optocoupler resistor

Thread Starter

jrekha

Joined Mar 31, 2010
11
i am using hcpl2630 opto. my input is 3.3V and i need to put a resistor at cathode to gnd, to limit the current.how do i fix its value.how do i get this opto's CTR value?
 

kkazem

Joined Jul 23, 2009
160
Hi,
I disagree with the sergeant. If you look carefully on the first page of the datasheet, they are recommending that you use a minimum of 6.3 mA, so that over time as the LED output diminishes by up to 20%, you'll still have the same output reception. Notice that 6.3mA * (1-0.2) = 5mA. The max input voltage of the device is 1.4 V forward on the LED, so R= (3.3V-1.4V)/6.3mA = 301 ohms, which is a standard 1% value.

You can probably get away with using 380 ohms, but an old engineer's rule of thumb is always design for the worst case. If you use 300 Ohms, you're covered; if you use 380, you may or may not have problems now or later as the Opto degrades, which they all do over time (mainly the LED section).
Regards,
Kamran Kazem
 
Top