I picked up some optocouplers cheap from the bargain bin. I was expecting to find 4 useful pins (2 in, 2 out, 2NC), but these turned out to be 4N32 units with a mysterious pin 6 connected to the base of the output phototransistor. Can I ground this pin to get the device to work as a 4 pin coupler? Connect it to supply voltage?
I just want to switch the output side on/off without an electrical connection to the circuit on the input side, no amplification, inversion, etc required.
Thanks, Chris
Chris,
You can use the 6-pin optocouplers just like the 4-pin ones if you leave the transistor base unconnected. In 4-pin optocouplers, the phototransistor turn-off time is usually quite a bit longer than the turn-on time. This limits the maximum repetition rate and distorts the waveform. For higher speed operation, the base connection of the 6-pin optocoupler is tied to the emitter via a resistor. This reduces the transistor turn-off time (drains charge from the base quicker). There are some good application notes on the Vishay-Telefunken web site that explain this and can help you figure out how to size the resistor if you are trying to speed up your optocoupler or reduce the distortion.
You can sometimes just leave it unconnected but it is better to connect it to the emitter via a 47k resistor.
The b-e resistor trades CTR for speed. It also increases the breakdown voltage of the transistor, which is sometimes useful.
John
Ah. This was probably Jagman's problem when he was using the same part...
I cant speak with experience but would imagine you could use them '4 pin' and should the need arise the base would give you an alternate switching method ?? (when in doubt experiment :-)
-- Regards ..... Rheilly Phoull
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