Opto Coupler vs Solid State Relay for low voltage application

Thread Starter

PastaPaul

Joined Dec 18, 2016
2
Hello,

I need to report the status of an old piece of industrial equipment back to an input zone on a security system. The industrial equipment has a very old and propriety PLC setup and so we are actually using the status LED from the industrial machine to drive our circuit. I've soldered wires to the LED and when the LED is lit I've got 2 volts to work with.

As I'm no expert with transistors, I've looked at doing this with HFS2/A213DN solid state relay (https://www.digchip.com/datasheets/parts/datasheet/922/HFS2_A213DN-pdf.php)

I rigged up a little test and this actually works fine. The 2 volts will activate the relay and let the system know the on/off status of the LED on the machine.
My problem is that when the HFS2 relay is connected the relay doesn't illuminate. I think the SSR is simply taking too much voltage/current from the circuit and the relay.

My question is can I add a resistor and/or capacitor that might allow the SSR and the LED to work at the same time. Or should I consider a different device like a 6N138 (https://www.jaycar.com.au/medias/sys_master/images/h1c/h05/8838539739166/ZD1940-dataSheetMain.pdf

Thanks,
Paul
 

MaxHeadRoom

Joined Jul 18, 2013
19,175
Is the issue that you just want to report when the PLC is active? or a certain function is ON?
What exactly the Status LED? On at all times when the machine is running? or just when a particular function is on?
What is the make of PLC?
If forced to use the LED, it may be wiser to use the opto in series with the LED.
Have you confirmed that there is no output on at the time you want to detect the condition?
Max.
 

Thread Starter

PastaPaul

Joined Dec 18, 2016
2
Hey Max - thanks for the reply!

We want to report on a function. I've just re-read my original post and to be clear the SSR I'm using does work for my application, it's just that the LED on the control panel wont illuminate when I'm powering the SSR.

More Details:
The unit is actually a refrigeration container from the 90's (full of ice cream!). We've been looking for a way to monitor it for a while and the guy who services it for us said if we damage the controller it wont be worth fixing and we would have to spend a fair bit of money rewiring the whole thing with a conventional refrigeration controller. We don't want to do that because the system is quite reliable. It's given minimal trouble over the years and has some good built in checks and redundancies.

The control panel (which is separate from the PLC) has 3 status LEDs we want to report on (cool, heat and alarm). The lowest risk approach has been to open the this panel and I've carefully soldered wires across these LEDs. As I mentioned each pair of wires will give me 2 volts when the noted condition is active, and there's virtually no risk to the actual control system or the panel with this approach.

My test confirms that the HFS2 solid state relay will switch on from this 2 volts and gives me optically isolated, voltage free contacts to wire into our alarm system (which then lets me write rules based on conditions like if the Alarm light is on send an SMS to...)

What I'm wondering is whether there's a way to keep the panel LED working while powering the SSR or an alternate device like the 6N138.

Thanks again,
Paul
 

crutschow

Joined Mar 14, 2008
23,508
What I'm wondering is whether there's a way to keep the panel LED working while powering the SSR or an alternate device like the 6N138.
As Max stated, one way would be to put the LED in series with the SSR input.

The other would be to add a simple transistor amp to the LED terminal.

What's your choice?
 

MaxHeadRoom

Joined Jul 18, 2013
19,175
I agree I would tend to try an Opto, 4N35 etc, or SSR in series with the LED, it should not affect the LED circuit at all.
The opto may offer the lowest impedance of the two.
Max.
 

AnalogKid

Joined Aug 1, 2013
8,140
Do you have a schematic of the controller, or at least the front panel? There are many ways to detect 2 V as present or absent, but how the three indicators are wired together, such as do they share a common ground or V source, make or break some of the methods. Since the voltage drop across the LED is 2 V, it almost certainly does not have any current limiting built in. Wherre that current limiting is located affects possible solutions.

ak
 
Yup, that will happen or at least it will be less bright. What you can do, if you have access is replace the LED with something like this: http://www.ixysic.com/home/PDFs.nsf/www/Lcc110.pdf/$File/Lcc110.pdf That has a from A and a Form B contact. The NO contact would just light the original lamp with the appropriate sized resistor from whatever supply is is available to get the same brightness. Current can be lowered with some series resistance it needed.

Now you have two isolated contacts to do with what you want. One lights the original lamp through some contortions like a resistor and some supply voltage.

Without "our circuit" who knows what you need. Just pull down +5 on your end?

The other choice is more complex where you actually look at the voltage across the LED and compare with a comparator. Lots of unknowns at this point.
 
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