[SOLVED] LED Driver by using PNP transistor

Hi,

I need to drive LED by using PNP Transistor.
For that PNP Transistor - Emitter connect to VCC OR Collector connect to VCC?
Please find attached schematic for your reference



Please explain both method difference and when (which application) we need to use Emitter connect to VCC and Collector connect to VCC?

Current Flow:
PNP Transistor is -> Emitter to Collector
NPN Transistor is -> Collector to Emitter

So that PNP Transistor, we should connect Emitter as VCC?

Please explain and provide your suggestion

Thank You
Rajan

Hi,

I assume there are many many internet sources explaining bjt circuits in detail.
And there are a lot of pnp circuits driving a LED ....without going into detail.

In short:
* option A does not work
* option B will work.

May I ask why you don't want to use the usual circuit: NPN low side switching.

Klaus

Circuit A is operating the PNP transistor inverted. It generates collector current with current gain of about 1 only.

KlausST said:

Hi,

I assume there are many many internet sources explaining bjt circuits in detail.
And there are a lot of pnp circuits driving a LED ....without going into detail.

In short:
* option A does not work
* option B will work.

May I ask why you don't want to use the usual circuit: NPN low side switching.

Klaus

Click to expand...

Hi,
For my application, Initial power on state LED should not glow, so that i have chosen PNP transistor.
Earlier i am also using PNP transistor for LED driver purose and i have connected Emitter point as VCC and its working fine.

But one reference circuit design mentioned (Option A) PNP Transistor - collector connected to VCC. so that i have confused.
I have simulated Option A circuit in Proteus and LED is glowing.

So i want to confirm PNP transistor current flow direction (E to C OR C to E)

Thank You,
Rajan

- - - Updated - - -

Hi,

Could you please explain little bit brief for my understanding
PNP Transistor Current flow is E to C only? Then How option A circuit is working?
NPN transistor only i have connected VCC at collector point.

Thank You for your support,

panneerrajan said:

Earlier i am also using PNP transistor for LED driver purose and i have connected Emitter point as VCC and its working fine.

But one reference circuit design mentioned (Option A) PNP Transistor - collector connected to VCC. so that i have confused.
I have simulated Option A circuit in Proteus and LED is glowing.

So i want to confirm PNP transistor current flow direction (E to C OR C to E)

Click to expand...

You do not need to know current direction. Instead, think about if the base-emitter or base-collector junction is conducting and is forward biased. When the emitter of a PNP is positive then the base-emitter is forward biased when the base voltage is low then the transistor conducts collector to emitter with a high current gain. But when the collector and emitter are backwards then the base-collector conducts and is forward biased when the base voltage is low then the transistor conducts emitter to collector with a low current gain of about only 1. The datasheet for the transistor warns that the emitter-base junction has avalanche breakdown at a low voltage of typically 7V but some break down with only 5V. 2N3906 and BC557 have a maximum allowed emitter-base reverse voltage of only 5V. Then it is best to use a transistor properly, not backwards.

But one reference circuit design mentioned (Option A) PNP Transistor - collector connected to VCC.

Click to expand...

It's mistaken.

I have simulated Option A circuit in Proteus and LED is glowing.

Click to expand...

As said, current gain B is about 1 in this configuration, so the LED would be in fact "glowing". Not sure if Proteus is modelling inverse operation correctly. Base-emitter breakdown voltage mentioned by Audioguru won't be reached in a 3.3V circuit, in so far it's safe, but still useless.

Beyond the scope of this thread, there are a few useful application of inverse bipolar operation, e.g. for analog switching with less offset.