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Current and voltage expansion for 6N136

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stoyanoff

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Greetings! I`m looking for schematic for 6N136 so I can control a current loop up to 20mA. So I need a high speed optocoupler such as 6N136 because of the baud rate, but it`s max current is not enough. So is there a way to achieve this by adding and additional VT for example? My supply voltage will be up to 15V. Is there a schematic? Or should I use another optocoupler?
Thanks!
 

Hi,

You say
* "current loop": analog or digital?
* "high speed": what does this mean in: delay, rise/fall time?
* "supply voltage 15V": Output side? your load is driven by 15V/20mA?

How do you want to drive the load? High side, low side, push-pull...?

Mind: 15V / 20mA can cause 300mW of power dissipation.

Klaus
 

I would imagine it best to use the opto in emitter-follower
mode, with a load resistor that puts the device in the
sweet spot for speed, and feed it to a 20mA current
loop driver IC as the logic input. Be sure the input will be
happy (i.e. not pulling excess Idd, from sub-spec Vih)
with the opto's VOH, or buffer as appropriate.
 

Hi. It will be digital current loop with baud rate up to 19200. First I've tried with regular optocoupler but it has very long switching time and I don't like it in spite of fact it's working. So I have current source, income data optocoupler, another node and TX optocoupler. I am not quite sure have to use an opto here because the current source shares common GND with the logic and a simple VT will do the work. So the current source is working with 15V supply but it's been distributted all over the line. The voltage drop on the last opto(the one we are talking to) depends on the another node which I have no control on.
Thanks!
 

Hi,

Your description is vary vague ... as if you are talking about anything top secret.
"Regular optocoupler" --> why don't you give the type and yor test circuit?
"Long switching time" --> why no value?
"I have current source" --> how and where connected? What current?
"another node" --> which node are you talking about.
...and so on....
********

6N136 datasheet tells 0.3us switching time.
This isn't slow at a 19200 baud signal.

If you see someting slower, then it's not the problem of the optocoupler ... it's the problem of your circuit and the part values you used.
Thus I assume you didn't care about the datasheet ... how to get fast signals.

If so, then even a very fast optocoupler can't speed up your circuit.

If you really need help, then it's time to show us a sketch what you really want to achieve.

Klaus
 

Sorry! I`ve written through a phone! My test circuit is with EL817S. The switching time is close to 20us(per optocoupler). I think this is a lot. My current source is simple VT based with current limiter and temperature correction. It`s on 15V. The current loop should work between 4-20mA.
I want to decrease the switching time because it potentially can cause errors in the receiver`s side... I have no control on the another node. So I don`t know what resistance it will add to my circuit. So can I expand 6N136 somehow with external VT?!
 

I think I did it! I`ll add a diode and transistor on the 6N136 output and I`ll supply the optocoupler directly from the power supply. I think this will do the trick!
Thanks!
 

Hi,

no idea how your circuit looks like. Only vague informations.
I can´t imagine how the diode is involved.

Klaus
 

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