# power supply max current

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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?

Re: power supply max current

"Warren Thai"

BTW

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

......   Phil

Re: power supply max current

so power supplies don't apply to electronics huh?
why be so rude for?
everyone should ignore this guy

current

Re: power supply max current

Everyone does ignore him.

Re: power supply max current

"Friday"

** FUCK  OFF   -  WOG   TROLL.

.......  Phil

Re: power supply max current

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

Re: power supply max current

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

Re: power supply max current

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

Re: power supply max current

"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

Re: power supply max current

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

Re: power supply max current

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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