I have a 2-terminal solid-state (pressure) switch device which is known to be a bipolar PNP or NPN device. The 2 terminals are emitter and collector with known pin positions. My question is:
In the case of INACCESSIBLE base terminal of a bipolar device, is there any easy way to identify whether it is a PNP or NPN type of transitor?
Thanks for any help.
One trick that I like, that often (not always) works: grab the tip end of a scope probe with the fingers of one hand; you'll usually see a volt or more of 60 Hz hum. Now touch the node in question with a finger of your other hand. If you now see positive half-wave humps, it's an NPN, and vice versa. The gadget should be grounded, of course.
John
Depends what's connected to the base.
If the base were effectively open-circuit and you know which is the collector and which the emitter, you could check which way round the device exhibits the lower breakdown voltage.
Use a voltage of say 12V, with a series resistor to limit current - something like 22k-ohm, but it's not particularly critical.
If the device exhibits a breakdown voltage less than 12V with the collector negative, then it's an NPN, otherwise it's a PNP.
(Note that this test relies on reverse-breakdown of the b-e junction which is not particularly good for a transistor and causes progressive degradation of several parameters, so don't take any longer than necessary).
Do you know what's connected to the base? It could be that a test along the lines described above would give adequate indication, depending what base connection is present.
-- Ted Wilson Senior Circuit Design Engineer BAE Warton edward.wilson@baesystems.com "The gods do not deduct from man\'s allotted span the hours spent in fishing."
Hello John,
Unless you live too close to Telegraph Hill.
Regards, Joerg
If you know where it goes in the circuit, you can just check the voltage polarity from emitter to collector. NPN will be positive on collector, and PNP will be negative.
-Bill
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