I have a monitor that requires 18v at 3amp DC and I have a power bank that provides 20v at 5amp. Is it safe to power the monitor from this? Does the monitor only draw what is required or is the over voltage and over ampage forced into the monitor?
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Welcome to EE.SE! You’re out of specification but not by much. The 2 A extra current capability does not matter, but the 2 extra volts are forced into your monitor. I would guesstimate a 90 % chance it works fine, 5 % chance of instant smoke and 5 % works but very short lifespan. Expensive item or a throwaway? – winny Nov 01 '20 at 08:55
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Thought experiment again: My car battery can supply > 100 A. If I switch on the sidelights will the 6 W (0.5 A) bulbs only draw what is required or will 100 A be forced into the bulb? What happens with two bulbs? Four bulbs? – Transistor Nov 01 '20 at 08:57
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This cannot be answered, we would only be guessing. We don't know how the monitor is built and what will be the result if you feed over 10% of overvoltage into the monitor. So assume it is not safe, unless the manual says it's OK. – Justme Nov 01 '20 at 08:58
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1You could place a couple silicon diodes in series to drop around 2V, give or take. I doubt it's overly critical. – Unimportant Nov 01 '20 at 08:58
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1Does this answer your question? Choosing power supply, how to get the voltage and current ratings? – JRE Nov 01 '20 at 09:14
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@Unimportant, heat sinking those diodes could get untidy. – Jasen Слава Україні Nov 01 '20 at 19:22
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1@Jasen Plenty of diodes in handy packages such as TO-220. – Unimportant Nov 01 '20 at 19:25
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1true, TO220 is good for 2 watts by itself, and even better with a clip-on, so that could work. – Jasen Слава Україні Nov 01 '20 at 19:48
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With the wrong voltage it's hard to say, 20V is 10% too high. maybe it will work, maybe it will do damage.
having spare amps at the supply is not a problem,
were I in your position I would be looking for a way to convert the 20V into 18V
LM350K perhaps, perhaps not, they're into scarcity pricing on those
LM338T looks better, you'll need with a heatsink big enough for 6W
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