Combining the Ampere of two voltage regulator?

I need a 12VDC voltage regulator that can produce at least 3A. My DC Source is 18.5volts 3.5A. I'm powering gsm shield, arduino, solenoids and etc. I only have here two LM317 regulator and their max current is only 1.5a based on the datasheet. The electronic shop near me doesn't have regulator that can produce more than or equal to 3A. My circuit has a master switch and backup battery that will turn on when the main supply is off. This is the schematic if i will use two regulators with master switch and back up, it is very complicated, and i want to reduce it as shown in the left of the picture by using only one regulator.

my bad, i forgot the voltage drop of zener diode. so the left schematic in the picture is invalid. -_-

You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

highOut.PNG|704x411

Well the following are 12vdc 3 amp linear regulators in TO-3 package: 78H12 NTE933

and lambda sometime ago use to have a 12vdc 5 amp regulator in a TO-3 package that you can sometimes find sold as new old stock:

http://www.ebay.com/itm/LAMBDA-LAS1912-12-VOLT-5-AMP-TO-3-REGULATOR-NOS-78H12-NTE933-EQUIVALENT-/380547135339?pt=LH_DefaultDomain_0&hash=item589a60776b

Lefty

be80be: You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

can i use npn transistor only? i have a couple of TIP120 here.

retrolefty: Well the following are 12vdc 3 amp linear regulators in TO-3 package: 78H12 NTE933

and lambda sometime ago use to have a 12vdc 5 amp regulator in a TO-3 package that you can sometimes find sold as new old stock:

http://www.ebay.com/itm/LAMBDA-LAS1912-12-VOLT-5-AMP-TO-3-REGULATOR-NOS-78H12-NTE933-EQUIVALENT-/380547135339?pt=LH_DefaultDomain_0&hash=item589a60776b

Lefty

do you have alternative solutions than buying a new regulator? :D

Get 2 tip42

be80be: Get 2 tip42

so i cannot use npn transistor?

be80be: You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

i simulate that circuit using circuit wizard and it is not working.

I guess they put that in there data sheet for looks It works.

Let's go back a few steps

Question - Why do you think you need a 3A regulated supply ? What is your actual load ? Just because the power supply can provide 3A+ doesn't mean your regulator has to be rated at that figure. If 6 volts meets you back-up needs than "losing" 12.5 across a linear regulator is a shear waste of power (12.5 x 3 = 37.5 watts). If you really need 6 or 12 volts (you seem to be a bit mixed on voltages) then I'd suggest a switched mode regulator that offers over 90% efficiency with very little heat.

my bad, i forgot the voltage drop of zener diode.

Not to mention that finding zeners that can flow 1.5a/each is quite hard.

If you really need 3amp, you can look into the datasheet of your voltage regulator to see how to expand their current capabilities.

If you really need 3amp, you can look into the datasheet of your voltage regulator to see how to expand their current capabilities.

I posted that for the OP he said in some kind of simulator it didn't work

it didn't work

thank god we don't live in a simulator.

so i cannot use npn transistor?

Yes, you can.

jackrae: Let's go back a few steps

Question - Why do you think you need a 3A regulated supply ? What is your actual load ? Just because the power supply can provide 3A+ doesn't mean your regulator has to be rated at that figure. If 6 volts meets you back-up needs than "losing" 12.5 across a linear regulator is a shear waste of power (12.5 x 3 = 37.5 watts). If you really need 6 or 12 volts (you seem to be a bit mixed on voltages) then I'd suggest a switched mode regulator that offers over 90% efficiency with very little heat.

I have a solenoid lock,alarm,relay rated at 12v. A couple of 5 inputs to arduino. The gsm shield required is 1.5A. Assuming the arduino is 500mA, gsm is 1.5A, the solenoid lock is 200mA, alarm is 300mA, relay is 50mA then the total is 2550mA or 2.5A. The remaining 500ma is just for extra. The purpose of the back up battery is not to power them all but to only power the arduino and gsm, so that the gsm can still send message and call.

The gsm shield required is 1.5A.

A nice heater element.

Here a NPN setup

Here a NPN setup

The output would fluctuate with the load.

You need 1 small signal pnp, + 2 resistors.

The GSM shield requires 5v @ 1.5A? Really?

I would use an unregulated PSU for the solenoids, because there's not likely to be any good reason to worry about the exact voltage going through a coil. Same for the relays. As long as it's rectified and filtered to remove any serious ripple, they should be perfectly happy. A simple transistor (and protection diodes) to fire them would suffice.

That leaves the control circuitry and the GSM module. At a load of 1.5A, you really need to move to switching regulators. That's way, way too much current to stick with linear. You'll be generating a TON of heat. That means, you're wasting quite a bit of power, and you need sufficient means to remove that heat. Those LM78xx chips say they'll drive 1.5A, but don't even think about asking them to do it IRL.

Yes, because i will use calls too. http://tronixstuff.wordpress.com/2011/01/19/tutorial-arduino-and-gsm-cellular-part-one/

The GSM shield can often require up to 2A of current in short bursts – especially when turned on, reset, or initiating a call. However your Arduino board can only supply up to just under 1A. It is highly recommended that you use an external 5V power supply capable of delivering 2A of current – from an AC adaptor, large battery with power regulator, etc. Otherwise there is a very strong probability of damaging your shield and Arduino. Ignore this at your own risk. When connecting this supply DO NOT use the DC socket on the Arduino. Instead, connect the 5V (positive) from the supply to the 5V pin on the GSM shield, and the negative to the GND pin.

I'm using linear regulator because it is easy to make and it is only a school project. So i will only use it once. I already have a big heatsink and thermal paste so generating heat is not a problem.