Re: How do I measure the output of a
Hello there,
In addition to the other post that provided some good
information i'd like to add a little too...
It's very hard to determine the output voltage
of a wall wart if you have no written info on it
(label, spec sheet, etc.), but that doesnt mean you
cant use it.
They usually come in two types:
either regulated or unregulated.
In general, when you test one you need a variable load
so you can decrease the resistance while measuring
output voltage (and possibly current with a 2nd meter).
First you determine if it's regulated or not:
hook up the voltmeter to measure voltage, and
carefully connect a load of 400 ohms.
Log the voltage measurement down on paper, then
decrease the load to 200 ohms. Note the new
voltage measurement and log that down too.
If the second measurement was very close to the
first, chances are it's a regulated wall wart.
If not, you can then do the unregulated tests:
<font class="small">Code:</font><hr /><pre>
ohms ..Vout ..Iout
XXX.X 14.00 0.000
400.0 13.83 0.035
200.0 13.66 0.068
100.0 13.33 0.133
050.0 12.73 0.255
025.0 11.67 0.467
012.5 10.00 0.800
</pre><hr />
The above is a table that could have been constructed
by measuring a wall wart's output with various load
resistors connected.
From the table,
at 10 volts, it puts out 0.8 amps, so this means
if we had something that ran at 10 volts and only
drew 0.8 amps this wall wart might work for that
device.
If the device we wanted to run needed 12 volts at
0.8 amps this wall wart wouldnt work, because
at 11.67 volts it was only putting out 0.467 amps.
Now lets say that we only needed 10 volts at 0.8 amps,
so we would like to use this wall wart.
Next, we would need to determine if the open circuit
voltage is too high for the device.
To do this, we look at the first measurement using
no load (in the above table, this looks like XXX.X).
Since that measurement was 14 volts, we would need to
decide if our device could withstand 14 volts for
a short time period.
The next thing to find out is if the internal capacitance
is enough to run our device. The easiest way to do this
is to connect it up and see if it runs properly.
If it's an audio device we want to run, it may exibit
quite a bit of humming out of the speaker if the
wall wart isnt regulated, or has very little output
capacitance.
Last note:
The table above was generated using a computer, setting
the wall warts internal resistance to 5 ohms and with
a constant open circuit voltage of 14 volts.
In an actual unregulated wall wart, the peak voltage is
related to the rms voltage by a factor of 1.41, so
the actual wall warts voltage would have went down
much more then the table indicated, however this
is still a good enough example to use as a guide.
You have to measure each and every wall wart separately
of course.
Good luck with your LED circuits,
Al
