"Andy" wrote in message
...
On Jul 19, 12:37 pm, BCDrums wrote:
I have a Jensen CD 60 ortable CD player that came without a power
supply. It calls for a 4.5V/500mA adapter. I have a 4.5V .8mA
adpapter, and the pin polarity matches. Can I use it with the Jensen?

Thanks.

BC
adapted

Andy comments:

It depends. You can test a dozen different "4.5 volt" wall warts
and have a dozen different open circuit voltages and a dozen
different internal resistances.
A wall wart is designed for a particular product that has a certain
load draw, and, typically, one get a LOT higher voltage than 4.5
when unloaded and approx 4.5 volts when loaded by the intended
device.

It has everything to do with the internal resistance of the wall
wart.

For instance, suppose the intended load is 1 amp and the
wall wart, unloaded, puts out 12 volts with an internal resistance
of 7.5 ohms. If this supplies a device that draws 1amp, there
will be a terminal voltage of 4.5 volts. If it drives a device that
draws only 100ma, the terminal voltage will be 11.25 volts.

Depending on the voltage rating of the device the wall wart
is driving, something like this could break down an internal
capacitor or transistor....

So you see, it just depends........

Here's how to find out, in your case. :

Connect three D cells, which provide 4.5 volts, and hook it to
the device and measure the current drawn. Then use Ohm's
Law to determine the load resistance the device provides.

THEN take a resistor of this value and connect it to the
wall wart in question. If the loaded voltage is around 4.5 volts
with the resistor, that means it will be 4.5 volts with the device
you want to power.....

One caveat , tho.... the device you want to power probably draws
different currents depending on whether a motor is running or
stopped, or the audio is cranked up high or not. That could cause
the terminal voltage to go up and down..... since the load is
not constant.....

So you see, your wall wart MIGHT work OK, or it might blow out some
part in the device powered. That is why wall warts are generally
mated
to the device being powered. It is very unusual for a wall wart to
have
an internal voltage regulator to supply the same voltage independent
of load. If the one you want to use actually does have an internal
voltage regulator, you are all set.....

Andy in Eureka, Texas Licensed EE and
retired design engineer.

Andy is correct on all counts. You will find a slightly higher voltage from
the new wall wart because of a lower current draw than it was designed
for.WW