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info on optocouplers needed

M

michael nikolaou

Jan 1, 1970
0
Hi to newsgroup

I want to use one optocoupler type LTV-8x6 from LITE-ON as a simple switch .
Output transistor drives one 4.7 K load @ 3,3V and directly to a mcu pin.
Input led is biased from 12 or 24 volts. As i see i can use a resistor to
bias the led at 12 volt and have double current at 24 volts . I imagine 6 ma
for 12 volt is ok . This calculates to 12 ma for 24 volts with the same
resistor.My question is after 10 years of operation if the led deteriorates
is the current sufficient ?. Do i need absolutely a reverse polarity diode
for the led?. Finally what is the simplest configuration in the input led
circuit so it can be driven with active low or active high signals . The
thing is i need an array of 4 optocouplers and only one common input per
opto for both types of logic .
Any help would be appreciated .
 
J

James Arthur

Jan 1, 1970
0
michael said:
Hi to newsgroup

I want to use one optocoupler type LTV-8x6 from LITE-ON as a simple switch .
Output transistor drives one 4.7 K load @ 3,3V and directly to a mcu pin.
Input led is biased from 12 or 24 volts. As i see i can use a resistor to
bias the led at 12 volt and have double current at 24 volts . I imagine 6 ma
for 12 volt is ok . This calculates to 12 ma for 24 volts with the same
resistor.My question is after 10 years of operation if the led deteriorates
is the current sufficient ?.

Correct. Your LED drive level has to allow for aging.
And if you want the LED to last longer, don't drive it
as hard.

The worst current transfer ratio I saw in that series was 20%.

Datasheet here:
http://optodatabook.liteon.com/DataBookFiles/13403/LTV-8x4.pdf

With your load, for 20% CTR, you do need about 3mA drive minimum,
plus aging margin.

10 years = 90.000 hours. Aging depends on LED duty cycle.
Choose accordingly.

10 years continuous duty? Cut the current! Use the 50%
min. CTR version, and increase the load resistor.

Do i need absolutely a reverse polarity diode
for the led?.

A back-to-back LED isolator needs no reverse protection
diode, if that's what you were asking.

Finally what is the simplest configuration in the input led
circuit so it can be driven with active low or active high signals . The
thing is i need an array of 4 optocouplers and only one common input per
opto for both types of logic .

It's not clear what you want. What does the signal driving the opto
look like electrically, and how do you want the opto to respond?

(Is the signal source a voltage or current source? Or sink? Of
fixed, or variable, or unknown polarity?, etc.)

Any help would be appreciated .

HTH,
James Arthur
 
M

michael Nikolaou

Jan 1, 1970
0
Thanks James

I need a lowest cost solution so i'll use a quad optocouler .
The type is LTV-846S .It has a 50% CTR @ 5mA. The load i'll fix to 0.8 mA .
This means i'll need 1.6 mA .I'll keep it safe at 3 mA led current
@ 12V .So finally this is 6 mA @ 24 Volts.With aging it should be Ok
since until the light intensity divides by two should last continously for 10 years . Am i correct ?.
Now regarding polarity i meant what happens if accidentally somebody connects the led input in reverse ?.In that case is it absolutely necessary i use a diode in shunt with the led Or the reverse breakdown voltage is related to current also so with 6mA we are ok?
Now regarding configuration i meant how can somebody detect a contact closing with common to ground or +V easily. Imagine an array of n optocouplers with n inputs detecting n number of switches and one common
for all switches. How can we detect without extra connections GND or +V common connection?
 
J

James Arthur

Jan 1, 1970
0
(un top-posted. James)



michael said:
Thanks James

I need a lowest cost solution so i'll use a quad optocouler .
The type is LTV-846S .It has a 50% CTR @ 5mA. The load i'll fix to 0.8 mA .
This means i'll need 1.6 mA .I'll keep it safe at 3 mA led current
@ 12V .So finally this is 6 mA @ 24 Volts.With aging it should be Ok
since until the light intensity divides by two should last continously for 10 years.
Am i correct ?.

That's the right method, but we have to check on the LED aging rate.
Also, you've not given the duty cycle. The problem is trivial if the
LEDs are only on intermittently, possibly serious for 90.000 hours
of continuous 12mA illumination, probably okay at 6mA--not sure.

For comparison, last time I faced this requirement I used 100%
CTR optos, 22k pullups to +5v (220uA load), and 2mA drive.
Now regarding polarity i meant what happens if accidentally
somebody connects the led input in reverse ?. In that case is it absolutely necessary
i use a diode in shunt with the led Or the reverse breakdown
voltage is related to current also so with 6mA we are ok?

Either in shunt or series, but yes, the protection diode is
needed. 12v reverse will kill the LEDs. Unacceptable.

Now regarding configuration i meant how can somebody detect a
contact closing with common to ground or +V easily. Imagine an array
of n optocouplers with n inputs detecting n number of switches and
one common for all switches. How can we detect without extra
connections GND or +V common connection?

This is what I did, with jumpers to reconfigure each input:


FIGURE 1 Contact closure to ground. (view schematic in Courier font)
========
LTV-846S
___ . . . . . . . . . .
V+ >------>>---|___|---+-------. .-----
R1 | . | | .
--- . --- |/ .
D1 / \ . \ / ~~> | .
--- . --- |>. .
| . | | .
| . | '-----
O-----------------+-------' .
. . . . . . . . . .
O---- GND




FIGURE 2 Contact closure to V+.
========

O----V+ LTV-846S
___ . . . . . . . . . .
O---------|___|---+-------. .-----
R1 | . | | .
--- . --- |/ .
D1 / \ . \ / ~~> | .
--- . --- |>. .
| . | | .
| . | '-----
GND--------------+-------' .
. . . . . . . . . .


