Transistors

Umiq88

Maybe not the correct place but im hoping i can get an answer.

I have a variable speed drive controlling a motor and it contains a transistor switch.

The switch consists of two transistors in i think its called a darlington-pair

connections as follows

first transitor (T1)
b - collector T2
c - 24V
e - output/switch

second transistor (T2)
b - diode symbol beside it
c - base T1
e - 0V

I could have the collector and the emmitters mixed up i have the one with the arrow pointing into the line as the collector and its on top which i think is an NPN but im not to sure.

Anyway problem is its working find on one of the VSDs but not on the others is it possible that it was connected in some way wrong before and that its has blown the transistors.

Only working with 24V here btw.

watty

Not a darlington

     | /  C
B _ _|/
     |_|
     |  \  E
    
NPN
(E= emitter to OV, arrow points out, current flows IN to base, out of emitter, 
collector current from Supply  = base x Gain)


     |  /  E
B _ _||_
     |\
     | \  C
    
PNP
(E= emitter to +V, current flows out of base toward 0V, into emitter,
Collector current to ground = base x Gain) 

A darlington has both transistors same kind, PNP OR NPN. The emitter drives base and both collectors are tied together. Externally only base, collectors and emitter.

old Germanium transistors only easy to make as PNP, so old radios used -9V and OV. NPN Silicon transistors are better performance and cheaper than PNP, hence modern gear uses + supply rails and PNP transistors thus often look "upside down " in circuits.


There may be a better forum.

Umiq88

Ok so from your drawing they're both PNP transistors with the emmittor being what i though was the collector and vice versa.

Do you reckon its possible that the transistors could have been blown from connecting the leads in the wrong way

watty

Possibly. Hard to tell via text chat. A short across motor or a stalled moter due to too much load are ways to wreck them.

My drawings are very bad.

NPN ->-- Arrow on Emitter points away from bar to pin

PNP -<-- Arrow on Emitter points to bar away from pin

The arrow is direction of Emitter current flow.

Using a Digital Meter on the ->|- diode (or 2K/2000 Ohms) setting

NPN test

Red lead to base: Low reading with black wire on collector or emitter (High on either = broken)
Black lead to base: High/open reading with red on collector or emitter (low on either = broken)

collector to emitter. Red to Black. High/open reading. if low it is faulty.


PNP test

Black lead to base: Low reading with red wire on collector or emitter (High on either = broken)
Red lead to base: High/open reading with black on collector or emitter (low on either = broken)

collector to emitter. Black to Red. High/open reading. if low it is faulty.


Most old analogue meters have the voltage reversed on Ohms. Above only applies to modern Digital meters.