# 12VAC to 12VDC Converter

Discussion in 'General Electronics Chat' started by parklol, Oct 12, 2012.

1. ### parklol Thread Starter New Member

Aug 17, 2012
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Hi, I am trying to design a 12VAC to 12VDC converter. The simplified circuit is attached below.The problem is that I have to use SMD component. The capacitor I can only find is 16V 220uF, and it is not safe to just put the capacitor right after the rectifier. Are there any way to solve this problem? -Leo

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2. ### bertus Administrator

Apr 5, 2008
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Hello,

The peak voltage of a sinus is sqr(2) * the input voltage , the voltage drop over the diodes is about 0.7 Volts
So the output voltage will be around 12 * 1.414 - 2 * 0.7 = 15.6 Volts

Bertus

3. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,791
976
What Bertus said is obviously correct..

I would just use the bridge.. and a LM7812 12V regulator or similar with the required input/output caps as per the datasheet.

You didn't specify a current requirement though..

4. ### chrissyp Active Member

Aug 25, 2008
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If the current is low a zener diode could be used to pull the voltage back down to 12vdc

5. ### MrChips Moderator

Oct 2, 2009
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Why do you say "it is not safe to just put the capacitor right after the rectifier"?

6. ### ScottWang Moderator

Aug 23, 2012
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I' afraid the Cap value is not enough, you may use 4 pieces of 220uf/16V, each two capacitors series and then parallel, the cap will become 440uf/32V.

7. ### parklol Thread Starter New Member

Aug 17, 2012
16
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The capacitor I found is 16V, but the peak voltage of 12VAC is about 17V. Even there is some voltage drop through the rectifier, it is still not safe to put the capacitor right after the rectifier.

8. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Use a 25V capacitor. They are as readily available (in SMD) as 16V caps.

If you load the circuit with current, voltage will drop anyway.

It takes about 20 or 22V to blow up a 16V eletrolytic cap. At least.

9. ### parklol Thread Starter New Member

Aug 17, 2012
16
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Yes, you are right. this seems work based on the calculation. The power supply I am using is a 120VAC to 12VAC transformer. However, the line voltage varies, so sometimes the output of transformer will give like 12.7VAC.

I can add another diode to reduce more voltage or add a regulator in between. The maximum current I need is 1.5A, adding these components will produce too much heat.

Are there any other solutions?

10. ### MrChips Moderator

Oct 2, 2009
12,642
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All linear voltage regulators have to give up excess power as heat.
You have to switch to a switching regulator if you want to be cool.

11. ### takao21203 Distinguished Member

Apr 28, 2012
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463
60 Hz transformer (for 1.5A, so 20W class) and small SMD cap. Why this particular requirement? SMD circuits are often supplied by electronic transformers.

12. ### parklol Thread Starter New Member

Aug 17, 2012
16
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I am trying to design a LED circuit. The LED and other components are on the same PCB, and there will be a lens on top of it. Eletrolytic capacitors are too big. I found tantalum capacitors are smaller, but these capacitors are very expensive when having a high rated voltage.

13. ### takao21203 Distinguished Member

Apr 28, 2012
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Oh I see. Use a smaller capacitor value maybe. Can you post the circuit? Some circuits can live with relatively high ripple current.

14. ### parklol Thread Starter New Member

Aug 17, 2012
16
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Due to limited PCB space I cannot use electronic transformers. So I have to use a power transformer to supply many single units.

15. ### takao21203 Distinguished Member

Apr 28, 2012
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If it is a toroidal transformer, you could take off some of the secondary windings. I guess under load, voltage will go down a little as well.

16. ### parklol Thread Starter New Member

Aug 17, 2012
16
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I used a 12V regulator and six 220uF capacitors to regulate the voltage. It works right now, but I am trying to find a way to not use the regulator.

By the way, do you know how to use a 2 line inductor? I am trying to add an inductor to reduce current ripples. If the inductor is 1.5A, do the both modes in the attached image work for 1.5A?

Thank you

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17. ### takao21203 Distinguished Member

Apr 28, 2012
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Obviously the copper resistance is 4 times as high for the differential mode. I have not used an 1.5A common mode choke in that manner, even if I have a few around here.

Indeed 220uF Tantalum caps rated at 25V are not common for 60Hz filtering (because of their high price). There are also very small chinese electrolytic caps (220uF) for SMD. They are usually blank, with a black plastic washer at the bottom.

TRIAC regulators are not so good for transformers however if you only need to reduce a few volts, it is not a big issue (for 20W class). There are very small 400V TRIACS, same size as a 2n3904 (TO 92 I think).

I'd experiment with that, if the particulary arrangement has to be used. This would be some kind of a fix, not a production level solution. I used a TRIAC regulator one time for a 50Hz transformer experiment and yes they can regulate the primary voltage. Commercially it is not done that way, because it causes an assymetric load, among other things. As I say, for a few precent reduction, and 20W class, neglible.

I found a photo with one of these electrolytic caps. They are not reasonable larger for 220uF/25V, just a bit.
Indeed I bought a kit which contained 10V, 16V and 25V caps.

• ###### PICT1344.jpg
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Last edited: Oct 16, 2012
18. ### parklol Thread Starter New Member

Aug 17, 2012
16
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Thank you very much for the information.