Right or wrong?

Thread Starter

MikeK

Joined Apr 1, 2016
34
A page in the 'IC Timer Cookbook'. On item 'B' is an NPN the correct transistor? The diode seems to be keeping current from getting to the base. If this was a PNP setup, I would understand it. A 2N6055 is NPN.
 

#12

Joined Nov 30, 2010
18,210
It might be a 2N3055, one of the most used transistors ever, but we can't tell because we don't have the book. If you post a drawing of the circuit you might get some good answers.
 

Thread Starter

MikeK

Joined Apr 1, 2016
34
It might be a 2N3055, one of the most used transistors ever, but we can't tell because we don't have the book. If you post a drawing of the circuit you might get some good answers.
I'll try again, I think I messed up the first time.

upload_2016-4-22_18-16-40.png
 

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#12

Joined Nov 30, 2010
18,210
The 680 ohm resistor is supplying the base current so this Darlington transistor is normally on. The timer is in charge of shutting it off by dumping the base current through the diode.
 

Papabravo

Joined Feb 24, 2006
13,909
The reason it might look strange is that the transistor is a Darlington, but the schematic symbol does not show that fact. A Darlington transistor has a higher Vbe than an ordinary transistor. With the base at 0.7V the Darlington transistor will be OFF.
 

dannyf

Joined Sep 13, 2015
2,197
Npn is the right type there.

Also, the diode is a must in that circuit. Alternatively, you can tie the base to pin 3 through a resistor and get rid of the diode.
 

Thread Starter

MikeK

Joined Apr 1, 2016
34
Yeah, in the 'A' the 'out' was high, so when I looked at the 'B' I presumed it was high again. But no, it was low. No fair changing these things without telling someone.
 
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