I am using heater circuit for bio-medical application ( to achieve 37 degree Celsius for blood samples ). I am using opto-isolator P781F to isolate control circuit ( Micro-controller I/O pins – H1G shown in schematic) and heater.
Link for opto datasheet:
http://www.farnell.com/datasheets/90914.pdf
My schematic is as shown below:
H1G= I/O pin from LPC2138 controller
Heater = 12V,23 Ohms Silicon Heater
But, I am little worried as I have connected controller I/O pin directly to cathode of LED of opto. So, is there any loading issue I can face, what parameter will come in picture to check I/O pin source/sink capacity with OPTO pin.
Will this cause any loading or impedance problem? Do I need any buffer circuitry from controller I/O to opto circuit.
Thank you in advance.
Best Answer
Assuming you're using a 5V-tolerant output in 3-state mode, and that the 5V is only present when 3.3V is, then the voltage should be okay.
Current will be about 7mA, which is almost twice the current at which the output voltage is guaranteed to be 400mV. Chances are very good that it will work okay, but a very conservative approach would be to buffer the output with something like a transistor.
Given the implied supply sequencing requirement, it would simplify your life and eliminate a worry if you just put the transistor in there (to drive the opto). An MMBT4401 with a few K base resistor would work fine. The 1K resistor is not necessary- but if you wanted to protect the MOSFET gate (against nasties on the 12V bus) by adding a zener it would be useful.
Your margin on CTR looks appropriate.
Here's a schematic showing Q1/R1 to buffer the output and an optional 2-BJT MOSFET gate driver. Note that the transistor inverts the logic (GPIO high = ON) compared to your original circuit.
simulate this circuit – Schematic created using CircuitLab
Outside the scope of this question, but you may need to add a driver circuit on the MOSFET side of the opto to make it switch faster. A couple BJTs would do it.