On Jul 19, 7:56*pm, wrote:
On Mon, 19 Jul 2010 13:52:44 -0700 (PDT), Andy
wrote:





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.

*The interesting thing is, most of these units that use a 4.5 volt
power supply run on 2 AA batteries - which is only 3 volts. There is a
voltage regulator built in to reduce the "unregulated" 4.5 to the
required 3 volts.

A note...the Panasonic portable player the Jensen replaces used three
AA batts, for a total of 4.5V. But yes, the Jensen says two AAs.

BC
low voltage