Maker Pro
Maker Pro

Protect existing power supply with switch or diodes



Jan 1, 1970
When you go to tie in the battery source of power, you may want to
consider adding a switch to select between either the rectified power
from the AC/DC converter or battery power.
Alternately you can add two rectifying diodes, such as a 1N4001 or
similar part. The rectifying diodes only allow current flow in one
direction. So without needing to throw a switch, power to the speakers
can come from the AC/DC or batteries.

If a switching method similar one of the two mentioned above is not
done, there is risk of damaging the output of the existing AC/DC
rectifier if the rectification is done with 3-terminal voltage
regulator such as the LM338. This device has a low output impedance
(resistance). Current from the battery would flow to the speakers and
into the output of the LM338 and potentially damage the part. Then
you'd have to use the battery source all the time.

Take a look an see if there is a 3-terminal device or just a resistor
and zener diode used for the rectifying. If it's just a resistor and
zener, then you'll just end up wasting a portion of the battery energy
in the zener.

Hopefully you can see better what I am talking about with the diagram
"drawn" below in text.

AC power-->AC/DC rectifier-->Anode-Cathode--\
Battery power--------------->Anode-Cathode--/

Adding a SPDT (single-pole double throw) switch of pair of rectifier
diodes would require cutting the existing connection between the AC/DC
rectifier and the speakers.

Good luck,