PNP Switch Issue

Thread Starter

Gord11

Joined Nov 27, 2015
14
Hello,

I have a problem with a PNP switch, it is an FMT722 configured as follows:
The emitter is tied to 5V, the collector feeds a 240R resistor which in turn feeds an IR LED to ground. The base is driven by a PIC, via a 1K resistor.

I know the 1K is not strictly the correct value, but i have tried various values.

The PIC drives the led at 38khz 50% duty, and it appears to work correctly, however noticed an issue when looking at the transistor output on a scope, in that the output is not cleanly switch 0-5 (or slightly less than 5) it is actually seems to be switching to around 5V down to 0.7-1V - not off!

If i disconnect the drive from the PIC and manually pull it low, it cleanly switches to the output to 5V, if i then pull the base to 5V, the output is off.

If i then use a function generator set to 38Khz square wave and feed this into the 1K base resistor, i get exactly the same problem.

However, if i reduce the frequency to 1Khz the output looks correct, i.e. clean transitions 0-4.8V.

I have also added a speedup capacitor across the base resistor, as i noticed the switch on / switch off times were not equal, this is now fixed.

I also tried changing the transistor for a IRML2244 fet, and i see the same issue.

So....

If i manually pull the base low, so the transistor is switched on, and i measure the voltages i get - around 1.2V across the LED, around 3.6V across the 240R resistor.

With the 38Khz running i see, 1.2V across the LED, but around only half of it is actually switching.

I am really confused about this, perhaps i am measuring something wrong, or don't understand what i am looking at.

Any suggestions would be appreciated .
 

Thread Starter

Gord11

Joined Nov 27, 2015
14
Remove the 10K base pull up and replace the base diodes with a short, then report back.

Hello ErnieM,

I have no base diodes on the circuit - i noticed the diodes on Bordodynov's simulation, but they are not on my base. I just have the base resistor.

Thanks
 

Thread Starter

Gord11

Joined Nov 27, 2015
14
Is this the sort of effect you're seeing?
View attachment 95437

Yes! That looks exactly it, although my On/Off pulse widths were not equal, so i added the speedup capacitor across the 1K resistor.

I am really interested in the software you have used to simulate this?

Are you able to help me understand why i am seeing this?

Thanks
Six more responses, still no schematic.
 

Thread Starter

Gord11

Joined Nov 27, 2015
14
Six more responses, still no schematic.

Really sorry about the schematic - i am on holiday at the moment, with just some books, pens, and paper. No drawing software on this machine, when i get back home i will post one.

Sorry!

The circuit is exactly as kindly drawn by Alec_T and Bordodynov above...
 

crutschow

Joined Mar 14, 2008
24,047
The PNP is switching fine and requires no changes for your application.
The observed waveform is because there is nothing to rapidly discharge the LED capacitance/stored-charge after the LED stops fully conducting below its normal ON voltage.
At that point the LED forward current becomes increasingly smaller due to the LED's logarithmic voltage-current relationship, and this give the long tail observed as that last volt or so of diode capacitance is discharged.
It has no significant effect on the switching of the LED so it can usually be ignored.
 

Thread Starter

Gord11

Joined Nov 27, 2015
14
The PNP is switching fine and requires no changes for your application.
The observed waveform is because there is nothing to rapidly discharge the LED capacitance/stored-charge after the LED stops fully conducting below its normal ON voltage.
At that point the LED forward current becomes increasingly smaller due to the LED's logarithmic voltage-current relationship, and this give the long tail observed as that last volt or so of diode capacitance is discharged.
It has no significant effect on the switching of the LED so it can usually be ignored.

Thank you crutschow,

I had no idea that the LED would store charge in this way.

The circuit is just for my learning benefit, and does appear to work. I thought it was an issue with the transistor.

Many thanks for the explanation.
 
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