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120VDC in to appliances and devices?

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Zero999:

--- Quote from: langwadt on September 07, 2023, 09:43:12 pm ---
--- Quote from: Siwastaja on September 07, 2023, 06:10:49 pm ---I have ran random switch-mode stuff out of DC and usually it just works. BUT:


--- Quote from: Berni on September 07, 2023, 12:28:39 pm ---The diodes won't be run that close to the rating to be a problem.

--- End quote ---

That's quite an assumption. Cheap consumer electronics is often dimensioned close to destruction. Diodes specifically are annoying to design in because they dissipate surprisingly lot of power for quite small packages. Suddenly doubling (+100%, not something like +10%) the average power dissipation in a component is something I would not classify as "definitely no problem". I would keep an eye on it, or just bypass the bridge.

--- End quote ---

but with ac the diodes are carry a higher peak current at the peaks of the period, with DC the diodes carry current all the time but it must be lower

--- End quote ---
I just said that.

Don't forget though that with DC, only two of the diodes in the bridge are conducting continuously, whist with AC, all four are used, with each pair conducting alternately.

I suspect the losses are over all lower with DC, probably not quite half though. If it's a bridge rectifier module, rather than discrete diodes, it's even better because the connections to the unused diodes will still provide a bit of heat sinking.

amyk:

--- Quote from: Siwastaja on September 07, 2023, 06:10:49 pm ---I have ran random switch-mode stuff out of DC and usually it just works. BUT:


--- Quote from: Berni on September 07, 2023, 12:28:39 pm ---The diodes won't be run that close to the rating to be a problem.

--- End quote ---

That's quite an assumption. Cheap consumer electronics is often dimensioned close to destruction. Diodes specifically are annoying to design in because they dissipate surprisingly lot of power for quite small packages. Suddenly doubling (+100%, not something like +10%) the average power dissipation in a component is something I would not classify as "definitely no problem". I would keep an eye on it, or just bypass the bridge.

--- End quote ---
I've seen this in cheaper PC PSUs that are based on a voltage doubler, where in 120V mode two of the diodes in the bridge rectifier are not used, so to save the miniscule amount of extra cost of two higher-rated diodes, they use two bigger and two smaller diodes instead of a single bridge rectifier.

Berni:
The diodes are on the AC side where the continuous current is pretty low due to the high voltage. These diodes also have to survive the inrush current of the capacitors filling up if potentially plugged in at the peak of the sine wave. So they have to be a reasonably robust power diode rather than some tiny fragile signal diode.

Fair point on the capacitor doubler input stages. I live in the 230V land so i don't come across those (apart from perhaps the 110/220 switchable PSUs). Those would definitely not work with HV DC on the input. But id think those are pretty rare these days since most modern switching PSUs seam to be universal input 90 to 250V and those won't have one.

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