Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

PNP transistor constant current source

tip35

Junior Member level 1
Joined
Sep 3, 2019
Messages
15
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
116
Hi, I got the attached schematic and I would like to know if Q1 needs a base resistor ?
 

Attachments

  • lm317_preregulator.png
    lm317_preregulator.png
    44.5 KB · Views: 67

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
20,504
Helped
4,463
Reputation
8,935
Reaction score
4,492
Trophy points
1,393
Activity points
135,592
Hi,

Every BJT needs a current limit on the base. It may be a resistor or any other circuitry.
In your case R2 and R3 and the BJT´s gain will limit the base current to a save value.

Klaus
 

tip35

Junior Member level 1
Joined
Sep 3, 2019
Messages
15
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
116
In your case R2 and R3 and the BJT´s gain will limit the base current to a save value.
This happens regardless the part number of Q1 ? For example it will also work if I will use BC556B ?
 
Last edited:

00kam

Member level 2
Joined
Dec 17, 2020
Messages
46
Helped
8
Reputation
16
Reaction score
5
Trophy points
8
Activity points
279
hello
i am wonder what is this circuit . and why you feed the LM317 unregulated input pin which is 1.5 Amp max. current output from two BJT 2N3055 connected in parallel those can supply more than 3Amp .
and i have calculated the the output regulated voltage of LM317
VO = 1.25 (1 + R4/R10) =1.25 (1+0.1/100)=~1.25 . why this ratio R4/R10
anyhow any BJT transistor needs base current only on "class C amplifier" (which is for R.F amplifier ) class C amp. works on cutoff region .

kamal
 

tip35

Junior Member level 1
Joined
Sep 3, 2019
Messages
15
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
116
Hello @00kam ,
This circuit is a linear power supply. It uses a preregulator for LM317, which has the role to keep the LM317 input at about 4-5V above LM317 output. The preregulator is made using 2 2N3055. I used 2 x 2N3055 because the short circuit current through the LM317 is abot 2.7A (max) and I wanted to keep the 2N3055 in the SOA somehow. Regarding the VO, in the schematic is just a simulation, in reality there will be a pot connected (2k pot).
 

vfone

Advanced Member level 5
Joined
Oct 10, 2001
Messages
5,347
Helped
1,553
Reputation
3,109
Reaction score
1,165
Trophy points
1,393
Activity points
33,882
LM317 has an internal short-circuit current limiting, so theoretically, will never be in situation to have 2.7A at its input.
Is no reason to use two 2N3055 in parallel BEFORE a LM317 regulator.
Anyway, there are many other simple solutions to build this kind of linear regulator.
 

Attachments

  • PS.gif
    PS.gif
    49.4 KB · Views: 30

LaTeX Commands Quick-Menu:

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top