## CNY74-4H max output current

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Hi Guys,

I have a very basic electronic background unfortunately. I am not familiar either with symbols in datasheets.

What is the maximum current between Collector and Emitter in a CNY74-4H ?

Datasheet:

http://www.vishay.com/docs/83526...

Can I "squeeze out" 4 x 7mA from it ?

It is not clear to me based on the datasheet :(

Can I connect 7.5V to the collector ? What is the max voltage ?

Really appreciate the help.

Thanks,

Mitya

This topic has a solution.
Last Edited: Fri. Mar 16, 2018 - 01:36 AM
This reply has been marked as the solution.

Hello, Mitya =

The amount of current available from any transistor depends on the current though the corresponding LED. There is a lot of variability in the sensitivity of these things, though. If you look at the Current Transfer Ratio, it says that it can be anywhere from 50% to 600% with 5mA of LED current. This means 2.5mA (5ma x CTR=50%) to 30mA! There ARE power limits, both individually and for the whole package.

Jim

Now,

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

Hi ka7ehk,

I would like to control a 7 segment led display (SA40-19SRWA) with an ATmega1824P via these optocouplers.

As I saw around 7mA - 10mA would be enough for each segment. So 2 x CNY74-4H  would be enough...

Vc would be 7.5V. The pull down resistor is about 1k as I calculated...

I am not sure how could I interpret this CTR. What is the connection between the INPUT and OUTPUT ? If the input LED opens, it closes the CE right ?

So this is what I don't understand (yet).

Thanks,

Mitya

Last Edited: Thu. Mar 15, 2018 - 10:25 PM

ZMitya wrote:
I would like to control a 7 segment led display (SA40-19SRWA) with an ATmega1824P via these optocouplers.

Why do you want isolation for a 7-segment LED display??

Why that display in particular?

As I saw around 7mA - 10mA would be enough for each segment.

Where did you see that?

Driving that from an opto-isolator is probably a bit optimistic.

You'll probably need some "drivers" on the LED side.

So 2 x CNY74-4H  would be enough...

For just a single digit

If you used an I2C driver, you'd only need to isolate 2 wires.

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Hi awneil,

awneil wrote:

Why that display in particular?

This project is a learning project, so I build a gear display for my car simulator game. This one is huge, this is why I chosen that.

awneil wrote:

Where did you see that?

I cound not wait for the answer and I connected it.

awneil wrote:

Driving that from an opto-isolator is probably a bit optimistic.

You'll probably need some "drivers" on the LED side.

I have this for each segment:

So I have an external power for the LEDs.

Do you guys think that it can work ?

awneil wrote:

If you used an I2C driver, you'd only need to isolate 2 wires.

I am not sure what does it mean. I'll check what is I2C.

Thanks,

Mitya

Last Edited: Thu. Mar 15, 2018 - 11:13 PM

You missed the big question:

Why do you want isolation for a 7-segment LED display??

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I would suggest using a uln2804 and series resistors for the display you linked to. The series resistors are needed as you need to limit the current through the leds. You cannot simply use a constant voltage source.
There are also many other ics specifically for driving leds. TI would be one supplier.

awneil wrote:

You missed the big question:

Why do you want isolation for a 7-segment LED display??

The MCU gives me 5V only and limited current (20mA / pin) as I can know.

The LED display is a "big one", it requires 7.5V, if I would use it in MAX power it would require 60mA / segment.

I am a beginner in this world and this is the only method I know to protect the MCU.

Kartman wrote:
I would suggest using a uln2804 and series resistors for the display you linked to. The series resistors are needed as you need to limit the current through the leds. You cannot simply use a constant voltage source. There are also many other ics specifically for driving leds. TI would be one supplier.

I will check this IC, thanks.

Why I can't use a constant voltage source ? What does it mean ?

Sorry for the dumb questions :(

I am really confused... :(

I don't believe that you could run seven segments at about 7-10ma per segment with the output transistor on this optocoupler.  The examples show about 7 mA going through the transistor (using a 1K current limiting resistor).

It would be better to use the optocoupler output to switch a second transistor that would provide all the current for the 7-segment units.  Does your LED display have internal resistors?  Usually they don't. But they usually have only seven inputs for segments (one for each (a-g) segment) and a single input that provides current for each 7-segment digit.  Plus the set of digits are multiplexed so that each digit is only on for a short period of time.

If there are four digits and each digit's segment would use 7mA if it were on constantly, then you can send 20-28 mA to that segment for a few milliseconds if it is being multiplexed with a 1-to-4 duty cycle.

You could simply have the MCU drive external transistors - there is no need at all for isolation

I am a beginner in this world

You do appear to be rather "jumping in at the deep end".

