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[SOLVED] Can i convert 12v AC to DC?

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Jack.Straw

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I am working on a project where i have a 120v AC to 12v AC transformer (60w max) powering 2x 20w 12v MR11 bulbs.

In the same space I also have a PIC circuit and a couple relays that require 5v DC. I've already made a 5v regulating circuit using a 7805 voltage regulator, but i'll need to supply it with 8v-12v DC. For testing I have been using a 120v AC to 9v DC wall-wart to supply the PIC circuit and relays, but as my project enclosure space is limited, i'm wondering if there is an easy way for me to simply use the 12v AC already present to provide the DC power i need.

Any advice you can offer would be appreciated... thanks!
-Scott
 

You can use the same Lm2805 with the full-wave rectified 12 VAC (20 VDC) as long as the 5 V load current x 15 V is less the max power rating of the LM7805 (with heatsink)
 

A little research came up with the "Diode Bridge". It seems like this is what i'm looking for, but I am unsure as to the capacitor value for the smoothing. Recommendations?

smoothing.jpg
 

Kak111... thanks!!!

I think i understand everything except the cap values.

You have 470uF... 2200uF for C1 and 470nF... 2200uF for C2. I don't understand... why there are 2 values for each?

Thanks again,
-Scott

---------- Post added at 21:55 ---------- Previous post was at 21:38 ----------

Another quick question: Since i am already regulating the voltage to 5v using the 7805, is the 7812 really necessary? It seems like with this circuit i would be regulating to 12v with the 7812, then sending that 12v to be regulated to 5v with the 7805. Is that important, or can i just use the 7805 (rated up to 35v 1a) to regulate from 17v down to 5v?

Thanks!
-Scott
 

Jack.Straw said:
I think i understand everything except the cap values.
... why there are 2 values for each?
Click to expand...
If you consult the manufactuer's data sheet for a voltage regulator you will often see that they recommend a small value capacitor which is not an electrolytic, in parallel with a large value electrolytic. The latter is for smoothing, the former is to eliminate the risk of oscillation. Electrolytic capacitors exhibit a small value of inductance which makes them unsuitable for filtering any high frequency components.

Since i am already regulating the voltage to 5v using the 7805, is the 7812 really necessary?
Click to expand...

No, it isn't. Rectify the 12Vac with a bridge and a large value electrolytic. The d.c. across the C will be about 15V (the peak of the a.c. minus two diode drops). Feed that to the 5V regulator. Continue to use the unrectified 12V for the two bulbs.
 
If you dont need 12Vdc for relays, there is no need to regulate 12V first.

Just remember that 7805 is serial analog regulator ie. the power dissipation
in regulator is (Udc supply - 5V)*I.

That is for heatsink calculations.

In case 17Vdc in , 5V / 1A out, dissipated power in regulator is 12W.
 
Awesome, everything is working perfectly now. Thank you guys SO much for your time and help!
 

Yes just look at the datasheet for your voltage regulator it usually lists everything you need to know to finish the circuit.
 

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