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Topic: optoisolators. (Read 341 times) previous topic - next topic

cjdelphi

Anyone know if it's a special type of pnp/npn transistor inside of one?

I might just grab an led and ldr later on, and experiment, what i see from scematics is an phototransistor (?)  Which is the base pin, but i don't see any current limitors (resistors) either high or low side just a direct short..

Am i right in presuming this is not an issue because the current is very small in the base pin?

For example.

1 phototransistor
1. Torch / flashlight.

I can create a short by turning the light on ... and collector emitter will flow even though i don't have ground tied both ends.

jremington

Most optoisolators that I've used have an NPN phototransistor as the detector and provide just the collector and emitter connections. The light acts to inject base current, allowing current to flow from the collector to emitter. There is an effective beta parameter, but it is based on illumination intensity rather than base current. No current limiting resistors are included in the package because those will depend on the circuit design and light levels anticipated.

cjdelphi

Ahh ok...    I suppose if i cut into the right place on the transistor, i could make my own phototransistor (and have the 3rd base pin D:)




Here's what concerns me..



Notice on the camera circuit, there's a dead short between + and - of the camera circuit, sure the current will be limited by the intensity of the light shining on the detector (making it low) esp with a 270ohm resistor...

But what happens if more current was to flow, is that not a direct short on the camera side?  or is the current just too low to worry?




cjdelphi

A heavy night of drinking does no good for ones brain.

I got it now.... the resistor should be  on the camera's side to limit the current (as it's a direct short to trigger off the camera) sorry for being so slow lol

jremington

The first circuit posted is commonly used. The resistor is necessary to limit the LED current. There may be no need for a current limiting resistor on the camera side for two reasons. One, the light from the LED is rarely enough to allow damaging currents to flow in the phototransistor and two, the camera undoubtedly has current-limiting resistors in it's input circuit. After all, switches are used to trigger cameras.

And yes, some optoisolators do give access to the base lead. I've never had reason to use that feature.

The advantage of a phototransistor over a photodiode detector is related to the current gain of the transistor -- far less light is required to achieve a certain current flow with a phototransistor. Phototransistors are slow and nonlinear in their response, so photodiodes are preferred for linear intensity measurement and speed.

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