# 24vac to 12vdc converter circuit

#### shahnawazpathan7

Joined Jan 14, 2019
19
Hi all,

I am working on a task to convert 24vac/3A from a transformer giving into 12vdc . This 12vdc will be given to an induction sensor device and to draw a 12v relay. So the current drawn from the circuit wont exceed 200mA. I guess I require a circuit related to LM7812 . since the device to be connected is sensitive and expensive I dont want to take any chances of experiment. I scanned some threads but i didnt find satisfactory solution of my use, so I decided to open a new thread.
I will be really thankful if someone just draw this circuit for me as I am really confused about calculation of capacitor.

Warm regards.

#### ericgibbs

Joined Jan 29, 2010
12,884
hi s7,
Welcome to AAC.
A 7812 would be OK, consider that it is dropping 12V at 0.2A, so that is 2.4Watts, so it may require a small heat sink for continuous operation.
Get the 7812 datasheet and fit the recommended capacitors on the 7812 inp/out pins.
E

Look at fig #17 on the 78 App PDF, I would add a100uF on the 7812 output.
Put this PDF in your database, lots of interesting circuits.

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#### bertus

Joined Apr 5, 2008
21,362
Hello,

The rectified voltage from the 24 Vac will give you a DC voltage of about 32 Volts on the buffer capacitor.
There will be a voltage drop of about (32-12) = 20 Volts accross the 7812.
At 200 mA the 7812 will dissipate about 4 Watts, a heatsink will be required.

Bertus

#### crutschow

Joined Mar 14, 2008
27,170
Use a bridge rectifier with a filter capacitor to convert the 24Vac to DC.
Here's a discussion on calculating the value of that capacitor.
For your circuit, 270uF will give a ripple of about 5Vpp, which should be more than sufficient.

The average DC output from that bridge-rectifier will be around 29V, thus the dissipation in the 7812 @ 200mA will be about 3.4W.
The 7812 will thus need to be mounted on a small heatsink.

If you want to protect the system from an overvoltage failure, google "overvoltage crowbar circuit" such as the circuit below:
It would be connected after the 7812 output.

#### Dodgydave

Joined Jun 22, 2012
9,951
Is the transformer a Centre Tap type, you can get 12V from that.

Or use Half Wave Rectifier and capacitor .

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#### ericgibbs

Joined Jan 29, 2010
12,884
hi s7,
Please post details of the 12V relay ,[its resistance if possible] I suspect it will require less than 200mA, say approx 50mA.
E

#### shahnawazpathan7

Joined Jan 14, 2019
19
hi s7,
Welcome to AAC.
A 7812 would be OK, consider that it is dropping 12V at 0.2A, so that is 2.4Watts, so it may require a small heat sink for continuous operation.
Get the 7812 datasheet and fit the recommended capacitors on the 7812 inp/out pins.
E

Look at fig #17 on the 78 App PDF, I would add a100uF on the 7812 output.
Put this PDF in your database, lots of interesting circuits.
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Hi eric,

Thanks for your reply. To make it quick can you plz check attached circuit diagram. If it is right or else plz mark the necessary corrections. In case of caps values plz specify voltage ratings too with farads etc.

thanks.

#### shahnawazpathan7

Joined Jan 14, 2019
19
hi s7,
Please post details of the 12V relay ,[its resistance if possible] I suspect it will require less than 200mA, say approx 50mA.
E
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Yup, It will require less than 50mA.
I have also an induction sensor device which consumes around 100mA so I calculated approx 200mA for both.

#### shahnawazpathan7

Joined Jan 14, 2019
19
Hello,

The rectified voltage from the 24 Vac will give you a DC voltage of about 32 Volts on the buffer capacitor.
There will be a voltage drop of about (32-12) = 20 Volts accross the 7812.
At 200 mA the 7812 will dissipate about 4 Watts, a heatsink will be required.

Bertus
You right Bertus. A heat sink would be required. I'll keep that in mind.
Thanks for writing.

regards.

