### Author Topic: Optocouplers -- Collector dark current vs Off-state collector current  (Read 972 times)

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#### MicroControllerGuy

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##### Optocouplers -- Collector dark current vs Off-state collector current
« on: March 16, 2019, 05:35:12 pm »
If i place a voltage across the Collector-Emitter of an octocoupler when the LED is completely off, what is the datasheet term for the Collector-Emitter leakage current?
Is it the "Off-state collector current", or the "Collector dark current?

I found this definition of collector dark current on a website:
"When the phototransistor is placed in the dark and a voltage is applied from collector to emitter, a certain amount of current will flow. This current is called the dark current (ID). This current consists of the leakage current of the collector-base junction multiplied by the dc current gain of the transistor. The presence of this current prevents the phototransistor from being considered completely "off", or being an ideal "open" switch."

If the quote is correct, what is  "Off-state collector current"?

Here is an example of a datasheet specifying both parameters (Toshiba LTP185): https://toshiba.semicon-storage.com/info/docget.jsp?did=14111&prodName=TLP185(SE

Can someone please tell me the difference between these two currents, I'm confused?

Thanks!
« Last Edit: March 16, 2019, 05:44:26 pm by MicroControllerGuy »

#### magic

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##### Re: Optocouplers -- Collector dark current vs Off-state collector current
« Reply #1 on: March 16, 2019, 05:48:18 pm »
One is specified for Vf=0.7V, the other presumably for Vf=0.
Extrapolate figures 11.4 and 11.9 to see what's likely to happen at Vf=0.7V.

Friendly reminder to always look at test conditions, including those "unless stated otherwise" at the top. These really define what the spec means in practice, not just the spec's name.

That being said, I think in most optocoupler datasheets "dark current" will be the exact thing you are looking for.
« Last Edit: March 16, 2019, 05:55:28 pm by magic »

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#### MicroControllerGuy

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##### Re: Optocouplers -- Collector dark current vs Off-state collector current
« Reply #2 on: March 16, 2019, 06:13:32 pm »
Thanks for the reply, Magic, this makes sense now

I guess if Toshiba had specified 'VF = 0" in the Test Conditions for the Dark Current, or had specified IF down to 0.0mA in figure 11.9, it would have been more obvious to me.

Smf