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optocoupler question

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David Romero

New Member
Hi, I am trying to use this circuit. Can anyone help me with the 4N35 spice model? I haven't been able to find it. Was trying to use the 4N28 but is not as good. Also, I am running an application at around 10kHz and i need precise switching. Can anyone recommend a fast opto-coupler that won't result in big rise and fall time delays? Cheers
 

David Romero

New Member
Here is the circuit, I'm using the 4N28 opto-coupler, I think the output should give me a square wave of 25volts amplitude but it's just giving me around 6volts and the shape is really bad too. Is this something to do with the optocoupler or what can be the error?
Cheers
 

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alec_t

Well-Known Member
Most Helpful Member
No error. The 4N28 is rather sluggish. A PC817D would be faster.
 

earckens

Member
Maybe connect the base with a 50k or so resistor to ground: I have seen its transition behaviour improve (on a CNY17) with that addition (also see datasheet about this subject).
 

fourtytwo

Member
Here is the circuit, I'm using the 4N28 opto-coupler, I think the output should give me a square wave of 25volts amplitude but it's just giving me around 6volts and the shape is really bad too. Is this something to do with the optocoupler or what can be the error?
Cheers
Why is your emitter load resistor so low (1K) no wonder you are only getting 6V on the output as even that represents 6mA! An opto-coupler is a curent transfer device and generally if you want them to operate fast the lower the current the better. Typical turn-off times are 5-10uS unless you do something to minimise it (lower current, lower gain etc). If you want faster try the digital output type.
 

ronsimpson

Well-Known Member
Most Helpful Member
Basic opto-coupler:
DC Current Transfer Ratio; When you put 10mA into the LED of a typical 4N28 the very best you can hope for is 3mA (30%) out the emitter. Some parts will be as bad at 1mA and some as good (we don't know). With your 1K resistor 3mA = 3V. The 10ma and 10V is the very best. Less current reduces the CTRdc. Temperature effects the number. A Base to Emitter resistor (post # 6) will greatly reduce the ratio.
upload_2018-3-18_6-40-54.png
Speed: The speed of many optos are measured with 100 ohm resistor. But you can only get a fraction of a volt output at that rate.
Here is a graph of delay verses resistor. 1K seems a good choice for speed. A 10k will give you the increased voltage but the Low to High delay will get much slower.
upload_2018-3-18_6-55-37.png
I hope this helps explain your trade offs for the resistor.
 

crutschow

Well-Known Member
Most Helpful Member
The 4N28 and 4N35 do not appear to be fast enough for your purpose.
Here's a simulation using a faster 6N136.
Since the 6N136 is rated at 15V max. I had to drop its voltage to 10V with a zener and add a small MOSFET to get the 25V signal.

I attached the LTspice model and symbol files for the 6N136.

upload_2018-3-18_9-41-40.png
 

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crutschow

Well-Known Member
Most Helpful Member
No error. The 4N28 is rather sluggish. A PC817D would be faster.
Why do you think it's faster?
From the data sheet, it appears fairly slow, comparable to the 4N35.
upload_2018-3-18_11-43-44.png
 
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