Elementary Transistor Problem

Yeah these questions are really hitting the fundamentals, but thanks for anyone who helps me :)

I have a typical NPN transistor, hook a battery to the collector, add voltage to the base connect an LED to the emitter then to ground and horray, it lights up.

I try the same with a PNP minus voltage to the base, and the LED doesnt light up (contrary to what i thought was supposed to happen). If i hook up the base to ground then the LED lights up.

I thought that with no voltage to the base the PNP stays on and allows voltage to pass, unless no voltage means connect to ground which might makes sense concidering ground is 0 volts or somesuch. Am i at least on the right track here? Thanks for any help.

Jesse

Reply to
ragtag99
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Nah :-) The pnp needs current flowing through the base like all transistors of that ilk. Mebbe you are confusing it with a p channel mosfet ? Incidentally (ignoring the lack of resistors used) conventionally the load goes above a npn and below a pnp.

Cheers ......... Rheilly P

Where theres a will, I want to be in it.

Reply to
Rheilly Phoull

You need to select a resistor to use on the base to limit the current to something the base can withstand. That info is in the data sheet.

The way of bipolar transistors: if the base is made positive on an NPN

-With respect to the emitter- by its turn on voltage ~ plus point six with silicon, the transistor is biased on.

PNP works the same way but the base needs to be minus point six with respect to the emitter.

Self biasing the transistor - connecting the collector of an NPN to positive and putting the load on the emitter to ground requires about

4 volts of power supply voltage to make it work.

PNP is the same - feed a negative voltage to the collector and put the load on the emitter and the power supply needs 4 volts or so to turn it on (the base-emitter junction in each case is still turned on by six tenths of a volt - it just takes more PS voltage to work)

The reason you see NPN's in the ground leg and load positive to the collector - it is more efficient. The PNP's emitter would go to the positive supply rail and the load be between collector and ground - for efficiency. Grounding the base via a current limiting resistor of the right size would turn it on - or connecting the base to a voltage (through a resistor) more minus than ground would also turn it on.

Mosfets are similar - but take about four volts on the gate to turn on (with respect to the source) and use virtually no turn on current so the gate resistor can be a relatively high value compared to bipolar.

Touching one's finger to the gate will usually pick up enough AC from the surroundings to have it switch on and off at the power line frequency or nearby radio stations. (but that is not good practive since the gate has to be protected from static electricity to keep the initial touch from blowint it out) N channel is similar to a NPN, P channel is similar to PNP.

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Reply to
default

What's the circuit you made with the PNP transistor?

You should really use a resoistor on the base to limit the amount of current that flows to/from it, the LED usually needs a resistor too - if you're runnin the circuit from 2.4V maybe not.

ground is not always "no voltage" compared to the transistor.

To turn a NPN transistor on you need to feed the base until it's more positive than the emitter, to turn it fully on you need t makr the base more positive than the collector (for a PNP is the same but more negative instead)

these voltages are measured ftom the other pins of the transistor not from "ground" (unless the transistopr is connected to ground)

In your first circuit the current to light the LED could have been flowing through the base. put a resistor in series with base wire, 10K would be a good size probably - it won't pass enough current to light a LED brightly (from ordinary batteries) so it'll be obvious when it's the transistor running the LED.

Bye. Jasen

Reply to
jasen

You've built an emitter follower. To do it with a PNP, use _exactly_ the same setup, but turn the battery, LED, and meter leads around: in a PNP, the current flows the opposite way of an NPN, and so all of the polarities are upside-down. :-)

I'd also use some kind of current limit in series with the LED; just a resistor should be fine.

Good Luck! Rich

Reply to
Rich Grise

Just wanted to clarify: P-Channel MOSFETS will not conduct with no voltage to the gate. The only difference between P and N channel, enhancement mode MOSFETs is that N channel devices requires a positive gate drive voltage for conduction while the P channel device requires a negative gate drive for conduction.

JFETs and Depletion mode MOSFETS, both N and P channel, will conduct with no gate voltage. But enhancement mode MOSFETS will not. (;

Dorian

Reply to
Dorian McIntire

It's that, or he's using the B-E junction as a diode.

Bye. Jasen

Reply to
jasen

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