# 12v ac to 12vdc with bridge rectifier and capacitor

Discussion in 'General Electronics Chat' started by collyd21, Jul 27, 2012.

1. ### collyd21 Thread Starter New Member

Jul 2, 2012
14
2
I HAVE MY CIRCUIT WIRED UP AS SUCH IN THE PICTURE, WHEN I TURN IT ON I GET 12VDC ACROSS THE CAPACITOR WITH NO LOAD CONNECT. BUT WHEN I CONNECT THE LOAD IT DROPS TO 9VDC, CAN SOMEBODY PLEASE TELL ME WHY THIS IR AND WHAT NEEDS TO BE DONE TO RECTIFY THE CIRCUIT TO GIVE 12VDC ACROSS THE LOAD?

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2. ### mkbutan Senior Member

Sep 30, 2008
270
16
what's the input voltage?
and what's the load on the Ckt.?
which diodes are u using?

Apr 5, 2008
15,536
2,302
Hello,

A 42 μF capacitor is a very low value for a buffer capacitor.
You will need about 1500 μF for each ampere.

Bertus

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4. ### collyd21 Thread Starter New Member

Jul 2, 2012
14
2
hi yes i was thinkin the problem was the the size of the capacitor alright, thanks for your help

5. ### #12 Expert

Nov 30, 2010
16,031
6,542
If you really have 12 VAC, this should charge the capacitor to about 15.5 or 16 volts. You need to get picky about your measurements so they wil lead you to the error.

6. ### collyd21 Thread Starter New Member

Jul 2, 2012
14
2
i put in a 1000uF capacitor and getting 11 volts across the load, but when i put in a 2200uF cap the voltage across the load begins to jump randomly up and down from 2v up to 6v down to 4v up to 7v??

7. ### mkbutan Senior Member

Sep 30, 2008
270
16

1000μf is good but it should be min. 25v(1000μF/25v)
just make sour that the capacitor voltage should be twice of the operating I/P voltage(secondary from transformer and after the rectifier diodes)

8. ### gerty AAC Fanatic!

Aug 30, 2007
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9. ### mkbutan Senior Member

Sep 30, 2008
270
16
but whats the difference between these two

or whats the use of inductor (COIL/CHOKE)?
pl explane

10. ### zubair306 New Member

Dec 21, 2013
1
0
i have 4 lights installed with 12vdc battery.when i turn on the lights ,the get blink.i think voltage drop.can anybody suggent me what will the capacitor rating to use for it.?

11. ### gootee Senior Member

Apr 24, 2007
447
50
What is the rating of the input source? It seems to not be able to supply enough current. There should be be at least 15v with no load, if the input is 12v RMS. 12v RMS should give 17v peaks. Maybe you need a heavier transformer. Or maybe your input is only 13.5 to 14 volts peak?

Anyway, once you have the correct transformer, you can calculate the capacitance needed:
Since i = C dv/dt we can make some assumption and get close-enough with C = i dt/dv where i is the maximum load current, dt is the time between charging pulses which is 1/2fmains, and dv is the voltage dip you want to allow, or the amplitude of the ripple voltage.

12. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Jul 18, 2013
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The Capacitor choke filter was used for many years an amplifiers and radio recievers.
Often seen with a cap - choke - cap, it is known as a Pi filter because of the obvious configuration.
The capacitor stores energy, the choke has the effect of high impedance at the ripple frequency allowing DC to pass un-reduced, therefore filtering or removing the AC ripple .
Max.