Hi, I am facing annoying troubles converting 24V AC to 12 V DC. The goal is to supply 1.2A current to 12V DC load for my project. I am both limited in input power (I can only use 24V AC output of a transformer that can supply up to 15W), size and possibilities of heat dissipation (the circuit will be placed within a sealed box with almost no air circulation). I've got some help in building the converter (See picture), but the problem is that it is way too demanding on transformer power input. I've added Proteus simulation screenshot, you can clearly see the power that is taken from the transformer (V1 on the screenshot). Any suggestions on how to make it more efficient or is there any other way for me to reach my goal?
Thank you.
That's interesting. I wouldn't have guessed that.
Normally I wouldn't say this , but I am inclined to doubt the accuracy of your simulation.
Online ac to dc conversion calculator shows 2.94 ac amps generating 6.49 dc amps @ 12V.
I know you are inclined to believe your simulation but people have been building that kind of power supply for many years and never needed a 24V transformer rated for 70W to get 15W dc .
It just doesn't add up. Perhaps there's an error in the schematic.
Try removing R1
Use this
PW1.5 C-TON Industries | Power Supplies - External/Internal (Off-Board) | DigiKey and then a 24V to 12V DC/DC converter.
...and possibilities of heat dissipation (the circuit will be placed within a sealed box with almost no air circulation).
Any linear regulator is going to dissipate (about) the same amount of heat. (Power = Voltage x Current, where the voltage is the voltage dropped across the regulator and the current is slightly more than the current through the load).
I have no idea why your simulation shows way more current going-in than coming-out...
When you rectify and filter AC, the capacitors charge-up to the peak AC voltage which is about 1.4 times the RMS voltage (minus the drop across the diodes). That means you're feeding about 33V into the regulator and dropping about 21V across the regulator. With 12V on the load, the regulator is consuming almost twice as much power as you're delivering to the load (and generating almost twice the heat as the load).
And, with your load requiring 1.2 Amps (14.4W) and almost twice that "eaten" by the regulator (21 x 1.2 = 25.2W, you're at more than twice the 15W limit of your transformer!
A switchmode regulator (AKA "DC-to-DC converter) can be nearly 100% efficient. That means you can get more current out than you feed-in (at less voltage out).
But size wise I would just grab a 2A 12V switch mode power brick.
I think a transformer, DC-DC buck converter, diode bridge and at around 3000uF of capacitor will be bigger then a 12V 2A brick.
slawa92:
Hi, I am facing annoying troubles converting 24V AC to 12 V DC. The goal is to supply 1.2A current to 12V DC load for my project. I am both limited in input power (I can only use 24V AC output of a transformer that can supply up to 15W)
Then you must use a DC-DC converter after your rectifier/smoothing cap.
Proteus is notorious for bogus simulations. Over in the PIC (CCS) forums, that is one of the "stickies" - no more proteus questions. All sorts of strange things with it. As for the circuit you show, it is basically a series circuit and as such, the current will be essentially the same all the way through - it is not possible for you to have double the current out as going in that way. You will also run into heat problems with the series linear regulator as others have indicated. A switching type regulator can drop the voltage and multiply the current effectively (the power is the same less conversion losses ). A switching type regulator is small and also does not waste anywhere the energy as a linear regulator will in your situation.
Nothing has been said about the fact that the 7812 is not supplying the load. It is only controlling the 2n3055 which is supplying the load.
If you do not have a lot of electronics experience and don't understand what DVDoug said, in a nutshell, what he is saying (I beleive) , is that if you want to make a 12V dc power supply, you are SUPPOSED to start with a 12V transformer, NOT a 24V transformer because what you have done is make a wall more than twice as high as it should be and you have to do WORK to chop it down to the size you wanted.
In laymen's terms, the fact that you chose the WRONG transformer causes the power supply to have to DISSIPATE the EXCESS HEAT, WASTING POWER.
Change your simulation to a 12V transformer (what you SHOULD hsve started with) and rerun the simulation.
FYI, most people use 12V transformers for 12V power supplies. That shouldn't come as a surprise, if you think about it.
Except the OP seemed to indicate he was stuck with the 24v transformer with his statement "I can only use 24V AC output of a transformer that can supply up to 15W". Otherwise, you are correct - start with a 12v transformer (although a switching wall wart would be even better).
xcept the OP seemed to indicate he was stuck with the 24v transformer with his statement "I can only use 24V AC output of a transformer that can supply up to 15W". Otherwise, you are correct - start with a 12v transformer (although a switching wall wart would be even better).
Well then I guess he's SOL , right ?
Maybe the power supply Crossroads linked would not dissipate as much heat. I
raschemmel:
Well then I guess he's SOL , right ?Maybe the power supply Crossroads linked would not dissipate as much heat. I
Well, he does have a problem with a linear regulator, but a switching regulator could solve the problem for him - both for the current and the heat dissipation issues. That is the way I would go if I was stuck with the 24v transformer. There is another possibility - not sure if they are out there but a 24v - 12v (2:1) stepdown transformer could work or a 24vct transformer used to get 12vac from one side and the center tap is also a consideration although, the best solution still looks like a switching converter. There are 24vac to 12vdc converters out there - a quick search turned up THIS ONE
slawa92:
Any suggestions on how to make it more efficient or is there any other way for me to reach my goal?
Thank you.
Yeah...... much more efficiency can be achieved if you keep the 24 AC supply, keep your rectifying stage, and take the DC output of that - and feed it into a switching DC to DC converter (aka switch mode power supply, switching regulator supply etc).
Google something like.... "5 Amp DC/DC step-down converter" or "Robust adjustable DC/DC converter"
Google something like.... "5 Amp DC/DC step-down converter"
And this is what you get:
Not very helpful.
Any $1 medium-sized DC/DC buck/stepdown converter can do what OP wants.
Like this one.
Measure the unloaded voltage on the 1000uF buffer cap first.
Some transformers have a 10-15% higher unloaded voltage.
If it's >35volt, try to find one with ~50volt input capability.
I would drop the idea of building that supply.
Modern fully encapsulated 12volt switch-mode bricks are much better and safer.
Leo..
And just read some discussions about that at...... click here just for heads-up. Good price though.
You get what you pay for.
One advantage to the one I referenced in #10 was that it takes 24v AC as the input and gives 12vdc as the output - that eliminates the need to supply some sort of rectifier and filter before the converter.
I didn't see one that combines rectification with regulation. This is no doubt why someone suggested keeping the rectifier and adding a dc-dc converter. What I don't like about that idea is the size of the OP's rectifier vs what is available online. The ideal solution is compact rectifier + compact dc-dc converter.
The one I linked on Amazon specifically says "24V AC to DC 12V Power Converter Transformer Box" which was why I showed that one. It includes (if the description is correct) rectification and conversion in one little box.