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chandu13 · 26th July 2007, 08:07 AM

Hi

I required 12V 3A. For this one iam using L7812 CT (TO-3)

But this component is not working properly.

I have 12V, 1A (L7812CV) Regulator, this is working well.

Can I use L7812CV for 3A load applications?

Please suggest me how to use 1A regulator for 3A load applications.

I have TIP122 transistor, please provide me circuit-using TIP122 .

Regards

chandu

Papabravo · 26th July 2007, 10:55 AM

In general you can't make a part designed for 1A handle a 3A load. I suppose you could have three of them in parallel with their outputs combined by diodes.

After that you lost me. What is the connection between the 7812 regulator and a TIP122. They are different animals.

ericgibbs · 26th July 2007, 12:06 PM

Quote:

Originally Posted by chandu13

Hi

I required 12V 3A. For this one iam using L7812 CT (TO-3)

But this component is not working properly.

I have 12V, 1A (L7812CV) Regulator, this is working well.

Can I use L7812CV for 3A load applications?

Please suggest me how to use 1A regulator for 3A load applications.

I have TIP122 transistor, please provide me circuit-using TIP122.

Regards

chandu

Did you want to use a bypass transistor with 7812?

Eric

Papabravo · 26th July 2007, 11:56 PM

With 0.5A going through R1, that is 0.25 watts per ohm. Isn't R1 going to get kinda toasty?

chandu13 · 27th July 2007, 04:55 AM

Thanks for reply

It is better to use three 7412 regulators Parallel

Iam getting confusion to arrange the regulators and Diodes in Parallel

Please can you provide circuit diagram

Regards

chandu

blueroomelectronics · 27th July 2007, 05:13 AM

What source voltage are you using? Why not just replace the defective 7812CT? Do you have the required caps in your circuit?

ericgibbs · 27th July 2007, 06:44 AM

Quote:

Originally Posted by Papabravo

With 0.5A going through R1, that is 0.25 watts per ohm. Isn't R1 going to get kinda toasty?

hi,
Woops,,, of course you are right, he would get the 'dark brown' smell.

Thats the trouble with these 'green' solar powered calculators, just as you hit the equals key, the Sun goes in!

A 0.5W or 1W rating would be the right choice.

Eric
Thanks

Papabravo · 27th July 2007, 11:21 AM

Take the output of a regulator to the anode of a diode. Connect all the cathodes together. The output of a 7812 is now 11.3 Volts for silicon diodes or 11.7 for Schottky diodes. So if you use adjustable regulators and boost the input a bit to compensate you can get 12V out.

audioguru · 27th July 2007, 11:49 PM

The resistor has 0.8V across it and 0.5A through it. Then it dissipate only 400mW.

If you connect regulators in parallel then the one with the highest voltage takes about 1.2A and current-limits and drops the voltage down until the one with the next-lowest voltage begins to conduct. Then the voltage regulation is lousy.

Papabravo · 28th July 2007, 12:15 PM

Quote:

Originally Posted by audioguru

The resistor has 0.8V across it and 0.5A through it. Then it dissipate only 400mW.

If you connect regulators in parallel then the one with the highest voltage takes about 1.2A and current-limits and drops the voltage down until the one with the next-lowest voltage begins to conduct. Then the voltage regulation is lousy.

So the half-watt would be marginally OK but the 1 Watt would be better.

I didn't say parallel regulators combined with diodes was a good idea, it just answers the original question which was "is there any way to use 1A parts for a 3A application.". As you quite rightly point out the performance is ...ah...sub-optimal.

mcs51mc · 8th August 2007, 07:45 AM

Still need some schematics?
Here are two:
one for 24Vdc/8A
one for 12Vdc/1.7A

I think you can adjust the last one to 3A by adjusting current feed back resistance. A 2N3055 can deliver up to 8A with proper heat sink of course.
You can also use the 8A principle to deliver max 3A in the 12V one.
Your choice

hotwaterwizard · 9th August 2007, 02:17 AM

What about the LM350K ?
It is 3 amps TO3 case.
Adjustable but you can make a circuit for steady 12v output.

hotwaterwizard · 9th August 2007, 02:24 AM

Here is a Handy circuit for 5 amps.
Use a LARGE Heat sink for High Current.

Oznog · 9th August 2007, 04:00 AM

Regulators CANNOT be put in parallel to increase their rating. One will usually end up taking all the current.

