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# optocoupler

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#### panfilero

##### Member
I was using an ILQ2 optocoupler the other day, and I wanted the thing to turn on when I put 24V across the diode and a series resistor, and I wanted it to turn off at anything less than 10V... So... I thought, I'll just select the resistor where at 24V it'll give ~10mA and under 10 it should be approx half the current and it'll not turn on... but I was wrong the thing turned on anyway... so I arbitrarily picked up a 15k resistor... the thing turned on at 24V.... the thing turned on at 8V... now 8V/15k = ~ 0.5mA... that's hardly nuthin.... 500uA.... but it still turned on (when I'm saying turned on I'm sayin that the LED inside the optocoupler was able to get the photo-transistor to turn on)...

my question: How is this possible? And is this not a good way to figure out a turn on and turn off voltage for the optocoupler... by picking a large series resistance?

Much thanks!

I was using an ILQ2 optocoupler the other day, and I wanted the thing to turn on when I put 24V across the diode and a series resistor, and I wanted it to turn off at anything less than 10V... So... I thought, I'll just select the resistor where at 24V it'll give ~10mA and under 10 it should be approx half the current and it'll not turn on... but I was wrong the thing turned on anyway... so I arbitrarily picked up a 15k resistor... the thing turned on at 24V.... the thing turned on at 8V... now 8V/15k = ~ 0.5mA... that's hardly nuthin.... 500uA.... but it still turned on (when I'm saying turned on I'm sayin that the LED inside the optocoupler was able to get the photo-transistor to turn on)...

my question: How is this possible? And is this not a good way to figure out a turn on and turn off voltage for the optocoupler... by picking a large series resistance?

Much thanks!

hi,
You could use a zener diode in series with emitter resistor, say 15V or so.

I did end up using a zener to solve my problem, but I don't understand why the resistor in series didn't work by itself

I did end up using a zener to solve my problem, but I don't understand why the resistor in series didn't work by itself

I haven't seen any shematics, but here's what I think:

If using a simple voltage divider, the divider circuit has an output impedance similar to both resistors in parallell, and any input to a led/optocouplers (hereafter I just call it led) input has very low impedance compared to the output impedance from the voltage divider.

Therefore, it's difficult to get the resistance values to match so that led/optocoupler turns on when voltage provided is 10 volts.

Since the diodes you ended up with probably has a very low forward impedance (if I can call it that), it's much easier to make the voltage match turn-on voltage for the led/optocoupler you're using.

However t's no problem using a voltage divider, but then you must use some voltage folower to get enough current into the led. I recomend you to use a regular npn bjt as common colector, with a resistor between emitter to led, or from colector to Vcc to prevent blowing the led.
Alternative you could use an opamp as comparator, and even make an positive feedback so that led could be either on or off. If supplied voltage is expected to vary, I think this would be the best option.

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