#### shahnawazpathan7

Joined Jan 14, 2019
19
Use a bridge rectifier with a filter capacitor to convert the 24Vac to DC.
Here's a discussion on calculating the value of that capacitor.
For your circuit, 270uF will give a ripple of about 5Vpp, which should be more than sufficient.

The average DC output from that bridge-rectifier will be around 29V, thus the dissipation in the 7812 @ 200mA will be about 3.4W.
The 7812 will thus need to be mounted on a small heatsink.

If you want to protect the system from an overvoltage failure, google "overvoltage crowbar circuit" such as the circuit below:
It would be connected after the 7812 output.

View attachment 167846
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That will be great. I will definitely check this.

thanks for writing in crutschow.

#### ericgibbs

Joined Jan 29, 2010
12,884
hi,
Note use at least 40Vdv working caps on the input side of the 7812, as the Vin will be ~30V

E

#### Bordodynov

Joined May 20, 2015
2,865
Input voltage is scattered. Hence the worst case may occur with a weak load. The input voltage of the stabilizer chip will exceed 35V.

#### shahnawazpathan7

Joined Jan 14, 2019
19
Input voltage is scattered. Hence the worst case may occur with a weak load. The input voltage of the stabilizer chip will exceed 35V.
View attachment 167869
Hi Bordodynov,
Well, We are discussing about 200mA as I am assuming it as the max but practically it will be 100+-10mA continuous and a relay would be operating on random pulse basis. So it would be between 150ma to 200ma range.
Can you plz explain the worst case scenario and weak load?

#### Tonyr1084

Joined Sep 24, 2015
5,965
My first thought was PWM (Pulse Width Modulation) with an on time of 36% and off time of 63%. All this after rectification. Input and output caps can smooth out the voltage while maintaining an average of around 12 volts.

Often people speak of car electronics as being 12 volt systems. While this is accurate, the actual voltage of such equipment can expect to see sustained voltages from 13.8 to 14.4 volts. If we're talking about a car relay, they will handily work at 12 volts. They may even work as low as around 8 volts, but I can't guarantee they'd pull in.

7812's are nice in that they maintain a steady output. But they must do something with the extra voltage that they convert to heat. Like has already been mentioned, you'd need a heatsink. But a heatsink inside an inclosed box can be ineffective. Since we don't know how you're building this circuit or what sort of enclosure - advice here may not be as useful as it may appear (including my approach). PWM does not produce the wasted heat a 7812 will. It also does not waste power, as when it is on it is on and when it is off it is off. the 7812 is always on (when the circuit is powered) and is always wasting some energy as heat. My approach, since this is such low power requirements, would be to investigate the feasibility of PWM.

#### crutschow

Joined Mar 14, 2008
27,170
Can you plz explain the worst case scenario and weak load?
Under a low (weak) load the transformer output will typically be 5-10% higher than the rated value (it's designed for 24Vac output at full load).
This means the rectified output voltage could exceed 35V, which is the 7812 limit.
You thus might consider Bordodynov's two 7812 circuit in post #6.
It distributes both the voltage and the power dissipated between the two.

#### ericgibbs

Joined Jan 29, 2010
12,884
hi 7,
This is another way, a very inefficient option, it will work, I am posting for reference only.
Its a resistive divider on the 7812 input, using 3-5W resistors.
Ask if you need explanation of the plots.
E

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#### MrChips

Joined Oct 2, 2009
23,505
24VAC/3A transformer is overkill. Change the transformer to a 12VDC 500mA wall-wart.

#### Dodgydave

Joined Jun 22, 2012
9,951
24VAC/3A transformer is overkill. Change the transformer to a 12VDC 500mA wall-wart.
Just what i thought in post #5

#### MisterBill2

Joined Jan 23, 2018
8,717
With the circuit in post#8, if you remove the C1 then the input voltage will be much lower, and you can use a larger capacitor , or both, at the C4 position and it will only charge to 12 volts.But you will need enough capacitance to hold the voltage up between cycles. This is another, less traditional method.