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Using DC supply voltage for standard wall wart PS

Using DC supply voltage for standard wall wart PS

(OP)
I was wondering if anyone knows if standard 120 VAC-rated wall wart power supplies can be sourced with 125 VDC instead of 120 VAC? Virtually all of these types of power supplies I see (think power supply for a wireless phone, USB cellular phone charger, etc.) indicate 100-240 VAC as the supply voltage, but I was wondering if anyone knows if these will actually work with an equivalent magnitude of DC voltage instead of AC? The application I'm using does not involve any AC voltage, therefore, all devices are fed from a 125 VDC battery. I know I can test this to see if the power supply would work with a DC supply voltage instead of an AC supply voltage, but I was wondering if, electrically, standard switching wall wart power supplies simply work out of the box this way? I have searched Google high and low and cannot find anyone who has written about this. Of course, this is a very unusual application, so it probably hasn't been done that many times, but figured if anyone know, it would definitely be someone on this forum! :)

Thanks.

RE: Using DC supply voltage for standard wall wart PS

2
There are two answers: No and Yes.

No for the "Heavy" wall warts - because they use a transformer that cannot work with low frequencies or DC.

Yes for most other "Light" wall warts- because the first thing the input voltage sees is a rectifier and a smoothing capacitor. The rectifier makes the polarity right regardless of how you connect the power, "hot" being plus or minus then doesn't matter.

There may be a few other components like snubbers or MOVs.There, the snubbers dont mind to see DC, but the MOV may have a problem with a continuously high DC voltage because the heat dissipation goes up incredibly fast when you get close to the protection level and that doesn't happen continuously when fed with AC but all the time when fed with a very high DC voltage.

The MOV is not a problem as long as you keep the DC voltage at or below the AC equivalent RMS voltage. And 125 V DC is definitely below the 90 - 250 V seen on most wall warts.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Using DC supply voltage for standard wall wart PS

When you go to test, I'd recommend using an extension cord so your hand isn't holding it at first test.

We've done similar 400 Hz testing of power supplies rated for 50-60 Hz. We took many precautions for safety (even if it was known to be low risk).

RE: Using DC supply voltage for standard wall wart PS

(OP)
You guys answered my question. Thanks so much for the input, I very much appreciate it.

RE: Using DC supply voltage for standard wall wart PS

I would add the caution that besides 'heavy' and 'light' if it is a 'powerful' wall wart then it too might have problems because then they don't have a rectifier front-end they have a power-factor correcting switching front-end that could be very unhappy with DC.

I vote for extension cord testing also. :)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Using DC supply voltage for standard wall wart PS

There are plenty of DC/DC converters out there.. What are you trying to do/power? voltage/current ratings?

RE: Using DC supply voltage for standard wall wart PS

I have a solar system with 52VDC from the panels and I find most electronic 100-240V wall warts still work at that reduced voltage at reduced output current. A pretty handy trick. Current gets limited by the chip FET that has about 35 ohms on resistance and inline EMI filter resistors that can be up to 50 ohms. I bought a number of defective inverters and the some with bad H bridges just got the FETs ripped out of them and the 140V DC was just run directly to the socket. TV, DVD players and electronic CFL lamps work just fine on DC when they have electronic conversion.

Don't expect you will have any problem with an electronic wall wart on DC. Of course just when you say that there will be some phase triggered device to prove you wrong.

RE: Using DC supply voltage for standard wall wart PS

One concern if there's a 2-diode full wave rectifier at the wall wart's input.

With AC, the input current will be split between both diodes. If you apply DC, one diode will see all of the input current, and it might not be able the handle all of the rated current itself.

If the diode burns out, it could cost 50 cents to replace it :( or $1.99 at Radio Shack.

-neil-

RE: Using DC supply voltage for standard wall wart PS

You've forgotten, Radio Shack is dead and gone.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Using DC supply voltage for standard wall wart PS

The shack is not completely gone. The ones that survived now sell Sprint phones too. We have one left in town.

Z

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