# Power Supply Question - RF Cafe Forums

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ice745
Post subject: Power Supply Question
Unread postPosted: Sun Jul 17, 2005 4:22 pm
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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?

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IR
Post subject:
Unread postPosted: Sun Jul 17, 2005 11:40 pm
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Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi ice745,

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 a
heat-conductive paste to the plate.

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- IR

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ice745
Post subject:
Unread postPosted: Mon Jul 18, 2005 2:01 pm
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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 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 works.

Thanks for the help :)

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IR
Post subject:
Unread postPosted: Mon Jul 18, 2005 2:05 pm
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Joined: Mon Jun 27, 2005 2:02 pm
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Location: Germany
Hello,

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 posted!! :smt023

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- IR

Posted  11/12/2012

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