pnp test

[NoRk1]

Hi all,

Just wandering should a continuity test with a DMM across
the E and C terminals of a pnp transistor return true for
current?

Thanks
nork1

 

[Pi]

Just wandering should a continuity test with a DMM across
the E and C terminals of a pnp transistor return true for
current?

Hi, it should not conduct. Here is an overview:
Whyen you measure PNP transistors you measure a diode conducting from
C to B, and from E to B. NPN has the diodes conducting from B to E,
and from B to C.

C to E, and E to C, should never conduct, but do not touch the base. A
very small base current is enough to let the transistor conduct.

Regards,
Pieter Hoeben

 

[Gareth Magennis]

This is the way I test unknown transistors, takes about 10 seconds.

Take the positive DVM probe and connect to one terminal. Touch the other
probe on the other two terminals in turn. If both show a typical diode
reading, the positive probe is on the B terminal and it in NPN. Then put
the probes on the C and E and try them both ways round - you shouldn't get
anything at all as they are 2 back to back diodes.
If you don't get 2 diode readings, move the probe to another terminal and
try again. If you still don't get 2 diodes, move to the third terminal. If
still no 2 diodes, reverse the probes and do the same again. When you find
2 diodes, the negative probe is on the base and it is PNP. Again you should
get no conduction at all on the C and E terminals. Any reading other than a
typical diode means a blown transistor.

Unfortunately this method doesn't tell you which of the C and E terminals is
which. I have a transistor tester for that or look it up in my equivalents
book. This method is really for checking quickly for blown devices.

You should also be aware of Darlington transistors that are actually more
than one transistor on the same chip. These will read differently to the
above.


Gareth.

 

[NoRk1]

Hi, it should not conduct. Here is an overview:
Whyen you measure PNP transistors you measure a diode conducting from
C to B, and from E to B. NPN has the diodes conducting from B to E,
and from B to C.

C to E, and E to C, should never conduct, but do not touch the base. A
very small base current is enough to let the transistor conduct.

Regards,
Pieter Hoeben

Thx Pieter,

I did what u said and the pnp test worked fine. But that leaves me
with one more question... according to textbooks i've read pnp
should allow current to pass through E to C when there is no
current at B..

So why wouldn't the continuity test accross E to C work?

I constructed the most simple npn which works fine replacing the
npn with a pnp i excpected the reverse result but got nothing.

Thanks
Nathan

 

[Gareth Magennis]

Thx Pieter,

I did what u said and the pnp test worked fine. But that leaves me
with one more question... according to textbooks i've read pnp
should allow current to pass through E to C when there is no
current at B..

So why wouldn't the continuity test accross E to C work?

Either the textbook is wrong or you have misread the text. Read it again.


Gareth.

 

[Pi]

I did what u said and the pnp test worked fine. But that leaves me
with one more question... according to textbooks i've read pnp
should allow current to pass through E to C when there is no
current at B..
So why wouldn't the continuity test accross E to C work?

The current between C-E is the gain factor * Base current. If there is
no base current, there will be no CE/EC current.

I constructed the most simple npn which works fine replacing the
npn with a pnp i excpected the reverse result but got nothing.

Because the currents flow in the other direction. This will not work.

Regards,
Pieter