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Photocoupler Saturation?

mrblueblue1234

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Hello everyone!

There is a photocoupler (LTV-817) that I am using that is seeing a power glitch (~1 microsecond) and it seems to take a long time for the photocoupler to recover (~35 microseconds).

shunt_power.jpg


The blue trace is the power glitch that I am seeing and the yellow trace is the "output" of the photocoupler.

I was wondering if anyone had ideas on reducing the response time of the photocoupler (the yellow trace). At the output (yellow trace) what is happening? The part is saturated?

Thanks for any ideas!

Datasheet link:

http://optoelectronics.liteon.com/upload/download/DS-70-96-0016/LTV-8X7 series 201610 .pdf
 

KlausST

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Hi,

Please show your complete input and output circuit of the photocoupler.
We need to determine where exactly your scope probes are connected
And we need to determine the photocoupler current (not the voltage) at input and output ... at normal conditions and at glitch...

Klaus
 

mrblueblue1234

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Hello Klaus,

This is the connection through the photocoupler:
opto.png

This is output of the photocoupler to our comparator:

comparator.PNG

The blue trace in the previous post is the measurement across R421 (1O OHM) with a differential probe when the high voltage lines (HV_P and HV_N are shunted to ground.)

The yellow trace is what is measured from the emitter of transistor across R422 to ground. (signal Over_I).

The purple trace is the output the comparator U94 meaused on test point TP59.

Thank you for any ideas.
 

SunnySkyguy

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Relaxation time during cutoff can be reduced by reducing Re to RL=100Ω and turn ON, VCE=2V, IC=2mA . Load capacitance affects the RC decay (pF) with emitter current shutting off, output impedance rises rapidly.
 

crutschow

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If lowering the load resistor value still does not give a fast enough response, you may have to use a faster opto such as a 6N136 (LTspice simulation below).

Capture.PNG

How fast a response do you need?
 

FvM

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If you read the datasheet of the used opto coupler, you realize that the shown recovery delay is in the expectable range. The fast attack time is in contrast suprizing, but it's only achieved by overdriving the optocoupler massively.
 

Easy peasy

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there is a fair chance your led will eventually die if subjected to unlimited current flow pulses ...

opto's turn off slowly due to carriers stuck in the base region of the o/p xtor - faster opto or better ckt or both, needed ...
 

mrblueblue1234

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Crutschow,

It would be nice to be as fast a possible. Closer to the 1 uS range instead of 35 uS? :)
 

schmitt trigger

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If you are willing to invert the signal by looking at a signal from the collector instead of the emitter, you could use a cascode NPN transistor.

But to reach 1 uS response time, perhaps you will require to connect the optocoupler in a photodiode mode.
 

KlausST

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HI,

your circuit:
I don´t like the idea to drive a LED without current limiting resistor.
I assume this makes the LED current unpredictable. In worst case you kill the LED.
When I see this correctly, then the pulse is about 6V .... this is way off specification, probably several Amperes of LED current.
--> Try to add a series resistor.

Klaus
 
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