Some beginner's Getting Started tips here: https://www.avrfreaks.net/commen... - including a basic electronics site.

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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
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6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...

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...
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awneil wrote:

You could simply have the MCU drive external transistors - there is no need at all for isolation

I am a beginner in this world

Ok, I can accept that the isolation is not required. But anyway, I bought those OCs already and as I am a programmer I don't even have a single transistor at home, so I would like to use them.

But if I had a transistor, I would have the same question :)

(probably not, because I assume that a datasheet of a transistor is much simpler and I could decide the max CE current limit.)

Quote:

You do appear to be rather "jumping in at the deep end".

Well, that is true but this project motivates me now :(

I can't wait to see it working.

So the question is still alive:

What is the max curent per OC I can use to drive my segment ?

ka7ehk said that it could be 30mA which is enough for me if I can get it 4 times as CNY74-4H has 4 OCs.

This is exactly what I have in my breadboard now, except that I don't have any transistor at home, but I bought these OCs.

There are loads of examples of driving big 7-segments displays on the 'net.

Do some research, compare some schematics.

Is your display CA or CC?

Using optocouplers here seem to be the wrong kind of component.

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

Paulvdh wrote:

There are loads of examples of driving big 7-segments displays on the 'net.

Do some research, compare some schematics.

Is your display CA or CC?

Using optocouplers here seem to be the wrong kind of component.

I can accept that 7 transistor would work, and a 7 segment driver IC would me the most elegant, but I don't have any of them.

I don't know which one would fit to me, so I would be in the same position as I am at the moment.

My display has common anode.

I know that this is not the optimal solution, but there is a room for improving and learning, so the next time I'll try to use a 7segment display driver IC for sure.

I am curious how they work.

Thank you very much for the help.

Mitya

If you apply 7.5V to the led, it will try to pull infinite current and quickly fail. Therefore you need to limit the current. Applying Ohm’s law, we can solve for the value of the resistor.
Assuming power supply of 12V and led current of 10mA we get R = (12-7.5)/0.01 = 450, so a common value of 470Ohms would be used. You need one of these resistors for each segment. The resistors will need to be rated at 0.25W.
Similarly, the leds in you optokoupler will need resistors as well. Assuming 5V and 20mA for the led, R=(5-1.2)/0.02 = 190. 180 or 220 Ohms are common values.

ZMitya wrote:

I can accept that 7 transistor would work, and a 7 segment driver IC would me the most elegant, but I don't have any of them. I don't know which one would fit to me, so I would be in the same position as I am at the moment.

If you parallel drive every OPTO LED with 20mA, you can get anything from  50% to 600% ie 10mA to 120mA (!) of current C-E on the output.

Notice the large variation means even with typical matching, it is unlikely your segments will match brightness.

ZMitya wrote:

My display has common anode.   I know that this is not the optimal solution, but there is a room for improving and learning, so the next time I'll try to use a 7segment display driver IC for sure. I am curious how they work.

Look at parts like TLC5916IN  - still available in DIP16, and has 8 current regulated drive pins.

You talk to it serially.

This specs 3% matching between LED channels, so your segments will match brightness, and you can DIM them easily.

ZMitya wrote:
I bought those OCs already and as I am a programmer

As a programmer, you should understand the importance of doing your design first - and implementation comes from that.

The same applies to hardware: you need to do your design first - and implementation comes from that.

Choosing components is an implementation detail.

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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...
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awneil wrote:

As a programmer, you should understand the importance of doing your design first - and implementation comes from that.

The same applies to hardware: you need to do your design first - and implementation comes from that.

Choosing components is an implementation detail.

It is always about to find the "balance" right ? I did not want to spend months to read 1000 pages prior to start this project, because it is eventually not that huge and complex stuff.

However I know that my knowledge is not good enough to properly design a circuit like this. It will not be optimal, I don't know the "components", I don't know several possible ways to solve a problem.

Last night I learned a lot again, so I really appreciate the effort from you guys helping me out with this.

In the mean time I read those tutorials and hopefully I will ask less and less dumb questions here.

Thanks,

Mitya

Who-me wrote:

Look at parts like TLC5916IN  - still available in DIP16, and has 8 current regulated drive pins.

Thank you very much for this, I am going to put this IC to my "need to check this part" list.

Thanks!

Kartman wrote:
If you apply 7.5V to the led, it will try to pull infinite current and quickly fail. Therefore you need to limit the current. Applying Ohm’s law, we can solve for the value of the resistor. Assuming power supply of 12V and led current of 10mA we get R = (12-7.5)/0.01 = 450, so a common value of 470Ohms would be used. You need one of these resistors for each segment. The resistors will need to be rated at 0.25W. Similarly, the leds in you optokoupler will need resistors as well. Assuming 5V and 20mA for the led, R=(5-1.2)/0.02 = 190. 180 or 220 Ohms are common values.

