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#1
10-12-2012, 06:39 PM
 parklol Junior Member Join Date: Aug 2012 Posts: 16
12VAC to 12VDC Converter

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
10-12-2012, 07:07 PM
 bertus Administrator Join Date: Apr 2008 Location: Amsterdam,Holland (GMT + 1) Posts: 12,263

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
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#3
10-12-2012, 07:28 PM
 mcgyvr Senior Member Join Date: Oct 2009 Location: North Carolina, USA Posts: 3,094

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
10-12-2012, 07:33 PM
 chrissyp Member Join Date: Aug 2008 Location: guildford,surrey, uk Posts: 82

If the current is low a zener diode could be used to pull the voltage back down to 12vdc
#5
10-13-2012, 02:59 AM
 MrChips Super Moderator Join Date: Oct 2009 Posts: 9,308 Blog Entries: 24

Quote:
 Originally Posted by parklol 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
Why do you say "it is not safe to just put the capacitor right after the rectifier"?
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#6
10-13-2012, 06:00 AM
 ScottWang Senior Member Join Date: Aug 2012 Location: Taiwan,Asia (GMT +8) Posts: 1,607 Blog Entries: 7

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
10-16-2012, 07:23 PM
 parklol Junior Member Join Date: Aug 2012 Posts: 16

Quote:
 Originally Posted by MrChips Why do you say "it is not safe to just put the capacitor right after the rectifier"?
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
10-16-2012, 07:27 PM
 takao21203 Senior Member Join Date: Apr 2012 Location: Ireland Posts: 2,443 Blog Entries: 16

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
10-16-2012, 07:36 PM
 parklol Junior Member Join Date: Aug 2012 Posts: 16

Quote:
 Originally Posted by bertus 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
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
10-16-2012, 07:43 PM
 MrChips Super Moderator Join Date: Oct 2009 Posts: 9,308 Blog Entries: 24

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.
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