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Need help converting 12V AC to 12V DC

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kenny782

New Member
Hopefully this forum is more helpful lol.
I tried another all I got was two smart a**es throwing out insults.
_______________________

Thanks in advance for any info.

So I'm trying to convert 12VAC coming out of a transformer to 12VDC.
Transformer has a 6V center tapped output, rated current is 1.2AMPS.

I can exchange it for a 24V with a 12V center tapped output if that helps.
I read in a few places that's better for this process but not sure.


I've never tried to actually build something to do this so bear with me.
Anyway I only expect it to be pulling about 500ma.

So I found this rectifier:
1.4A, 100V Full-Wave Bridge Rectifiers - RadioShack.com
I believe that should work?

Now from what I've read the output from that will be noisy and probably be around 15V. So it looks like a capacitor can smooth it out and give me my 12V.

What I can't figure out is what size capacitor I need. I know the uF is the measurement of capacitance, but how do you calculate how much you need?

I mean if I had a constant draw of 1.2amps I'd need more capacitance than say 100ma.


Thanks in advance,

Kenny
 

Hero999

Banned
Firstly you'll get about 16VDC minus some ripple.

The size of the smoothing capacitor will depend on the minimum voltage your equipment will work down to.

[latex]C = \frac{I \times 10000}{V_{ripple}}\\
\text{C is the capacitance in }\mu F\\
\text{I is the maximum current you plan to draw is in Amps}\\
V_{ripple} \text{ is the maximum allowable ripple}[/latex]

Note: this formula is only valid for 50/60Hz supplies.

If you need a smooth 12V power supply then you'll need a low dropout voltage regulator on the output of the rectifier.
 

kenny782

New Member
Gotcha, thanks alot for that info, I think I'll throw that one in "My Documents" :)


One more question, even with the voltage regulator. Do I still need the capacitor to the job to really do the job right?

If I do need both, the voltage regulator would go last right?


Thanks again,

Kenny
 
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Hero999

Banned
Yes, you do need a capacitor.

The AC voltage from the rectifier looks like the waveform on the right.

The regulator won't regulate the output voltage when the input voltage drops below a certain point.

For example, if you're using an LM7812 which has a dropout voltage of 2.5V, the output voltage will start to fall when the input voltage is less than 14.5V. This is where the filter capacitor comes in: the larger the filer capacitor the less ripple there will be and the capacitor needs to be large enough to make sure that it never drops below 14.5V.

The transformer voltage is specified as and RMS value, the peak value is higher by a factor of √2. The voltage at the output of the rectifier is equal to the peak minus any rectifier losses (normally about 1.4V).

Using the formula above for sizing the filter capacitor. If you want to use an LM7812 to give an output current of 1A.

Calcualte the voltage across the rectifier.

[latex]12 \sqrt{2} - 1.4 = 15.6\\
V_{ripple} = 15.6 - 14.5 = 1.1V\\
C = \frac{10000}{1.1} = 9091 \times 10^{6} = 9091 \mu F[/latex]

So use a 10,000µF capacitor and make sure it's rated to at least 25V.

Above you asked whether it would help using a 24VAC centre tapped transformer. The anser is it would help because you could use a bi-phase rectifier (see the schematic at the top of this post) which has a lower rectifier loss meaning more ripple is acceptable. Perfoming the same calculations as above but for a rectifier loss of 0.7V indicates the capacitor can be reduced to 6800:mu:F.

If you only ever plan to draw 500mA, the capacitor size can be reduced even further, try repeating my calculations with a bipase phase rectifier and a current of only 500mA.
 
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