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Topic: Power supply outputs 0mA ?? (Read 621 times) previous topic - next topic

guix

Hello :)

I have few power supplies ("wall wart") and tested them out of curiousity.

I was surprised by one of them, which should output 9VDC and 350 mA as written on the sticker, but in reality it outputs 16VDC and 600mA, nearly the double :smiley-eek:... Before I measured it, I used it to power the Arduino, hopefully only for a few minutes, and I will never use it again for this purpose...


On the sticker of another one, it's written 7VDC and 300mA. in reality it outputs 7VDC and.. 0 mA. How is that possible? Is it broken?


Anyway, lesson learned, always measure a power supply before using it...

outofoptions

#1
Apr 25, 2013, 04:12 am Last Edit: Apr 25, 2013, 04:14 am by outofoptions Reason: 1
It is up to the device to 'draw' current.   The obvious question is how did you measure current?  If you simply plugged your meter in without a load you were shorting the supplies.

fungus


Anyway, lesson learned, always measure a power supply before using it...


Lesson learned badly.

To measure a power supply, put a load on it (a known resistor) and measure the voltage across the resistor. Now use Ohm's law...
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

MarkT


It is up to the device to 'draw' current.   The obvious question is how did you measure current?  If you simply plugged your meter in without a load you were shorting the supplies.


Which you should not do, it should be said, unless you know the supply has automatic current limiting - you may have
blown a fuse or other protective component if you short a supply out - in other words it might have been working till
you "tested" it.
[ I won't respond to messages, use the forum please ]

semicolo

The 9V one that outputs 16VDC is probably not regulated, just a transformer with a diode bridge and maybe a capacitor, the voltage will drop, the more when more current is drawn.

majenko



It is up to the device to 'draw' current.   The obvious question is how did you measure current?  If you simply plugged your meter in without a load you were shorting the supplies.


Which you should not do, it should be said, unless you know the supply has automatic current limiting - you may have
blown a fuse or other protective component if you short a supply out - in other words it might have been working till
you "tested" it.

The same could be said for your meter... it may not be working now - the internal fuse may have blown...
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guix

Ok, thanks guys :(

The multimeter is still working hopefully :)

DVDdoug

#7
Apr 26, 2013, 12:43 am Last Edit: Apr 26, 2013, 12:49 am by DVDdoug Reason: 1
Quote
I was surprised by one of them, which should output 9VDC and 350 mA as written on the sticker, but in reality it outputs 16VDC and 600mA, nearly the double ... Before I measured it, I used it to power the Arduino, hopefully only for a few minutes, and I will never use it again for this purpose...
A power supply does NOT push-out current, and a power supply of the proper voltage and a very-high current rating will not harm your Arduino!    The current rating on a power supply is the maximum that you should take from it, and the actual current depends on what you've connected.

You can usually get more current out of a power supply than it's maximum rating... But, it might burn up.   

Per Ohm's Law, current flow depends on voltage and resistance (or impedance).   Resistance means "resistance to current flow".

In the USA household power is 120 VAC (RMS), and most power outlets are rated at 15 Amps.  With nothing connected the 120 Volts is there, but there is zero current.    A 100W light bulb "draws" about 1 Amp.  (100W/120V = 0.83 Amps).    A toaster or hair dryer might require the full 15 Amps.  If you run a toaster and a hair dryer at the same time from the same circuit, you'll "pull" more than 15 Amps, 'till the circuit breaker blows.

With nothing connected, there is infinite resistance and zero current.  With a short circuit (zero ohms) you theoretically get infinite current (as long as there is some voltage).   In the real world there is no such thing as zero-resistance.  The power supply has an internal resistance and if you short the output, the voltage will drop to zero and you may burn-up the power supply.    (Ohm's law is always true, so the voltage must drop if the power supply can't supply the "calculated" current .) 

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