# using tip31s to increase current from voltage regs ?

#### turbo1600

Joined Jan 12, 2016
7
i have a lot of TIP31 transistors , can i use them to increase the current handling capacity of LM78xx regulators ? i need the voltage reg to handle 3 to 4 amps. will this also work for the LM317 adj ?

any body provide diagram on how to do this. ? ( i have the voltage reg side sorted out , just need the current increased.)

thanks, mk

#### Alec_t

Joined Sep 17, 2013
10,909
Page 8 of the National Semiconductor datasheet for the LM317 shows how to do it.

#### bertus

Joined Apr 5, 2008
20,322
Hello,

To boost the current a PNP transistor is used.
The TIP31 is a NPN transistor of only 3amps.

Here is how the schematic looks like:

I also have attached the TIP31 datasheet.

Bertus

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#### dl324

Joined Mar 30, 2015
10,044
Welcome to AAC!
i have a lot of TIP31 transistors , can i use them to increase the current handling capacity of LM78xx regulators ?
To use power NPN transistors, you need this circuit (from a NatSemi Voltage Regulator Handbook):

Q1 is a general purpose PNP, Q2 is the power transistor.

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#### hp1729

Joined Nov 23, 2015
2,304
Welcome to AAC!
To use power NPN transistors, you need this circuit (from a NatSemi Voltage Regulator Handbook):
View attachment 100057
Q1 is a general purpose PNP, Q2 is the power transistor.
Questions on figure 7.4. How do you select resistor values?

#### hp1729

Joined Nov 23, 2015
2,304
i have a lot of TIP31 transistors , can i use them to increase the current handling capacity of LM78xx regulators ? i need the voltage reg to handle 3 to 4 amps. will this also work for the LM317 adj ?

any body provide diagram on how to do this. ? ( i have the voltage reg side sorted out , just need the current increased.)

thanks, mk
Super! I have used such designs often but never actually stopped to think about how they work. Attached is a few drawings. Current ranges have been brought down to something I can breadboard but the principle is the same.
"A" shows the ever popular PNP used for a positive regulator.
"B" shows how that PNP is controlled by the inerds of the LM78L09 I used.
"C" shows the suggested use of the NPN.
Testing the NPN version under different loads my voltage varied about 500 mV either way. I confess a lack of proper design in resistor selection.. I assumed they set the point where the transistors started conducting.

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#### crutschow

Joined Mar 14, 2008
24,354
So here's my LTspice simulation of a LM317 circuit boosted with a 2N2907 and two TIP31's, showing the output current and voltage versus the load resistance.
The maximum current for a regulated output voltage is about 4.5A with a 10V supply.
This will vary some with different supply voltaes.
The LM317 is configured for a nominal output voltage of 5V.

Note that with such a high output current the short-circuit dissipation can be quite high, depending upon the source voltage.

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#### hp1729

Joined Nov 23, 2015
2,304
So here's my LTspice simulation of a LM317 circuit boosted with a 2N2907 and two TIP31's, showing the output current and voltage versus the load resistance.
The maximum current for a regulated output voltage is about 4.5A with a 10V supply.
This will vary some with different supply voltaes.
The LM317 is configured for a nominal output voltage of 5V.

Note that with such a high output current the short-circuit dissipation can be quite high, depending upon the source voltage.

View attachment 100096
Thanks. How did you come to a value of 100 ohms for R6?
Do you have a 660 ohm resistor? Maybe 665 1%, or a 620 with a 100 ohm trimpot?
Or a 1K pot trimmed for 5.0 V out?

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#### bertus

Joined Apr 5, 2008
20,322
Hello,

@hp1729 , why do you answer for @turbo1600 ?
Are you related to him?

Bertus

#### crutschow

Joined Mar 14, 2008
24,354
Thanks. How did you come to a value of 100 ohms for R6?
Do you have a 660 ohm resistor? Maybe 665 1%, or a 620 with a 100 ohm trimpot?
Or a 1K pot trimmed for 5.0 V out?
R6 is just a bias leakage resistor to shunt away any high temperature leakage current.
It's value is somewhat arbitrary and can vary over a wide range. It never has more than about 1V across it.
Where does the value of 660Ω come from?
R6 has nothing to do with the 5V output which is determined by R1 and R2