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 .
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 .