Optoisolator with Transistor Out Problems

[jnnewton]

Hello, I have an output where I want to output a pwm signal (1.3kHz, 0-5V). I also would like to use a optoisolator on this output. the signal is generated via a **broken link removed**

**broken link removed**

I have a good signal coming off the microcontroller, but have tried varying the base resistor value and the pull-down to get a good output, but the problems are:

if there is no pulldown, the output sits at around 5 V. if the pulldown is as it is, the signal still only swings from around .3 V to 2.4V.

I do not have a good idea of how to fix this. Any suggestions?

 

[Mikebits]

Try putting the emitter to ground, adding a resistor from the collector and V+ then take the output from the collector.

 

[Torben]

Here's an illustration of what Mike suggested:

The resistor values will have to be tweaked to match what you have.


Torben

 

[Papabravo]

Hello, I have an output where I want to output a pwm signal (1.3kHz, 0-5V). I also would like to use a optoisolator on this output. the signal is generated via a **broken link removed**

**broken link removed**

I have a good signal coming off the microcontroller, but have tried varying the base resistor value and the pull-down to get a good output, but the problems are:

if there is no pulldown, the output sits at around 5 V. if the pulldown is as it is, the signal still only swings from around .3 V to 2.4V.

I do not have a good idea of how to fix this. Any suggestions?

You should look at the current transfer ratio (CTR) of the opto. It can be as low as 20%. What does that mean? It means if you drive the LED part of the opto with 10 mA, the the output transistor can only source or sink 500 microamps! Also CTR degrades with time so things only get worse.

BTW 5 V across 100 ohms is 50 mA!

 

[Mikebits]

The TLP283 part has a good CTR spec. Min 100 Max 400 @1ma

 

[audioguru]

The datasheet for the Toshiba opto-coupler is half written in Yapanese so I couldn't see its max ratings. Its LED might work at 25mA.

The 470 ohm input resistor limits its input current to only 6.8mA but the PIC has a max allowed output current of 25mA. The 470 ohm resistor should be changed to 100 ohms.

The opto-coupler has a minimum coupling of 100% so if the output resistor is 180 ohms then the max output voltage will be +4.3V.