Cheers,
James Arthur
 
W

whit3rd

Jan 1, 1970
0
 > i use a diode in shunt with the led Or the reverse breakdown
 > voltage is related to current also so with 6mA we are ok?

Either in shunt or series, but yes, the protection diode is
needed.  12v reverse will kill the LEDs.  Unacceptable.

A resistor-zener-resistor TEE would both reverse protect
and remove the sensitivity to drive voltage. Many very
inexpensive transistors have base-emitter breakdown
that can be useful here (because zeners are more
costly).
 
M

michael nikolaou

Jan 1, 1970
0
James Arthur said:
(un top-posted. James)






That's the right method, but we have to check on the LED aging rate.
Also, you've not given the duty cycle. The problem is trivial if the
LEDs are only on intermittently, possibly serious for 90.000 hours
of continuous 12mA illumination, probably okay at 6mA--not sure.

For comparison, last time I faced this requirement I used 100%
CTR optos, 22k pullups to +5v (220uA load), and 2mA drive.


Either in shunt or series, but yes, the protection diode is
needed. 12v reverse will kill the LEDs. Unacceptable.



This is what I did, with jumpers to reconfigure each input:


FIGURE 1 Contact closure to ground. (view schematic in Courier font)
========
LTV-846S
___ . . . . . . . . . .
V+ >------>>---|___|---+-------. .-----
R1 | . | | .
--- . --- |/ .
D1 / \ . \ / ~~> | .
--- . --- |>. .
| . | | .
| . | '-----
O-----------------+-------' .
. . . . . . . . . .
O---- GND




FIGURE 2 Contact closure to V+.
========

O----V+ LTV-846S
___ . . . . . . . . . .
O---------|___|---+-------. .-----
R1 | . | | .
--- . --- |/ .
D1 / \ . \ / ~~> | .
--- . --- |>. .
| . | | .
| . | '-----
GND--------------+-------' .
. . . . . . . . . .


Cheers,
James Arthur

James
The problem is if you want to use an array of inputs , with one common
reference either
V+ or Ground .So for n inputs you require n+1 cable connections .You have to
fill the board
with jumpers.Is there another alternative ?
Anyway James your help was great . I appreciate that.


Michael
 
M

michael nikolaou

Jan 1, 1970
0
That's a nice idea in this way you can work with higher voltages and
constant led current.
The thing is is there a sot-23 device with two zener's inside ? . Another
factor is the cost
of this device .Anyway the idea is fine


Either in shunt or series, but yes, the protection diode is
needed. 12v reverse will kill the LEDs. Unacceptable.

A resistor-zener-resistor TEE would both reverse protect
and remove the sensitivity to drive voltage. Many very
inexpensive transistors have base-emitter breakdown
that can be useful here (because zeners are more
costly).
 
W

whit3rd

Jan 1, 1970
0
On Jan 31, 12:39 am, "michael nikolaou"

[concerning two resistors-and-zener input limiter]
The thing is is there a sot-23 device with two zener's inside ? . Another
factor is the cost
of this device .

DigiKey lists MAZC062D0LCT for $112/thousand, a dual
6.2V zener. MMBT3904 variants are $30/thousand, and
the base/emitter breakdown is about 7V, should do fine.

The optoisolator is considerably more expensive, and a
service repair is much more than that.
 
J

James Arthur

Jan 1, 1970
0
whit3rd said:
A resistor-zener-resistor TEE would both reverse protect
and remove the sensitivity to drive voltage. Many very
inexpensive transistors have base-emitter breakdown
that can be useful here (because zeners are more
costly).

Nice idea.

Cheers,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
James
The problem is if you want to use an array of inputs , with one common
reference either
V+ or Ground .So for n inputs you require n+1 cable connections .You have to
fill the board
with jumpers.Is there another alternative ?

No alternative that I see with your isolators and signal sources.
In my case, I designed in jumpers to configure my inputs as needed.

If you wanted to spend the money you could do something
like this (below) and sense contact closures to either rail:

FIGURE 3
========
LTV-846S
___ . . . . . . . . . .
V+ >------|___|--------+-------. .-----
R1 | . | | .
--- . --- |/ .
D1 / \ . \ / ~~> | .
--- . --- |>. .
| . | | .
CONTACT | . | U1a '-----
INPUT O----------------+-------' .
| . . . . . . . . . .
|
|
| . . . . . . . . . .
+-------. .-----
| . | | .
--- . --- |/ .
D2 / \ . \ / ~~> | .
--- . --- |>. .
| . | | .
___ | . | U1b '-----
GND-----|___|-------+-------' .
R2 . . . . . . . . . .

Decode the outputs of U1a-b to detect contact input status.


Regards,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
James said:
That's the right method, but we have to check on the LED aging rate.

Googling "optocoupler LED degradation" turned up several sources,
but, basically, optocoupler IRLED degradation at 10mA drive appears
to be minimal. Fig. 6 of this source:

http://www.fairchildsemi.com/an/AN/AN-3001.pdf

shows 2% LED degradation with 100.000 hours' operation at 10mA.

Fairchild's H11L1 datasheet only suggests a 10% guardband
(overdrive) to compensate for LED degradation over time.

By comparison, just 1.000 hours' operation at 30mA and +55°C
degrades an emerald green LED by 28%:

http://www.avagotech.com/docs/5091-7778E


So, all of this reminds me that my earlier referenced
application had visible LED indicators in series with
the optocoupler LEDs, giving me different aging considerations
(visible LEDs age faster). I was also trying to save power,
much of which went to the many channels of isolated inputs.

You haven't mentioned those requirements, so even 6-10mA
wouldn't seem to be a problem for you.

Best,
James Arthur
 
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