be sure to have a video camera running, that could be YouTube gold!!
#5 21666395 04 Feb 2013 20:35

Frank Bushnell Frank Bushnell

Anonymous

Post #5
21666395 04 Feb 2013 20:35

And if you want any eyes left afterwards, safety goggles would also be appropriate.

And don't try this at home. Use someone else's home.
Then you'll have a place to sleep tonight.
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#6 21666396 05 Feb 2013 00:25

JAWED MATEEN JAWED MATEEN

Anonymous

Post #6
21666396 05 Feb 2013 00:25

If voltage source are lab Power supplies than out put will be 15 volt .
How ??
generally DC for power supplly is built by diode or rectifiier bridge
so higer voltage can not flow into the low voltage source as diodes
will be reverse biased. see the attached diagram to understand.
#7 21666397 05 Feb 2013 00:55

stephen Van Buskirk stephen Van Buskirk

Anonymous

Post #7
21666397 05 Feb 2013 00:55

Most power supplies do not have diodes in the output. Bridge rectifiers are used to rectify AC on the input. The diodes have a drop (0.6 to 1v commonly) which affects the output voltage. The drop generates heat which requires a big diode and heat dissipation stuff. If the diode drops 0.6v at 10A that is 6w. Just try and buy a simple 6W diode.
#8 21666398 05 Feb 2013 01:29

Earl Albin Earl Albin

Anonymous

Post #8
21666398 05 Feb 2013 01:29

This is the same person who keeps on getting into these discussion disrespecting us all. it's perfectly obvious.
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#9 21666399 05 Feb 2013 02:12

stephen Van Buskirk stephen Van Buskirk

Anonymous

Post #9
21666399 05 Feb 2013 02:12

Say what? How did I disrespect anyone, let alone EVERYONE? Did I say anything that wasn't true?. I was just trying to explain in a simple way why the statement was incorrect. Is this forum supposed to let incorrect statements pass for fear of hurting someones feelings? I thought the point was answering questions. How do you see it? Is this a new age forum where there are no wrong answers?

Ididn't say "Are you joking?" which may be an honest response but is not much of an explanation. I don't understand how your answer is respectful and not personal, though I imagine you can enlighten me.
#10 21666400 05 Feb 2013 02:49

Steve Lawson Steve Lawson

Anonymous

Post #10
21666400 05 Feb 2013 02:49

You're making a lot of assumptions about these power supplies. What about regulation? What about switch mode supplies? Hell, he didn't even use the term "power supply". These are "voltage sources". Could be batteries. Could be solar panels -- hell, they could be very talented Electric Eels! Or a mixture, thereof.
#11 21666401 05 Feb 2013 02:55

Earl Albin Earl Albin

Anonymous

Post #11
21666401 05 Feb 2013 02:55

Technically this is correct way to achieve this and is a very usefull circuit in UPS, and is actually found in most all laptop PCs.

It on the other hand it's like the old how do you tell the difference between a Norton black box and the Thevenin equal, open circuit? In theory you can't distinguish this two port network unless you are very crafty. Thake both of them, toss them in the snow, the one that melts the snow is the Norton. Norton dissipates power, Thevenin doesn't.
#12 21666402 13 Feb 2013 00:14

john Poole john Poole

Anonymous

Post #12
21666402 13 Feb 2013 00:14

just read the load voltage after the fire goes out... when i was 8 (in the 50;s) i connected a d cell to a 6 volt car battery to see if it would charge.. i still have the scar...
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Topic summary

✨ Connecting multiple voltage sources of different voltages (5V, 10V, 15V) in parallel is generally unsafe and not recommended. The resulting voltage at the common point depends on the internal impedances, current limits, and protection features of each source. Lower voltage sources may be damaged due to reverse voltage stress, capacitor failure, or overcurrent conditions. Power supplies typically regulate their output voltage and current, but when paralleled with higher voltage sources, the lower voltage supplies can be forced into shutdown, fuse blowing, or catastrophic failure including smoke or fire. Most DC power supplies do not have output diodes to prevent backflow; diodes are usually on the AC input side for rectification. The voltage at the common node will likely be dominated by the highest voltage source if it can supply sufficient current, but this can cause damage to other sources. Safety precautions such as using current limiting, isolation diodes, or dedicated load sharing circuits are necessary. Experimental attempts without proper safeguards can result in hazardous conditions including fire and equipment destruction.
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FAQ

TL;DR: Paralleling 5V, 10V, and 15V sources is unsafe; diode drops are 0.6–1 V, and “Most power supplies do not have diodes in the output.” [Elektroda, stephen Van Buskirk, post #21666397]

Why it matters: This FAQ helps students, hobbyists, and engineers avoid blown supplies, fires, and injuries when mixing DC sources.

