# Power supply design questions.

Discussion in 'The Projects Forum' started by dswartz2, May 19, 2009.

1. ### dswartz2 Thread Starter Active Member

May 19, 2009
31
0
Hi,

I am in the process of trying to design a power supply that can take 230V down to 24Vdc.

I'm using a full wave bridge rectifier with a transformer that has a turns ratio of 0.1.
I am using a 7824 regulator to produce the constant 24Vdc output.

I've calculated the circuit and got the Vo(peak) to equal 31.1V.

I tried to solve for Cf, the first capacitor between the rectifier and the regulator, and got 2.03mF.

My question is, what values should I use for C1 and C2 (the caps on the input and ouput of the regulator)?

And does this overall design sound correct to anyone? I want to make sure i'm on the right track.

Thanks for any help

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Your current draw will be no more than an amp with the 7824, so figure on something on the order of 1000 - 2200 uF for the filter capacitor. Use a 50 volt rated capacitor at least. Capacitors are cheap, so it won't hurt to have more capacity than you really need.

Use the ones suggested by the data sheet up next to the regulator.

3. ### dswartz2 Thread Starter Active Member

May 19, 2009
31
0
Thanks for the info.

I wired this up, and I used a 3,300 uF capacitor for the filter. I didn't put a cap on the input of the regulator, but then I used a 4.4 uF cap on the output.

When I measure the voltage across the 3,300 uF filter, I get 24 Vdc with about the same ripple (200mV).

Why am I getting these values at the spots that I am getting them? and what should I do to fix it?

Apr 5, 2008
15,647
2,346
Hello,

What is the voltage on the buffer capacitor without the 7824?

Greetings,
Bertus

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
What is the output (AC) of the transformer? The DC on the filter should be close to 1.404 times the AC output. For a 24 volt output, you're going to need about 27 volts on the filter.

6. ### dswartz2 Thread Starter Active Member

May 19, 2009
31
0
The AC voltage coming out of the transformer is about 22.5V.

Apr 5, 2008
15,647
2,346
Hello,

With 22.5 Volts AC you wouls expect to have 31.5 Volts DC on your buffer capacitor.
Take a look at the attached PDF for some explanation.

Greetings,
Bertus

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