help needed for optocoupler

[istian92]

hi every one i am a new user of optocoupler pic817 and for my pwm based h-bridge driving circuit i am using it.. is it a right choice for this circuit?...or should i need to replace it with some other package?

one more thing that i want to clear is,currently i am using emiiters output which is being further fed into buffer . so what if i take output from collector?
my circuit diagram is given in attachment

 

[Tony Stewart]

Current gain is the same <1 but bandwidth or Tr is best on emitter since voltage gain is 1.
What is ESR of driver? 50? bipolar? R7 is over selected for faster turnoff at expense of current drain and less gain.
Unfortunately your opto load is TTL meaning asymmetric load, needing 1.6mA worstcase for "0", so emitter impedance on "1" is low but = R5 for "0" =3k3 so that wont be <0.4V , will it!, so for TTL inputs, it must be bipolar drive or open collector.

So consider https://www.fairchildsemi.com/datasheets/NC/NC7NZ17.pdf 3ns 1.5 pF $0.60(1pc) or a logic high speed opto.

 

[Tony Stewart]

In other words, Emitter follower wont work into TTL due to active low current 0.4V/1.6mA=250Ohm max impedance for "0" at std. Levels which guarantee std immunity, which would draw >16mA in the high state or not actually reach logic 1.

This is why bipolar drive or open collector with pullup works but has slower rise time than fall time.

But this design needs careful attention to prop. Delay on each edge to preserve and prevent shoot-thru with at least 1-2 us of deadband for temperature and back EMF affects.Otherwise. Smoke may appear from your IC's from shorted drivers. So use Ohm's Law and compute RC delays or rise time to threshold where possible.

 

[AnalogKid]

What is the purpose of D9?
What is the purpose of R23?
Which power supply runs the NOR gates and the high voltage logic?
You need to add separate ground symbols for your isolated and non-isolated circuits so we can tell what goes where.

ak

 

[dr pepper]

Have I seen this thread before?
Looks as though it'd work opto wise, though you probably wont need the resistors to ground on the leds unless your doing something fancy.
If you ground the emmiters and connect the resistors as pull ups yes that would work, however the logic would be inverted, you can correct this in software or use a inverting version of U7, in fact do you really need U7, if you want schmitt triggers a schmitt nand chip might even suffice.

 

[Tony Stewart]

What is the purpose of D9?
What is the purpose of R23?
Which power supply runs the NOR gates and the high voltage logic?
You need to add separate ground symbols for your isolated and non-isolated circuits so we can tell what goes where.

ak

U11,10 was some attempt to make a 5V regulator act as negative 7V regulator down from +15 or in others 8V as the ground rail. Very poorly thought out and has no active pulldown anywhere.