Electronics > Beginners

Please explain 'no' current to PNP transistor

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windmill john:
Hi All,

Please have a read of the attached picture.
I can see why the NPN transistor turns on the top half of the LED. Not totally sure why the PNP doesn't turn on the bottom half at the same time.
To clarify my lack of understanding, if that's not a contradiction in term! I guess this is an OHM's law question.
There is a positive feed to the base of the PNP via the 2.2K resistor. So is the reason it does not light the bottom half of the LED purely because the resistance 2.2K ohm is a lot higher than the 1K resistance to the base of the NPN?

The comment about the diode confuses me because surely current flow is left to right through it in the diagram?

Thanks.
Beginner section, so be gentle  ;)

langwadt:
flip the picture upside down then it is much easier to understand

shapirus:
When the switch is open, current flows (remember that current direction is positive->negative, and the arrows on the transistor symbols follow this convention) like this:


--- Code: ---battery_pos ->  1K resistor -> NPN base -> NPN emitter -> LED anodes -> LED cathode (COM) -> battery_neg
             `->     NPN collector    -^

--- End code ---

It can't flow via the PNP base, because its possible path is blocked with the diode.

When the switch is closed, NPN transistor can't turn on, because current is diverted back through the diode to battery negative, being the path of lowest resistance, instead of going into its base. But now current can flow via:


--- Code: ---battery_pos -> PNP emitter -> PNP base -> 4.7K -> switch -> battery_neg
                           `-> PNP collector -> LED anodes -> LED cathode -> battery_neg

--- End code ---

The 2.2K resistor is optional, as I see it. Possibly added to pull the PNP base high to prevent reverse current flow through the germanium diode (I don't remember off the top of my head what their typical values were).

It's easy to understand cicruits like this when you consider the possible paths and directions for current to flow.

Using animated simulators like https://everycircuit.com/app helps a lot in understanding the basics.

bdunham7:

--- Quote from: windmill john on February 02, 2023, 05:54:40 pm ---There is a positive feed to the base of the PNP via the 2.2K resistor.

--- End quote ---

And?  What is needed to turn on a PNP transistor according to the last sentence of the first paragraph?

The operation of the diode and the need for it to be germanium is a bit more complex, but yes current will flow through it from left to right when the switch is closed.

shapirus:

--- Quote from: bdunham7 on February 02, 2023, 06:32:37 pm ---The operation of the diode and the need for it to be germanium is a bit more complex, but yes current will flow through it from left to right when the switch is closed.

--- End quote ---
This is a good point btw. Considering that germanium diodes are now mostly phased out, this circuit isn't quite an ideal educational example, and even if they were still used, it would need an explanation. A good homework task will be to redesign this circuit with a requirement that the germanium diode must be replaced by a silicon one (no cheating! P-N diodes!), and still have it working properly :)

up: in fact, I forgot about those LEDs and their junction drop. This circuit, unless I'm wrong, will work as expected with a silicon diode too, provided that the NPN transistor is also a silicon one.

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