# Discrete Voltage Regulator

#### Daniel Dutu

Joined Oct 15, 2017
3
Hello!

My first post ever on electronics forums (so excited!).

I come here looking to find a solving at a friend's sugestion, a discret voltage regulator ( not sure is the same with series voltage regulator).
Vout should be between 5.5 - 11V
Vin should be between 19.8 - 22 V
I need to limit the transistor temperature at 100C and 0,4A current as a protection of overcharge.

Any idea of a schematic idea according to this?
So far I managed to found this.

Thanks!

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#### wayneh

Joined Sep 9, 2010
17,493
...a discret voltage regulator ...
May I ask why you want to avoid using a dedicated voltage regulator IC? They are very cheap, widely used and designed for this exact chore. As your schematic shows, designing and building your own can be complicated.

#### Daniel Dutu

Joined Oct 15, 2017
3
Hey wayneh!

I have a lot of these components and some spare time to invest in this. I know is kind of a chore but is worth trying for me.
But so far that scheme I found is a little complicated, wondering if anyone can drop one maybe a little bit easier to understand but doing the requirements from the first post.

#### panic mode

Joined Oct 10, 2011
2,636
this is linear regulator. output current also flows through regulator and will heat it up. the worst case scenario is when output voltage is low, output current is high and supply voltage is high.
the best way to limit heat dissipation of the regulator is reduce supply voltage to something reasonable.

it is not very reasonable to drop from 22V to 5V. in fact your schematic shows unregulated voltage is 45V which is close to insane:
45V - 5V = 40V drop....
40V drop * 0.4A = 16W. (some of my soldering irons are 8W, 18W, 25W)

linear regulator always need some overhead (2-3V) so if you use unregulated source of say 14-15V you would dramatically reduce voltage drop across regulator and eliminate most of the heat.
Suppose you use 16V, that would mean up to 11V drop and 4.4W heat. that is 1/4th of what we started with.

xox

#### dl324

Joined Mar 30, 2015
16,655
xox

#### Alec_t

Joined Sep 17, 2013
14,199
In the circuit in the post #5 link, R2 should be 1.5Ω if you want a current limit of about 0.4A.

#### crutschow

Joined Mar 14, 2008
33,977
The circuit in post #5 link uses a forward biased diode and base-emitter junction as a voltage reference, thus the output voltage will vary about -0.3%/°C with ambient temperature change.
Is that acceptable?

If not, you could substitute a TL431 adjustable reference for Q1 and D1.

#### Daniel Dutu

Joined Oct 15, 2017
3
Thanks a lot guys for you time and helpful information.

Bless you!

#### MSFTF

Joined Aug 11, 2017
33
An LED can work good for the diode between the ground pin and ground. Kind of gives free light at the same time. I used a white LED to make a five Volt regulator go up to eight volts.