Re: Speeding optoisolators



James Arthur wrote:



This will smoke eventually. The opto current is not limited.


Not so. The opto's current transfer ratio (CTR) limits the
phototransistor's current.

I feel suspiciously about the circuits with the potentially unlimited escalation of the current. The data*** guarantees the minimal CTR but it says nothing about the maximal.

Since the LED current is about 2mA, the
output current is about the same, and the opto's phototransistor will
dissipate 10mW, typical. The detector in the specified opto can
safely dissipate 150mW, which would be quite impossible given the
combination of limited LED current, CTR of roughly 100%, and 5v
supply.

Yes, it won't smoke at once, but it will smoke eventually.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com


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