# Building 24VAC to 12VDC power supply

#### faf79

Joined Jul 12, 2014
2
Hello forum members

I'm working on arduino project and I need to make small and efficient power supply for the arduino microcontroller.
The only power source I have is 24VAC line, I would like to use that line to convert the AC to DC and then step down to 12V via regulator.

I'm adding the electric schematic that I made for the power supply.
I have few question to ask you because I'm still new to all this subject.

1. I need help to determine which fuse value should I put in the system - if my regulator output is 12V 1.5A (MAX) should i put 1.5A ? (The reason I'm putting a fuse is because if something went wrong I need it to burn the fuse instead of burning my home)

2. I need help to determine the value of the c2 and c3 in my system, I researched a lot on google and found few people that did similar projects and each one of them used different capacitor values, then I tried to look for how to calculate which cap value do i need and i didn't found something that would be clear and understandable enough for me.

3. If you see something wrong with my schematic it would be great if you could tell me about it, also suggestions for improvements are always welcome

Thank you very much for your help
John

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#### shteii01

Joined Feb 19, 2010
4,644
The fuse is on the ac side. So the part you need to worry about is the bridge. How much current can the bridge take?

#### NorthGuy

Joined Jun 28, 2014
611
Not a good idea.

24VAC is 34V peak. Assume you undersize your capacitor a bit and get 30VDC. You want 12V. This means that 18V will have to drop on 7812. At 1.5A, it's 18x1.5 = 27W. I don't think 7812 can take this, even with a huge heat sink.

You need a switching regulator or a transformer.

#### ronv

Joined Nov 12, 2008
3,770
We either need some more numbers or we can make some assumptions. Lets assume.

Your input voltage is kind of high. 24 volts RMS turns into almost 34 volts once it charges up the filter cap to the peak voltage, so I'm going to pick 1/4 amp for the maximum output of the supply. What this means is your regulator needs to dissipate around 5.5 watts ( the difference between 34 and 12 X .250) so you need a heat sink for it. So now to your question for the caps. Take a look here so you can see how things look.

So using their formula and solving for say 17 volts (5 volts above the regulated voltage) we need maximum ripple of about 17 volts since we loose a little voltage across the diodes and the regulator. We would get about 120 ufd. So use 150 and the ripple will be around 14 volts.
For the other capacitor just use the one recommended in the data sheet. 0.1 ufd. I think.

#### faf79

Joined Jul 12, 2014
2
That's why I want to make one.. (and also because I want to learn)

The fuse is on the ac side. So the part you need to worry about is the bridge. How much current can the bridge take?
The bridge is build from 4 LN4007 Diodes so by the datasheet I assume it's 30A ?
www.diodes.com/datasheets/ds28002.pdf

Fairchild website says LM7812 does up to 1 A, not 1.5 A.
http://www.fairchildsemi.com/pf/LM/LM7812.html
You're right, I will fix it, thanks.

Datasheet list the capacitors you need.

For LM7812:
C in=0.33 uF
C out=0.1 uF

For LM7812A:
C in=0.33 uF
C out=0.1 uF

http://www.fairchildsemi.com/ds/LM/LM7812.pdf
I saw that, I need to know why they choose those and how can I calculate it by myself.

Not a good idea.

24VAC is 34V peak. Assume you undersize your capacitor a bit and get 30VDC. You want 12V. This means that 18V will have to drop on 7812. At 1.5A, it's 18x1.5 = 27W. I don't think 7812 can take this, even with a huge heat sink.

You need a switching regulator or a transformer.
You're right, it cannot do that, I took your advice and I will change it to switching regulator.
I found one that based on LM2575 - DATASHEET: http://www.ti.com/product/lm2575

Here: http://www.ebay.com/itm/3-Terminal-12V-1A-Switching-Voltage-Regulator-Power-Sup-/400079060937?pt=LH_DefaultDomain_0&hash=item5d2691ffc9
It's already assembled in a way that i have 3 pins just like the LM7812.

Thanks

We either need some more numbers or we can make some assumptions. Lets assume.

Your input voltage is kind of high. 24 volts RMS turns into almost 34 volts once it charges up the filter cap to the peak voltage, so I'm going to pick 1/4 amp for the maximum output of the supply. What this means is your regulator needs to dissipate around 5.5 watts ( the difference between 34 and 12 X .250) so you need a heat sink for it. So now to your question for the caps. Take a look here so you can see how things look.

So using their formula and solving for say 17 volts (5 volts above the regulated voltage) we need maximum ripple of about 17 volts since we loose a little voltage across the diodes and the regulator. We would get about 120 ufd. So use 150 and the ripple will be around 14 volts.
For the other capacitor just use the one recommended in the data sheet. 0.1 ufd. I think.
Regarding the regulator I will change it to switching regulator since it won't stand the hit.
The switching regulator will be LM2575 - DATASHEET: http://www.ti.com/product/lm2575

So from what you say I need to put 330uF cap after the bridge and 0.1uF after the regulator?
Also, the one that is located after the bridge should be 50V? 24V rms will be 34V so 50V is safe enough?
and the one after the regulator should be around 24V ? double of the 12V regulator input

Thanks guys for the quick answers

#### NorthGuy

Joined Jun 28, 2014
611
This one is only rated for 1A. If you need 1.5A, you need something rated for at least 1.5A (better 2A).