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Power Supply Question - RF Cafe Forums
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Post subject: Power Supply Question
Sun Jul 17, 2005 4:22 pm
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 11:40 pm
Joined: 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.
I hope 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
heat-conductive paste to the plate.
Unread postPosted: Mon Jul 18, 2005 2:01 pm
Joined: Fri Apr 22, 2005 12:22 am
Thanks. I 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
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 works.
Thanks for the help :)
Unread postPosted: Mon Jul 18, 2005 2:05 pm
Joined: 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 DC mode.
Good luck, please keep me