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Topic: PNP Transistor not 'shutting off' when increasing voltage. (Read 1 time) previous topic - next topic

Inevitableavoidance

I'm trying to multiplex small segments of LED Strips.
I'm sinking using TLC5940's, i'm sourcing using a ULN2803 driving PNP transistors. (BC577)

I started out at 12v, which neatly gave me about 20mA on each rgb led channel.
Since I'm multiplexing I wanted to increase the current going to the LED strips.
I did this by increasing the voltage to 18v.

Now, however, the transistors are not 'shutting down' completely, does anyone know how this happens?

I'm using a 2.5kohm resistor between the base and the ULN2803 outputs.

dhenry

Many things can go wrong. Without a circuit diagram, it is hard to know what.

Grumpy_Mike

Quote
I'm using a 2.5kohm resistor between the base and the ULN2803 outputs.

You need pull ups to the +ve and a resistor in series with the base.

Inevitableavoidance


You need pull ups to the +ve and a resistor in series with the base.

Could you please explain what kind of resistors from where to where that should be?

Grumpy_Mike

Put a 4k7 resistor one end to the base, the other end has a other 4K7 to the +ve and is also connected to the output of the ULN2803.
Values are not critical.

Inevitableavoidance

So then i'd have the the emitter to 18v, a 2k5 from base to the ULN, a 4k7 from the emitter to base, and another 4k7 from the emitter to the ULN?

Runaway Pancake

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Inevitableavoidance

Ik hooked up my scope to the collecter and to ground, and noticed something what i consider 'odd'.

When using 12v, I read about 3v when the transistor should be off, and 12v when it should be on.

When using 18v, i read about 12v when the transistor should be off, and 14v when it should be on.

What could this mean?

Inevitableavoidance


So then i'd have the the emitter to 18v, a 2k5 from base to the ULN, a 4k7 from the emitter to base, and another 4k7 from the emitter to the ULN?

Loose the 2K5 and yes.

Without the 2k5 nothing's happening at all..

Docedison

A PNP transistor is effectively a diode from base to emitter ( base cathode ) and pulling too much current from the base WILL destroy the transistor, Short the base to ground and the transistor WILL explode or shut down the power supply, in either case the transistor is toast. ALL PNP transistors need some method of current limiting (A resistor) in the base circuit. If it was an NPN directly pulling the base down, likely the NPN is toast as well.

Bob
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Grumpy_Mike

Then you are missunderstand what I was saying. Can you draw what you are doing.

Inevitableavoidance


A PNP transistor is effectively a diode from base to emitter ( base cathode ) and pulling too much current from the base WILL destroy the transistor, Short the base to ground and the transistor WILL explode or shut down the power supply, in either case the transistor is toast. ALL PNP transistors need some method of current limiting (A resistor) in the base circuit. If it was an NPN directly pulling the base down, likely the NPN is toast as well.

Bob


To be honest, i don't quite see how this is relevant..?

Also, when not connecting anything to the transistor except for 12v on the emitter and the collector to the 'anode' of the led strips, i measure about 11v from the collector to ground..

Grumpy_Mike

Quote
Also, when not connecting anything to the transistor except for 12v on the emitter

Transistors are current devices. Do not leave the base floating, the results you see will not make sense.

Inevitableavoidance

#14
Oct 15, 2012, 12:52 am Last Edit: Oct 15, 2012, 12:54 am by Inevitableavoidance Reason: 1

Then you are missunderstand what I was saying. Can you draw what you are doing.



My initial setup was:

Leds
|
|
Collector

Base ------2k5-----ULN2803 output

Emitter
|
|
18v


What i understood you meant was:

Leds
|
|
Collector

Base --- 2k5 --- ULN2803 output
|                      |
4k7                    |
|                      |
Emitter ----------4k7
|
|
18v

(edited, misdrew something)

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