Driving optocouplers

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
http://pdf.datasheetcatalog.com/datasheet/vishay/83725.pdf

The output transistor of the 4N25 optocoupler is spec?d at max 150 mA (no  
mention of on-resistance). I measure the resistance across the output  
terminals with the input driven with 17 mA and I see a little over 110 ohms.

This is going to operate the enable input of a VFD: 10 vdc high-impedance  
digital input needs to be pulled to ground. I?m concerned whether or not  
the 4N25 will do the job. With an on-resistance of 110 ohms, will this pull  
the input low enough?

Maybe I should be looking at another optocoupler? What spec is important look  
for?

Thanks,
Dave


Re: Driving optocouplers
On Thu, 06 Aug 2015 22:52:21 -0700, DaveC wrote:

Quoted text here. Click to load it

It's a bipolar junction transistor, not a FET.  "On resistance" isn't  
really a concept that applies -- figure that it'll pull down to about  
0.2V if you're pumping enough current in.

You don't specify what your "high impedance" is, but it'll probably pull  
the enable input down low enough.

--  
www.wescottdesign.com

Re: Driving optocouplers

Quoted text here. Click to load it

Thanks Tim. Good to know.

How would I determine in future whether or not such a OC is appropriate?  
What?s the thought process?

Thanks again,
Dave


Re: Driving optocouplers
On 08/06/2015 11:16 PM, DaveC wrote:
Quoted text here. Click to load it
?
Quoted text here. Click to load it

Many of the OCs specify if they are TTL or CMOS compatible. That would  
help as long as you know the ON/OFF voltages for the devices you are  
trying to control.

Speed is next, are you passing data or just signal levels? Data would  
require (depending on the rate) a much faster OC than something that is  
just isolating a signal to a relay, etc. In other words, what is the  
clock speed and is Rise Time important?

Hope this helps...

John :-#)#
--  
(Please post followups or tech inquiries to the USENET newsgroup)
John's  Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
We've slightly trimmed the long signature. Click to see the full one.
Re: Driving optocouplers
wrote:

Quoted text here. Click to load it

Quoted text here. Click to load it

Some claimed here that the old 4N25 is slow, but still it has been
used for  video (5 MHz) isolation. You have to control the
photo-transistor base in an proper bootstrap circuit.


Re: Driving optocouplers
wrote:

Quoted text here. Click to load it

Quoted text here. Click to load it

http://optoelectronics.liteon.com/upload/download/DS-70-99-0010/S_110_4N25 (26).pdf

The key thing is the minimum CTR = Current Transfer Ratio. That's the
current you get through the transistor for a given LED current.  

There are few phototransistors optoisolators worse than a 10-cent
4N25. It's 20% CTR but that's not taking temperature, different
currents, aging or good saturation into account.  

It's unwise to run them at much more than 10-15mA through the LED.  

So let's say we run at 12mA LED current. That would give us 2mA with
10V across the phototransistor. It might drop 30% at high temperature,
and maybe allow another 30% for aging, then halve that to have it
decently saturated and we're left with about 0.5mA.  

So if the pullup voltage is 5V it would work okay if the pullup
resistor is 10K or greater. It will be able to switch at a couple kHz
with a 10K resistor, less if the resistor is higher, but that's plenty
for an enable signal.  

A Fairchild FOD817B300 has 130% minimum CTR at 5mA LED current, so it
could be run at lower LED current and would switch more. It also
doesn't bring out the base connection, probably a good thing.  

http://www.fairchildsemi.com/datasheets/FO/FOD814A.pdf

--sp  



--  
Best regards,  
Spehro Pefhany
We've slightly trimmed the long signature. Click to see the full one.
Re: Driving optocouplers

Quoted text here. Click to load it

Quoted text here. Click to load it

Quoted text here. Click to load it

An SSR might be a better choice.


--  

John Larkin         Highland Technology, Inc
lunatic fringe electronics

We've slightly trimmed the long signature. Click to see the full one.
Re: Driving optocouplers
On 8/7/2015 12:52 AM, DaveC wrote:
Quoted text here. Click to load it

A VFD (Variable Frequency Drive) is usually designed to be an industrial  
controller. It usually has isolated inputs. That is, the inputs for the  
enable function is usually passed through an optocoupler inside the VFD.  
Furthermore, those digital inputs usually accept dry switch contacts.

If the manual recommends otherwise then ignore my response.

Re: Driving optocouplers

Quoted text here. Click to load it

That has been my limiated experance with the VFDs.  Some of them have  
adapter boards that use optical isolators.

 Ran into that problem when the company I worked for was in a big rebuilding  
project.  The engineer had just used the solid state output of a PLC to turn  
on a VFD.  The VFD wanted a hard contact, so I used an interposing relay as  
a quick fix as it was the weekend and needed to get it up and running.

It is really difficult to say the resistance of a solid state device is so  
many ohms.  As pointed out is is the ammount of currrent it can sink and how  
low the voltage will go.



Re: Driving optocouplers
On 8/7/2015 10:59 AM, Ralph Mowery wrote:
Quoted text here. Click to load it


You are correct, AFAIK. The important thing is to know the input circuit  
of the VFD. Without that, you are forced to do tests. Which is what I  
would do.


Re: Driving optocouplers
In article < snipped-for-privacy@news.eternal-
september.org>, snipped-for-privacy@home.cow says...
Quoted text here. Click to load it

150 mA (no  
Quoted text here. Click to load it
ms.
  

er or not  
Quoted text here. Click to load it
ll  
Quoted text here. Click to load it
look  
Quoted text here. Click to load it
I've used them for isolation of digital signals in the past.
I did run into a problem where, even though it is a bipolar transistor,  
in a noisy environment there could be enough noise conducted in by the  
bonded out base of the transistor to cause the thing to turn on  
sporadically.  For the speed of the signals I was interested in I moved  
to good old PC817 parts and haven't had any issues since.


Re: Driving optocouplers
snipped-for-privacy@mindspring.com says...
Quoted text here. Click to load it

Quoted text here. Click to load it

Quoted text here. Click to load it

 Excessive photons flying around the area?

Jamie

Re: Driving optocouplers
jamie snipped-for-privacy@charter.net says...
Quoted text here. Click to load it

max 150 mA (no  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
0 ohms.
ance  

hether or not  
Quoted text here. Click to load it
s pull  
Quoted text here. Click to load it
ant look  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it

Not that I know of, I think just good old near-field coupling or  
capacitive coupling to nearby traces. The output was acting like the  
opto was always in the linear region, where they needed off or  
saturated.
Cutting the Base pin off corrected the issue, presumably because of a  
smaller antenna length or exposure to capacitance between it and other  
traces on the laminate.
I didn't spend anymore time on it beyond changing the opto family and  
relaying the PCB.
Mind you, this was 20 some years ago, and I was working for a different  
company, so all the details are a bit dragged through the filter of  
time.

Site Timeline