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]
