Author Topic: Opto-Schmitt question  (Read 571 times)

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Offline Chris WilsonTopic starter

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Opto-Schmitt question
« on: July 29, 2019, 01:16:36 pm »
I am experimenting with an opto-Schmitt device, an H11L1M. If I input a sine or square wave of 50% duty cycle I can only get the same duty cycle on the output pins at a certain input LEVEL. If I go under or over that level the duty cycle changes from the original signal's 50%. I assume this must be normal? If so why does this occur please? Thanks.

www.farnell.com/datasheets/1793287.pdf
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Offline soldar

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Re: Opto-Schmitt question
« Reply #1 on: July 29, 2019, 04:42:05 pm »
While somebody else more knowledgeable comes along my guess is that all semiconductor switches have inherently certain Ton and Toff delays cause by different factors and only under certain circumstance would the delays be equal thereby preserving the duty cycle.
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Offline David Hess

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Re: Opto-Schmitt question
« Reply #2 on: July 30, 2019, 09:24:11 pm »
Square wave drive should produce a constant duty cycle on the output.

The input is suppose to be driven with a current and not a voltage.
 

Offline schmitt trigger

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Re: Opto-Schmitt question
« Reply #3 on: July 30, 2019, 09:38:29 pm »
Take a two-channel scope, and overlay the input and output waveforms, to determine the threshold changes.

If you have a trianglewave signal generator, it becomes far easier.
 

Offline magic

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Re: Opto-Schmitt question
« Reply #4 on: July 30, 2019, 09:45:29 pm »
I played with this device at one point. Both things are true.

There is some LED current threshold which turns the output on and another (lower) which turns it off. So if your current waveform isn't symmetric around those points, output duty cycle has no reason to be 50%.

At high speeds propagation delays become significant and they may be asymmetric and dependent on things like output pullup resistance and input current limiting resistor. Getting 50% duty cycle at 1Mbps takes some tweaking.

In fact, propagation delay is so long that it approaches one full bit time at circa 1.3Mbps, right before the device gives up and starts dropping bits. It's fun to watch on the scope as input and output change their state almost in unison, even though it is an inverting device.
 


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