Thank you very much for this. This is exactly what I did I was just not clear enough with my question.

Thanks,

Mitya

ZMitya wrote:
I did not want to spend months to read 1000 pages prior to start this project
But unfortunately that IS the job for both hardware and software engineers. You have to do a LOT of "up front" reading and making specifications and design documents before you commit to actual implementation. In the old days this usually (for hardware) involved having a "library" of 100's of databooks from all leading manufacturers and knowing a good range of what each had to offer (so you knew where to look). The internet these days has made that SO much easier for beginners as most of the research you need is just a couple of clicks away.

If for example you were planning to drive multiple 7 segment displays from an AVR you might just google "AVR 7-seg(ment) display" and hit things like:

http://maxembedded.com/2013/01/s...

(and tons of other similar "how to" articles)

It's very unwise just to start buying and putting things together (just as it is to start writing C) without a complete, well researched plan.

About 20%+ of engineering probably is reading and research (perhaps even "prototyping" things you aren't sure of) before committing to a design.

Sadly the modern world seems to be more about an expectation for instant answers/solutions. That is not how engineering works. So don't become an engineer if that's what you were hoping for ;-)

So you want to climb Mount Everest (because it's there, of course).

Do you:

1. Jump on Amazon and search for "mountaineering gear".
3. Get your flight to Nepal
4. Start climbing

You'd very soon find that

• You're missing some of the gear you need
• Some of the gear you have is the wrong gear, or unnecessary
• The gear that you do have and do need - you don't know how to use.

Would it not have been more sensible to

1. start out with some basic classes - probably at a gym with a climbing wall
2. as your skills progress, move on to some more adventurous climbs - on real rocks
3. once you've reached a suitable skill level, try for Everest.

#ClimbEverest

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Kartman wrote:
If you apply 7.5V to the led, it will try to pull infinite current and quickly fail. Therefore you need to limit the current. Applying Ohm’s law, we can solve for the value of the resistor. Assuming power supply of 12V and led current of 10mA we get R = (12-7.5)/0.01 = 450, so a common value of 470Ohms would be used. You need one of these resistors for each segment. The resistors will need to be rated at 0.25W. Similarly, the leds in you optokoupler will need resistors as well. Assuming 5V and 20mA for the led, R=(5-1.2)/0.02 = 190. 180 or 220 Ohms are common values.

ZMitya, I think the advise Kartman has provided will work for you, try it an let us know how it goes.

Your making good progress, seeking help / advise from others was wise.

Good luck with your project, and post a picture when you have it working!

Jim

Click Link: Get Free Stock: Retire early! PM for strategy

share.robinhood.com/jamesc3274

If you do not have transistors, then buy something like this:

https://www.aliexpress.com/wholesale?SearchText=transistor+assortment

And read some 1000 page manuals & books on how to use them.

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

ki0bk wrote:

ZMitya, I think the advise Kartman has provided will work for you, try it an let us know how it goes.

Your making good progress, seeking help / advise from others was wise.

Good luck with your project, and post a picture when you have it working!

Thank you very much for the words, I will do a video how it works for sure :)

Thanks,

Mitya

Thanks, I am going to order one box now...

I can absolutely see your points guys, and really thanks for those.

I think nowadays you can not be "tall" and "fat" at the same time. You can't be very good in too many fields because each territory is just huge.

After a few years SW development XP I learned that it is not possible to create a perfect system from the beginning. The good system is modular, you can throw and replace modules easily.

I thought that I just need to have the interfaces designed well and the modules can be finetuned done step by step.

So this is what I followed here, I don't mind if those OCs are overkill for this project. I just don't mind, I am just playing. In v2 I will do it using transistors, in v3, I'll switch the LEDs with those ICs. I am curious, I would like to learn them.

Sorry if it brakes some rules what you follow guys and really appreciate your patience.

Mitya

"We" re here to help.

"We" are not here to take "beginners" by the hand and lead hold them by the hand from the first step to a successfull commercial product.

"We" here have to make guesses about the motivation / lazyness / school projectes, etc of "everybody" asking questions.

ZMitya wrote:
Thanks, I am going to order one box now...
Does this relate to a box of transistors as I suggested?

You probably also want something similar for resistors & capacitors. Add in some breadboards and wires and maybe a soldering iron and 1-hole-per-pad pcb's.

But the amount of "info" and "stuff" is so overwhelming as you mentioned before that "we" can hardly give more specific advice to general questions.

Have fun.

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

Looks ugly, but works and does not smoke :)

Thanks guys..

https://goo.gl/4LRT19

Last Edited: Sat. Mar 17, 2018 - 12:21 AM