Typical silicon diode drop: 0.6–1.0 V; at 10 A that’s ~6–10 W heat per diode. [Elektroda, stephen Van Buskirk, post #21666397]
Outcome depends on internal impedance, current limit, and protection circuits; results vary widely. [Elektroda, Steve Lawson, post #21666394]
Common failure modes: overcurrent shutdown, blown fuses, “odd noises and smoke,” sometimes flames. [Elektroda, stephen Van Buskirk, post #21666392]
“Voltage source” could be battery, PSU, solar—type and regulation matter for behavior. [Elektroda, Steve Lawson, post #21666400]
Wear eye protection and keep a fire extinguisher handy if experimenting. [Elektroda, Frank Bushnell, post #21666395]

Quick Facts

Typical silicon diode drop: 0.6–1.0 V; at 10 A that’s ~6–10 W heat per diode. [Elektroda, stephen Van Buskirk, post #21666397]
Outcome depends on internal impedance, current limit, and protection circuits; results vary widely. [Elektroda, Steve Lawson, post #21666394]
Common failure modes: overcurrent shutdown, blown fuses, “odd noises and smoke,” sometimes flames. [Elektroda, stephen Van Buskirk, post #21666392]
“Voltage source” could be battery, PSU, solar—type and regulation matter for behavior. [Elektroda, Steve Lawson, post #21666400]
Wear eye protection and keep a fire extinguisher handy if experimenting. [Elektroda, Frank Bushnell, post #21666395]

What happens if I connect 5V, 10V, and 15V sources in parallel?

They fight. The result depends on each source’s internal resistance, current limit, and protections. Expect unpredictable currents and stress. [Elektroda, Steve Lawson, post #21666394]

What will the common-point voltage be?

It is undefined without source details. A higher-voltage source can dominate; lower-voltage units may shut down or be damaged. [Elektroda, stephen Van Buskirk, post #21666393]

Is it safe to parallel different DC supplies?

No. Most supplies lack output isolation diodes, so backfeeding can overvoltage and destroy components. Avoid direct paralleling of unequal rails. [Elektroda, stephen Van Buskirk, post #21666397]

Do lab PSUs have diodes that block reverse current on the output?

Not typically. Bridge rectifiers sit on the AC input, not the DC output. Output diodes add 0.6–1 V drop and heat. [Elektroda, stephen Van Buskirk, post #21666397]

What failures should I expect if I try this anyway?

You may trigger overcurrent shutdown, blow fuses, or hear “odd noises and smoke.” Flames are possible. Wear PPE. [Elektroda, stephen Van Buskirk, post #21666392]

How can I test safely if I’m curious?

Don’t. If you must, keep distance and a fire extinguisher ready, and record from safety. Prioritize personal protection. [Elektroda, Steve Lawson, post #21666394]

Can the highest-voltage supply force the bus to its value?

Yes. A 15 V source can pull the node to 15 V if lower rails shut down, risking over-voltage on 5 V parts. [Elektroda, stephen Van Buskirk, post #21666393]

Will surge clamps or filters save the 5V rail?

No. Over-voltage filters may appear as a short to 15 V, failing catastrophically and dragging rails down. [Elektroda, stephen Van Buskirk, post #21666393]

What about batteries in parallel at different voltages?

Dangerous. Large equalization currents can cause burns or explosions. One user reports a lasting injury from a mismatch. [Elektroda, john Poole, post #21666402]

Do switch‑mode and linear supplies behave differently here?

Behavior varies by regulation design and protections. Without specifics, you cannot predict a safe outcome. [Elektroda, Steve Lawson, post #21666400]

What is a Thevenin vs. Norton equivalent?

They are equivalent circuit models: Thevenin uses a voltage source; Norton uses a current source. Both model power behavior. [Elektroda, Earl Albin, post #21666401]

What should I do instead if I need multiple voltages on one system?

Use dedicated regulators or DC‑DC converters per rail and avoid cross-connecting outputs directly. Account for diode drops if OR‑ing. [Elektroda, stephen Van Buskirk, post #21666397]

How do I OR two supplies safely with diodes?

Choose diodes rated for your max current and account for 0.6–1.0 V drop.
Heatsink for ~6–10 W at 10 A per diode.
Verify load voltage still meets spec after the drop. [Elektroda, stephen Van Buskirk, post #21666397]

Why do people warn about smoke and flames specifically?

Directly paralleling unequal sources can exceed components’ ratings, causing thermal runaway and combustion. “Smoke” is an early warning. [Elektroda, stephen Van Buskirk, post #21666392]

How do I fix a mixed‑voltage parallel hookup I already made?

Disconnect immediately. Inspect for swollen capacitors, damaged clamps, and tripped protection. Replace stressed parts before reuse. [Elektroda, stephen Van Buskirk, post #21666393]

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