Optocoupler speed

Go To Last Post
14 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

    Hello

    Some transistor output style optocouplers have the base brought out. I wonder what would be the purpose of this ? Is there a way to avoid the output transistor to saturate in order to increase the speed ?

 

    Another question, from your experience, what would be the maximum baud rate a transistor based optocoupler could achieve, more than 9600 ?

 

    Thanks.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

angelu wrote:

Another question, from your experience, what would be the maximum baud rate a transistor based optocoupler could achieve, more than 9600 ?

 

1Mbps. ACNT-H50L-500.

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I like Brian's part!

 

That said, I used a TIL111 in the past.

It is certainly not a fast part.

I was driving it with a Mega168 USART, 5V system.

 

I ended up putting a 2N7000 NFet on the output to square up the output signal.

That then drove a Tiny's USART just fine.

Note that is this configuration the opto inverts the signal, and the NFet re-inverts the signal, so the isolated USART is happy.

 

It worked at 38400 and 57600 baud just fine, although the 38K baud rate signal "looked" better on the O'scope.

The Tiny's USART, however, read both signals just fine.

 

BTW, I still have one of your XMega PCBs on the bench, it gets powered up every onw and then for a project / test or two.

 

JC

 

 

Pretty ugly PCB, one of my earlier hand soldered SMD boards, but it worked well!

 

 

 

Last Edited: Fri. Nov 11, 2016 - 05:42 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Certainly more than 9600bps is possible, e.g. ICPL2630 / ICPL2631 (Features say 10Mbps, but I do not think that is a realistic spec for commercial product.)

David (aka frog_jr)

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

frog_jr wrote:

ICPL2630 / ICPL2631

 

Those are logic gate output parts though.

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

frog_jr wrote:
Certainly more than 9600bps is possible, 

Indeed: MIDI is opto-coupled and works at 31250 bps.

 

But if you actually want opto-isolated RS232, there are vendors with specific products for that; eg,

 

https://www.maximintegrated.com/...

 

http://www.analog.com/en/product...

 

And, nowadays, there are plenty of other isolation options; 

 

eg, http://www.analog.com/en/product...

 

Top Tips:

  1. How to properly post source code - see: https://www.avrfreaks.net/comment... - also how to properly include images/pictures
  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
  4. Difference between a crystal, and a crystal oscillatorhttps://www.avrfreaks.net/comment...
  5. When your question is resolved, mark the solution: https://www.avrfreaks.net/comment...
  6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Adding some resistance from the base to ground can significantly shorten the turnoff time

Some capacitance to ground can greatly reduce the effects of high frequency noise capacitance-coupled from the led

Tom Pappano
Tulsa, Oklahoma

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Unlike a conventional BJT, an opto-coupler speed is NOT limited by collector saturation. Instead, it is limited by the high inter-electrode capacitance due to the large area needed capture the light (photo-sensitive area). The base region of an opto-coupler can be thousands of times larger in area than a normal transistor. Thus, strategies for speed improvement in opto-couplers are quite different than in a normal BJT circuit. Often, your only viable option is to use a coupler that is optimized for higher speed.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

angelu wrote:

    Some transistor output style optocouplers have the base brought out. I wonder what would be the purpose of this ? Is there a way to avoid the output transistor to saturate in order to increase the speed ?

The really early opto couplers gave access to the base, as that gives more choices on how to use.

More recent ones tend to have no base bonded, as that also picks up noise.

 

The Collector to base junction has a quite high C, in opto Trx, as that is the large-area photo junction. 

 

angelu wrote:

    Another question, from your experience, what would be the maximum baud rate a transistor based optocoupler could achieve, more than 9600 ?

If you need moderate speed, (~1MBd) the H11L1 is worth a look.

A transistor optocoupler speed will come down to that Cbc mentioned above.

If you use a low load resistor, to reduce the miller effect, and a comparator, you can likely get > 100k in a transistor based optocoupler.

Also, lowering the RL and increasing the drive, will likewise increase the speed, but you will probably decide some more modest drive is more practical.

 

Most data sheets show tr,tf with load, and you can see the miller effect there.

 

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

If you can reach the base you can probably use it to build a schmidt trigger.

 

I have experimented with general purpose / low speed opto's and could get about 150kHz through them but no practical use ( jitter, aging, temperature effects, etc).

Optocouplers are also cheap & easy, but for higher speeds nowadays specialised ic's with inductive or capacitive barriers are used for 400MHz and faster.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Paul can you mention some part numbers "specialised ic's with inductive or capacitive barriers are used for 400MHz and faster.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

This EDN Design Idea significantly speeds up the turn-on time of a standard Opto-Coupler.

Unless this method appeals to the user for some reason, a faster part is probably a better choice.

 

Optocoupler speed-up also reduces power consumption by Marian Stofka -September 01, 2014
http://www.edn.com/design/analog/4433947/Optocoupler-speed-up-also-reduces-power-consumption

 

Stan

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I am not very good with the electronics side, what about these devices:

 

http://www.mouser.com/ds/2/678/u...

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

That is a nice multi-channel fast coupler. There is a really big caveat, however. Both sides need power. If you want to maintain the isolation, you will need isolated power on the input side. And, those input channels are NOT isolated from each other, they must share a common terminal. If you can live with those limitations, then its a great part.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net