What is your source voltage? There are a lot of thermal issues if you have say a 20V source. That would be 24W of heat!

You can always use an external transistor to create an emitter-follower circuit. The output will be 0.5v-1v less than the regulator's voltage. Yes the voltage will change a bit with load.

You can use an adjustable reg to start with 13v or whatever to get the nominal 12v you need. But then if you've got an adjustable reg like the LM317T, there's an "adjust" pin for feedback. Instead of dividing the 317's voltage output and feeding back to that pin, why not divide your big external transistor's output voltage?

But anyways there ARE bigger regs, this one handles 5 amps:
http://www.national.com/mpf/LM/LM338.html

But again pay CLOSE attention to its heat dissipation with the heatsink you have vs how much wattage your application will generate.

hotwaterwizard · 9th August 2007, 01:20 PM

Quote:

Originally Posted by Oznog

Regulators CANNOT be put in parallel to increase their rating. One will usually end up taking all the current..

Hmmmm, If this is completely true explain these Articles.

http://www.elecdesign.com/Articles/A...9270/9270.html

http://www.edn.com/index.asp?layout=...cleid=CA434875

http://www.edn.com/contents/images/434875f1.pdf
http://www.edn.com/contents/images/434875f2.pdf

http://www.edn.com/contents/images/72204di.pdf

26th July 2007, 08:07 AM	(permalink)
chandu13 Experienced Member	High Current Hi I required 12V 3A. For this one iam using L7812 CT (TO-3) But this component is not working properly. I have 12V, 1A (L7812CV) Regulator, this is working well. Can I use L7812CV for 3A load applications? Please suggest me how to use 1A regulator for 3A load applications. I have TIP122 transistor, please provide me circuit-using TIP122 . Regards chandu

27th July 2007, 04:55 AM	(permalink)
chandu13 Experienced Member	High Current Thanks for reply It is better to use three 7412 regulators Parallel Iam getting confusion to arrange the regulators and Diodes in Parallel Please can you provide circuit diagram Regards chandu

27th July 2007, 05:13 AM	(permalink)
blueroomelectronics Experienced Member Blog Entries: 4	What source voltage are you using? Why not just replace the defective 7812CT? Do you have the required caps in your circuit? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

27th July 2007, 11:49 PM	(permalink)
audioguru Experienced Member	The resistor has 0.8V across it and 0.5A through it. Then it dissipate only 400mW. If you connect regulators in parallel then the one with the highest voltage takes about 1.2A and current-limits and drops the voltage down until the one with the next-lowest voltage begins to conduct. Then the voltage regulation is lousy. __________________ Uncle $crooge

9th August 2007, 04:00 AM	(permalink)
Oznog Experienced Member	Regulators CANNOT be put in parallel to increase their rating. One will usually end up taking all the current. What is your source voltage? There are a lot of thermal issues if you have say a 20V source. That would be 24W of heat! You can always use an external transistor to create an emitter-follower circuit. The output will be 0.5v-1v less than the regulator's voltage. Yes the voltage will change a bit with load. You can use an adjustable reg to start with 13v or whatever to get the nominal 12v you need. But then if you've got an adjustable reg like the LM317T, there's an "adjust" pin for feedback. Instead of dividing the 317's voltage output and feeding back to that pin, why not divide your big external transistor's output voltage? But anyways there ARE bigger regs, this one handles 5 amps: http://www.national.com/mpf/LM/LM338.html But again pay CLOSE attention to its heat dissipation with the heatsink you have vs how much wattage your application will generate. __________________ I thought what I'd do was I'd pretend I was one of those deaf-mutes.

26th July 2007, 10:55 AM	(permalink)
Papabravo Experienced Member	In general you can't make a part designed for 1A handle a 3A load. I suppose you could have three of them in parallel with their outputs combined by diodes. After that you lost me. What is the connection between the 7812 regulator and a TIP122. They are different animals.

26th July 2007, 11:56 PM	(permalink)
Papabravo Experienced Member	With 0.5A going through R1, that is 0.25 watts per ohm. Isn't R1 going to get kinda toasty?

27th July 2007, 11:21 AM	(permalink)
Papabravo Experienced Member	Take the output of a regulator to the anode of a diode. Connect all the cathodes together. The output of a 7812 is now 11.3 Volts for silicon diodes or 11.7 for Schottky diodes. So if you use adjustable regulators and boost the input a bit to compensate you can get 12V out.

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