Re: optocoupler trouble


"ted" <strnbrg59@xxxxxxxxx> wrote in message
news:0da35bb4-12a5-4b99-84a2-977ebfa4d906@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I'm getting strange results from an optocoupler (aka optoisolator, and
this is the kind with transistor output). In brief: nothing I do
seems to make the transistor "conduct", except very feebly.

Here's my setup:

Diode anode: 0V/1.07V, 0.4mA
Diode cathode: ground

Transistor collector: connected to a 330 Ohm resistor which is
connected to 4.92V.
Transistor emitter: ground
Transistor base: open

When the diode anode is at 0V, the transistor collector is, as one
would expect, at 4.92V.
But when I give the diode anode 1.07V, the transistor collector drops
to just 4.88V. I want it to go down to 0V and I don't understand why
that's not happening.

The diode in an optocoupler is an Infrared LED. It takes more than 1.07
volts to turn on. You can see this with the feeble current of 400uA you
measure. Run the diode on 5 to 10 mA. The voltage should be nearly two
volts.

If this thing has to run fairly fast, place a 10 K resistor from the base of
the transistor to ground. The open base will slow operation appreciably.


.

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