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.

No base resistor for NPN driven by opto output?

cupoftea

Advanced Member level 4
Joined
Jun 13, 2021
Messages
1,034
Helped
24
Reputation
48
Reaction score
44
Trophy points
48
Activity points
5,747
Hi,
Please could i just do a reality check and confirm that no base resistor is needed in the attached NPN base drive?
LTspice and jpeg schem attached.

The NPN is just for a digital high/low output (from its collector) when the opto_diode current source is turned on (5mA) and off (0mA)
 

Attachments

  • opto.jpg
    opto.jpg
    166.6 KB · Views: 18
  • opto.zip
    581 bytes · Views: 10

danadakk

Advanced Member level 5
Joined
Mar 26, 2018
Messages
1,667
Helped
279
Reputation
572
Reaction score
356
Trophy points
83
Activity points
7,585
The design is depending on the CTR and input current. If limited on coupler input
current and the CTR limitations will limit current into external transistor
base. That being said I would think about using a R to limit input current
to external transistor should the coupler input driver fail with high current.
Keep in mind graphs are typical values, not worst case, in general in datasheet.


1652525395221.png


Regards, Dana.
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
21,809
Helped
4,580
Reputation
9,174
Reaction score
4,765
Trophy points
1,393
Activity points
143,900
Hi,

What´s the benefit of the circuit?
To drive a BJT with some miliamperes base current while the bjt just drives below 1mA...

The bjt "overdrive" will lead to quite unsymmetrical timing behaviour.

Klaus
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
49,894
Helped
14,483
Reputation
29,236
Reaction score
13,391
Trophy points
1,393
Location
Bochum, Germany
Activity points
286,515
Asymmetrical timing is primarily caused by the optocoupler itself, but no timing requirements have been mentioned at all. You can however omit Q1, it has no purpose with 10k load.
 

LaTeX Commands Quick-Menu:

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top