-2

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Why is it okay to connect the base of CNY17-1 to the gnd with a collector?

winny
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jji
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    I expect that it isn't OK. – Andy aka May 22 '23 at 16:19
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    https://electronics.stackexchange.com/questions/48462/optocoupler-with-phototransistor-base-lead – Chris Knudsen May 22 '23 at 16:22
  • Do it with some ten's of kOhm. – Antonio51 May 22 '23 at 16:31
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    But base is not connected to the collector. It is connected to the emitter. Connecting with collector is not OK. – Justme May 22 '23 at 16:52
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    Where did your circuit come from? What is the motivation or justification for the connection shown? Also why is Q1 backwards? – Tim Williams May 22 '23 at 17:36
  • The circuit as shown makes no sense. The opto's emitter LED is always on. U2 appears to be a switch that leaves U3A-2 input floating (because the opto is disabled). And Q1 seems to be upside down, so the B-E junction may be reverse biased. Finally, there seems to be no point in driving a string of silicon diodes D1-D3. – PStechPaul May 23 '23 at 07:41

2 Answers2

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Shorting the base to emitter directly will cause the optoisolator to stop coupling the input to output. It won't hurt anything. If that's your intention, it's certainly okay.

More often the base to emitter is bridged with a resistor of many kΩ, which speeds up the optoisolator, while at the same time decreasing the coupling (CTR). Somewhere around 10kΩ (wild guess) the opto will typically stop coupling input to output in the above circuit.

Spehro Pefhany
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0

It is ok to wire the base to the emitter through a "high" (10k to 100k) resistor.
here is an example with a 4N33 which can work until 100 kHz ...

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If you wire it directly, nothing will happen.

enter image description here

Antonio51
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