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ISOLATING 4-20mA INPUT SIGNAL USING IL300 Linear Optocoupler

A

alcuemar

Jan 1, 1970
0
Dear Friends,

I have read today that there is one possibility to isolate the 4-20mA
input signal using the IL300 octocoupler, I have read the application
note 54 from siemes but i have not too much clear the functioning of
the design.

http://crio.mib.infn.it/wig/electro...n/Accoppiatori Ottici/IL300 per industria.pdf

My design require 8 analog inputs from 4-20mA, with the possibility of
to detect the broken sensor or short-circuit sensor, and out of range
sensor, so the option to do that without isolation is to put simple
shunt resistor for 220 ohm, 4mA = 0.88V, and 20mA=4.4V, so we can
detect the out of range easitly, the short-circuit will be 0V, ... and
the broken sensor will be 0 too.

How to do the same using the IL300 linear octocoupler, in the
application note 54 page 11, they equation to join the input and
output is Vo/IL=(R3/R4)*R5*K3, what is the value for K3??.

I have some doubt about how to use this design.

If someone has make something similar I will appreciate the help.

Thanks in advance.

Alberto
 
D

Dan Hollands

Jan 1, 1970
0
I think the circuit of figure 19 is what you want. It is self powered on
the input meaning that up to 4 ma of current is used to power the input. The
ma from 4 to 20 creates a 1 to 5v signal isolated from the input.

If the input is shorted, open or out of range the output will be outside the
normal 1 to 5 volts.

Dan

--

Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com
 
A

alcuemar

Jan 1, 1970
0
Thanks Dan for your feedback.

I will test this figure 19 schematic in a new prototype, just i have
the doubt of how to calculate the Vo voltage depending the I input. I
have see the equation:

Vo/IL=(R3/R4)*R5*K3

But I have not clear the value of K3.

Thanks in advance.

Alberto
 
R

Rich Grise

Jan 1, 1970
0
Thanks Dan for your feedback.

I will test this figure 19 schematic in a new prototype, just i have the
doubt of how to calculate the Vo voltage depending the I input. I have see
the equation:

Vo/IL=(R3/R4)*R5*K3

But I have not clear the value of K3.

Thanks in advance.

From:
http://crio.mib.infn.it/wig/electro...n/Accoppiatori Ottici/IL300 per industria.pdf
From the paragraph fragment at the top of page 11-202, between equations (15) and (16), it
says:

Where: IP1 = feedback photocurrent
K1 = feedback gain
IP2 = output photocurrent
K2 = output gain
K3 = transfer gain.

The value of K3 will be determined by the implementation of your circuit.

In other words, let the math mumbo-jumbo go to the bit bucket, where
all mumbo-jumbo belongs, and lash up the circuit in figure 19, page
11-204, and calibrate it by putting a known 4 mA through "line", and
measure the voltage at "output", and put a known 20 mA through "line",
and measure the voltage at "output".

In short, K3 = K2/K1.

Hope This Helps!
Rich
 
S

Spehro Pefhany

Jan 1, 1970
0
From:
http://crio.mib.infn.it/wig/electro...n/Accoppiatori Ottici/IL300 per industria.pdf
From the paragraph fragment at the top of page 11-202, between equations (15) and (16), it
says:

Where: IP1 = feedback photocurrent
K1 = feedback gain
IP2 = output photocurrent
K2 = output gain
K3 = transfer gain.

The value of K3 will be determined by the implementation of your circuit.

In other words, let the math mumbo-jumbo go to the bit bucket, where
all mumbo-jumbo belongs, and lash up the circuit in figure 19, page
11-204, and calibrate it by putting a known 4 mA through "line", and
measure the voltage at "output", and put a known 20 mA through "line",
and measure the voltage at "output".

In short, K3 = K2/K1.

Hope This Helps!
Rich

"The remaining variable is the IL300’s transfer gain, K3. The part to
part variation of the transfer gain offers (sic) a range of 0.56 to
1.53"

K3 is a datasheet parameter. Nominally it's 1.0 (the ratio of the
small currents you get out of the two photodiodes) but it could be
anywhere between 0.56 and 1.65, according to the data sheet (0.56 and
1.53 sez the AN, but that's suspect).


Best regards,
Spehro Pefhany
 
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