Power Supply Question - RF Cafe Forums
Post subject: Power Supply Question
Sun Jul 17, 2005 4:22 pm
Fri Apr 22, 2005 12:22 am
I am trying to build a
power supply for a talking clock my father made in the 80's (unfortunately
he passed so I can't ask him). On the clock itself is a rectifier
followed by two 7805CT 5 volt regulators then large capacitors.
I just need to know what rating transformer both Volts and Amps
I should use to power the clock. 12 volts 9/10 amp works but excessively
heats the regulators; and 6.3 volts 2 amps works but only for a
few hours before the clock rapidly turns on and off and has excessive
static from the speaker. I hope someone can tell from just the rectifier
and regulators what transformer would be best. And should I put
a fuse on the 115 volt side or the lower voltage side of the transformer,
and what rating?
Thanks for any help. If you need more information,
please ask and I will try my best.
Unread postPosted: Sun Jul 17, 2005
Mon Jun 27, 2005 2:02 pm
You should add a high value capacitor between
the rectifier and the voltage regulators. The value of capacitor
should be of few hundreds uF (For example 220uF, 470uF, you can
connect in parallel couple of these values) and they should have
a working voltage higher than (At least 1.5 times) the voltage obtained
at the output of the rectifier (Vmax=1.414*output voltage from rectifier).
The capacitor keeps the voltage to the regulators' input with low
ripple, and doesn't allow it to swing from 0 to Vmax.
that you use full-wave rectifier. The rectifier itself should be
able to stand the total current that your clock consumes.
If your regulators are 7805CT, then the voltage at their input
should be at least 3V higher than the output voltage i.e. 8V. A
9V transformer is the choice (of standard voltages available). The
current at the secondary of the transformer should be higher (1.5
times at least) than the current your clock consumes. You should
check the current consumption of your clock, you mentioned that
the regulators are heating. Maybe you can add another regulator
to ease, or attach a heat sink to each regulator (If they are TO-220
package, then they have a tab with a hole exactly for that). If
not, then attach them to a metal plate and glue them with a
paste to the plate.
Unread postPosted: Mon Jul 18, 2005 2:01 pm
Joined: Fri Apr 22, 2005 12:22 am
am not sure how to check the current consumption of the clock. Would
I just try with my current power supply and use an ammeter on the
secondary side of the transformer?
It is a fullwave rectifier.
The volt. regs. only heat up if I use about 12 volts, when I
tried 6.3 they would stay really cool. And both of them already
have heat sinks. I may try a 9 volt transformer and see how that
Thanks for the help :)
Unread postPosted: Mon Jul 18, 2005
Mon Jun 27, 2005 2:02 pm
You should connect an Amper-meter in series at the
output of the regulators and measure the current consumption in
Good luck, please keep me posted!! :smt023