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Newbie Question on NPN Transistors

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Dragging up an old post!

Greetings everyone, I have been playing with the transistor switch circuit in Uncle $crooge's post for exactly the same application - switching segments of a 7 segment display (multiplexed) - from a 74HC595. The only difference between this circuit and mine is that I have put a 'pull down' resistor from the output of the 595 to ground to prevent the base of the transistor from floating.
I have a couple of questions;
Why do we need to try and pull almost max current from the 595? Surely this will eventually damage the chip.
I have experimented with various values for the resistances and found that what seems to work best is R(base) = 5.1k , with R(pull down) = 1K, R(load) = 510. I am using 3V to simulate the signal from the 595 and a different 3V source for load. I have found that using any R(base) value less than 5.1 K, causes the LED to light without the 3v for the load being connected (must be getting a leak through the transistor). Obviously I will have to change these values for the 5v that will be eventually used. I am just thinking that the 100 and 10 ohm resistors will cause to great a current, or does the transistor require this base current to switch properly.
Last one.......
It does not seem to matter if I use a BC337 (NPN) or a BC327 (PNP). I get exactly the same outputs. Why would that be?

Sorry for the 'noob' style questions, but that is what I am ;) Well not really, its just been over 20 years since I played with electronics!

Regards Jon
 
-jon- said:
No one able to help?

Guess I shall have to keep looking elswhere..........

Cheers
Post a schematic. I don't think your question has much if any relevance to the OP's question.
 
Well, the schematic is here......

https://www.electro-tech-online.com/attachments/pnp_led_driver-gif.6809/

The only difference between this circuit and mine is that I have put a 'pull down' resistor from the output of the 595 to ground to prevent the base of the transistor from floating.

Q1. Why do we need to try and pull almost max current from the 595? Surely this will eventually damage the chip. (32ma / 35ma)

Q2. It does not seem to matter if I use a BC337 (NPN) or a BC327 (PNP). I get exactly the same outputs. Why would that be?

Jon
 
-jon- said:
Q1. Why do we need to try and pull almost max current from the 595? Surely this will eventually damage the chip. (32ma / 35ma)
Transistors saturate best with up to 1/10th of their collector current as the base current.

Q2. It does not seem to matter if I use a BC337 (NPN) or a BC327 (PNP). I get exactly the same outputs. Why would that be?
Impossible, the transistors are completely different.
 
audioguru said:
Transistors saturate best with up to 1/10th of their collector current as the base current.


Impossible, the transistors are completely different.
If you substitute NPN for PNP in the schematic, the LEDs get driven directly through the forward-biased B-C junction, with no transistor action. However, the logic will be reversed (high level from IC --> LED on).

Jon, are you using the same resistor values as in the schematic? You don't need 32mA of base current if you are reducing the collector current. As Audioguru pointed out, a good rule of thumb for driving a transistor into saturation is to set Ib=Ic/10.
 
Thanks for the replies, that explains the NPN / PNP difference to me.

No I am not using the same resistance values as the diagram.

For the base, I am using 5.1k and for the collector 510 ohms. Both having 5 volts (app) creates the 10:1 ratio you mentioned. These values were decided upon using trial and error on a test circuit and worked on the test.

Jon
 
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