12v 2A Power Supply

[ParkingLotLust]

Can I use a circuit like this, and just leave off a few of the transistors (since I only need 2A), or do I need to keep them all? Just dont want to blow anything up. Im under the assumption that I can just remove a few and that will lower the amperage it can produce (aswell as the cost):

**broken link removed**

 

[microtexan]

Actually, there is a 5 Amp version of the 7812 regulator with which you would need no pass transistors at all.

 

[Willbe]

More transistors = longer life for each.

The surge current rating for that diode bridge must be very high.

 

[ParkingLotLust]

Actually, there is a 5 Amp version of the 7812 regulator with which you would need no pass transistors at all.

That seems like a much better way to do it. Thanks!

 

[saturn1bguy]

That seems like a much better way to do it. Thanks!

If you're just looking to build a 12V/2A supply, use an LM350T or LM350K. It's an adjustable regulator good from 1.2-33V at 3A. You need only two resistors to set the output, and a good heatsink

Go to nsc.com and search for the LM350 datasheet. Look at their examples, the circuit is quite simple...

EDIT: (Of course you wouldn't need as big a transformer, bridge rectifier, and capacitors as your above drawing indicates, as they are all sized for 35A. A 12V-16V transformer would suffice, at 2-3A.)

 

[Hero999]

The disadvantage of this circuit is that over current protection is provided by fuses which need replacing when blown.

You could replace them with PTC resistors but put them on the input side so they don't affect the regulation.

 

[ParkingLotLust]

I decided to drop to 10.5v, which means that I will only be drawing ~1.3A (ammeter shows 1.2ish but I want to go a bit higher just to be safe). Ive found an old AT power supply from a computer, so would I be able to use an LM317T to drop the 12v rail to 10.5v? They are supposed to be able to handle 1.5A, so it makes sense to me that it can handle 1.2A. If my logic is off can you please explain, or confirm that it will work (before I go blowing things up)? And yes, I know Im likely going to need a heatsink, that shouldnt be an issue.

EDIT or is there a simpler way to drop the voltage? Maybe by modifying something inside the PSU? I originally thought of a voltage divider but I dont have any resistors rated at a high enough power (P = VI = 10.5*1.2 = 12.6w)

 

[blueroomelectronics]

What was wrong with using a smaller transformer?

 

[ParkingLotLust]

'cause I have an old AT power supply lying around - easier and cheaper for me to use it versus buying a transformer, case, etc.

 

[saturn1bguy]

'cause I have an old AT power supply lying around

In that case just use the 12V output as is...

(I know you stated you were dropping to 10.5V, but it seemed you were doing that to be able to get away with using an LM317 instead of something larger. If 12V still works, just use your AT power supply as is.)

 

[ParkingLotLust]

Id have to remove all the resistors from the LED board Im powering and replace them all with 1/2w resistors (theyre all 1/4w now - miscalculated the power to be 200mW per resistor, when I checked again it came out to 270 or 280mw). Instead of desoldering 78 resistors and putting 78 back in, I figured it would be easier to drop the voltage Which is why I brought up dropping the voltage from 12v to 10.5v in the first place

 

[blueroomelectronics]

Some large diodes in series to drop the voltage...

 

[ParkingLotLust]

Will 1N400x diodes handle the current draw at 10.2v (~1.2A)? The datasheet says that "Average Rectified Output Current = 1A", so thats a no?

 

[blueroomelectronics]

They do make 3A diodes.