# can you parallel voltage regulators to increase amprage?

I

#### izzi4

Jan 1, 1970
0
Hello, i'm looking to put together a 12v 3amp power supply. Can i parallel
multiple regulators to add up to 3 amps or will this not work? Any other
simple suggestions to get this working?

J

#### John Popelish

Jan 1, 1970
0
izzi4 said:
Hello, i'm looking to put together a 12v 3amp power supply. Can i parallel
multiple regulators to add up to 3 amps or will this not work? Any other
simple suggestions to get this working?

It is difficult to get the regulators to share the output current
equally. One will tend ot pass most of the current till the load
causes it to current limit and the next one comes on, etc. So one
tends to overheat and shut itself down.

Better to buy a higher current device like an LM323 and modify it to
provide 12 volts.
http://cache.national.com/ds/LM/LM123.pdf

I

#### izzi4

Jan 1, 1970
0
any idea of where to get these inexpensivly? I always get stuck paying
double for shipping and for only one component it seems dumb.

J

#### John Popelish

Jan 1, 1970
0
izzi4 said:
any idea of where to get these inexpensivly? I always get stuck paying
double for shipping and for only one component it seems dumb.

Yeah. Digikey socks you with a $5 surcharge for an under$25 order,
though they do ship via first class to save some on shipping a very
small order.

Mouser has no minimum order surcharge but charges a bit for shipping.
The LM350K (diamond shaped TO-3 case for $5.70 and the power tab to-220 LM350T for$1.53 (harder to heat sink as well). You will have
to go through the order process to find out what shipping they
charge. I never buy one of anything. I always get at least one
spare.
http://www.mouser.com/

G

#### Geir Klemetsen

Jan 1, 1970
0
izzi4 said:
Hello, i'm looking to put together a 12v 3amp power supply. Can i parallel
multiple regulators to add up to 3 amps or will this not work? Any other
simple suggestions to get this working?

To let a power-transistor take most of the load:

.--------- --------------.
| \ v |
| --- |
| | |
| | ____ |
V_in | ___ | | | | V_out
o-----o-|___|----o----|7805|-----o--------o
|____|
|
|
|
===
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

B

#### Byron A Jeff

Jan 1, 1970
0
-
-"izzi4" <[email protected]> skrev i melding
--> Hello, i'm looking to put together a 12v 3amp power supply. Can i
-parallel
-> multiple regulators to add up to 3 amps or will this not work? Any other
-> simple suggestions to get this working?
->
-
-To let a power-transistor take most of the load:

That's one way to go. You size R1 so that the transistor
starts to conduct (Vbe of 0.6V) when the current across the small regulator
rises to a set point (say 200ma for a standard 1A 78XX part).
So using Ohm's Law:

R = V/I = .600V/.200A = 3 ohms

As the current demands of the load increases the small regulator will
cause more current to be drawn across the pass transistor.

Also note in the modifird verson below, there is a 7812 instead of a 7805.

BAJ

-
-
- .--------- --------------.
- | \ v |
- | --- |
- | | |
- | | ____ |
- V_in | ___ | | | | V_out
- o-----o-|R1_|----o----|7812|-----o--------o
- |____|
- |
- |
- |
- ===
- GND
-(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
-
-
-

C

#### Clarence

Jan 1, 1970
0
Byron A Jeff said:
-
-"izzi4" <[email protected]> skrev i melding
--> Hello, i'm looking to put together a 12v 3amp power supply. Can i
-parallel
-> multiple regulators to add up to 3 amps or will this not work? Any other
-> simple suggestions to get this working?
-To let a power-transistor take most of the load:

That's one way to go. You size R1 so that the transistor
starts to conduct (Vbe of 0.6V) when the current across the small regulator
rises to a set point (say 200ma for a standard 1A 78XX part).
So using Ohm's Law:

R = V/I = .600V/.200A = 3 ohms

As the current demands of the load increases the small regulator will
cause more current to be drawn across the pass transistor.
Also note in the modified version below, there is a 7812 instead of a 7805.
BAJ
-
- .--------- --------------.
- | \ v |
- | --- |
- | | |
- | | ____ |
- V_in | ___ | | | | V_out
- o-----o-|R1_|----o----|7812|-----o--------o
- |____|
- |
- |
- |
- ===
- GND
-(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
-

Maybe it is just the schematic is hard to read, but the transistor should be a
pnp power type with the emitter connected to V_in. R1 is only a return and
about 470 ohms to 1K is recommended. The regulator will draw as much current
as needed to raise the voltage on it's output pin, and by driving the base
current it is (I_out / Beta)
A MPS2955 would be a good choice for the transistor.
NOTE: minimum over head for the regulator is increased by the V_be drop.

J

#### Jamie

Jan 1, 1970
0
i wouldn't do that.
because the chance you will get them to share the load equally is
1:1000 and one them will end up doing most of the work and the 2
others just loafing along.

M

#### me

Jan 1, 1970
0
i wouldn't do that.
because the chance you will get them to share the load equally is
1:1000 and one them will end up doing most of the work and the 2
others just loafing along.

It works if you use equilizing resistors as if you were paralleling BJTs,
but an external pass transistor is a better way to go.

L
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