# power supply max current

• posted

suppose you have a power supply but don't know what is the maximum current it can deliver. how would you find out what the max current is?

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• posted

"Warren Thai"

** 1. Read the labelling.
1. Ask the makers.

BTW

Kindly only post questions with the relevant context included in future - dickwad .

...... Phil

• posted

Load it little by little until the voltage drops. Then back off about 10% for a safety margin.

• posted

OH SO SORRY!!!! YOUR MAJESTY! so power supplies don't apply to electronics huh? why be so rude for? everyone should ignore this guy

current

• posted

Everyone does ignore him.

• posted

"Friday"

** FUCK OFF - WOG TROLL.

....... Phil

• posted

What sort of power supply? More details would help, otherwise Lord Garth's reply is about the best you'll get. For instance, is it linear or switching?

... Johnny

• posted

Find the output power in watts and divide it by the output voltage.

• posted

Its a ac plug pack, the ones you use to power electronic keyboards or computer speakers etc. The label has been removed a long time ago. I know the voltage by measuring with a multimeter. I'm after an experimental way of determining its maximum output current.

current

• posted

"Warren Thai"

** So NOT a "power supply " at all - just a transformer.

Now, WHY was it Sooooooo hard to DRAG that tiny fact OUT of YOU ????

** The VA ratings of transformers are determined by temperature rise alone.

The primary side resistance is your guide to VA rating and temp rise.

6 VA = about 800 ohms at room temp.

10 VA = about 450 ohms at room temp.

15 VA = about 240 ohms, at room temp. 20 VA = about 140 ohms, at room temp.

Apply a load resistance that draws a VA in line with the above table, then let the unit heat up - check it now and then.

The temp rise is OK if the primary resistance increases by no more than 30 % of its room temp value after an hour or so - jut un-plug the unit and quickly apply your DMM leads to the AC plug to measure ohms.

A 30 % increase corresponds to a rise of 76 degrees C, since the tempco of copper is 0.39 % per C.

....... Phil

• posted

On Fri, 20 Oct 2006 19:15:59 +1000, "Warren Thai" put finger to keyboard and composed:

If it is linear, then it may have a current sense/limit circuit, in which case you would find the value of the appropriate resistor and consult the datasheet for the regulator. Otherwise, if the CL circuit relies on turning on a PN junction, then use 0.6V as the limiting voltage and do the arithmetic. You may need to do a little reverse engineering if other arrangements are used. Alternatively you could just keep loading the supply until it droops and hope nothing breaks.

If the supply is a switchmode type, then its absolute maximum rating would be less than or equal to the rating of the diodes on the secondary side. I suspect you may be able to estimate the max current by measuring the switching frequency, allowing for a certain max ripple, and then calculating the current draw that would produce this amount of ripple in the output cap.

- Franc Zabkar

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