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I have used an optocoupler/isolator before in a similar circuit, can't figure out why it isn't working in this instance, any help would be much appreciated.

I am using the Siemens SFH615A, link to spec below, and the following circuit which I am using to ground an Inhibit signal on a Servo drive.


enter image description here

Very much a newbie with electronics, any help would be great, thank you.

Inhibit line when grounded stops the servo drive from powering a connected motor. When the line is left open the drive is enabled. Output voltage of the Inhibit line is 3.56V.

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closed as unclear what you're asking by Olin Lathrop, Daniel Grillo, Joe Hass, Dave Tweed, Chetan Bhargava Feb 6 '14 at 19:36

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

You need to provide more detail of the inhibit line - opto outputs generally use a pull-up resistor and without bias (from a pull-up resistor) the opto's behavior is unpredictable. – Andy aka Feb 6 '14 at 13:47
Really!? You think "not working" is somehow a useful description of what this circuit is actually doing (duh)? – Olin Lathrop Feb 6 '14 at 13:47
Thanks Olin, very constructive, if you read the comments I'm very new to this and if you need any information that I have omitted you need only ask. I'll take it you are just having a bad day, but thanks for your input. – Newbie Feb 6 '14 at 13:49
Hi Andy aka, thank you for your reply, you have also helped me on previous questions so thank you. The Inhibit line stops the servo drive (and motor) when the inhibit line is grounded, otherwise, the servo drive spec states to leave the line open to enable. – Newbie Feb 6 '14 at 13:53
It doesn't take any electronics knowledge to know that "not working" is a pretty useless description. What do you think your mechanic would say if you brought in your car and only said it was not working? Same here. Working means behavior matches expectation. In this case we don't know what behavior you are actually seeing, nor is it clear what your expectation is. Again, none of this has anything specifically to do with electronics, so there is no excuse for leaving out such information, newbie or not. – Olin Lathrop Feb 6 '14 at 16:13

Note: You are driving that poor LED very hard with 60mA typical current (the absolute maximum number). I would suggest increasing the resistor to at least 300 ohms- but more information on the VFD input is required to determine if that is okay or not. Possible consequences include rapid aging of the LED (the isolator will weaken in CTR over time) or sudden failure of the LED. Note that the CTR is specified at 1mA and 10mA. If you have no info on the VFD input you could test it at (say) 2mA and if it works go to 10mA (add some at least 2.5:1 safety margin to account for time and temperature changes).

  1. When the switch is closed (LED should be energized) do you measure voltage across the 62\$ \Omega \$ resistor? If not, there is no current flowing and the LED cannot work. Possible reasons include wiring, a bad component, lack of 5V supply or (more likely) the LED is connected backwards. Measure the voltage across the LED- it should be about 1.25V.

  2. Assuming the LED is conducting and the drive input is the type that accepts a dry contact, there should be some positive voltage present at the transistor collector (several volts) when the switch is open (or when the opto is disconnected). The voltage should decrease to much less than 1 volt (still positive) when the switch is closed. If that is not happening, there is something strange with the input, the opto-isolator has been damaged, or (more likely) the transistor is wired backwards (emitter must go to ground).

  3. Some inputs are not designed to accept a contact (such as a mechanical switch) to ground and require a logic input-- which means you need a power source on that side of the LED. It's not so common with industrial equipment, but a link to the manual would help.

  4. Why is the opto in there? Will the switch not work by itself? Or is there some mains isolation issue?

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Hi Spehro, I'll do my best to answer :) – Newbie Feb 6 '14 at 14:15
Hi Spehro, Yes there is a voltage, very low 0.07V, across the 62Ohm resistor, voltage across LED is 1.07V. The switch is actually an I/O line, wasn't sure how to represent it, logic high is 3.3V-5V and low is 0.15V. – Newbie Feb 6 '14 at 14:26
Something is wrong-- LED and resistor voltages should add up to 5V. – Spehro Pefhany Feb 6 '14 at 14:36
Ok I will try replacing the 62 with a 300 ohm and see how it goes. – Newbie Feb 6 '14 at 14:56
I doubt it will make any difference to your present issue. – Spehro Pefhany Feb 6 '14 at 14:57

Is there any voltage on Inhibit signal on a Servo drive when opto-isolator is not connected ? If there is nothing there - your schematic ain't gonna work. This may be fixed by adding pull-up resistor from INHIBIT Line to appropriate voltage at Servo drive side. More details require knowledge of your servo model and connection schematic.

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Thanks x4mer, just read the Inhibit line, it is outputting 3.56V – Newbie Feb 6 '14 at 13:56
Then either your SFH615A is dead or you missed something in your connections – x4mer Feb 6 '14 at 14:01

Well I think your problem mus be the wiring or the optocoupler itself. You can perform the following tests:

  1. Test the device (unconnected):

a) Use a multimeter in the semiconductor test option (set the knob to the diode symbol) and measure with it the forward voltage across the LED (red probe to pin 1 and black probe to pin 2 of the opto). It MUST display between 1.0 and 1.6V (It is typically 1.25, see vishay datasheets). If you have something near 0V the opto's LED is short-circuited (burned up) and it's because the low resistor value (I use a 220 ohm resistor to this kind of circuit).

b) If the LED is still alive, test the photo-transistor: With the multimeter knob in the semiconductor test option (set the knob to the diode symbol) measure the voltage across the output transistor (red probe to pin 4, black probe to pin 3) in two times: first with the four pins unconnected multimeter MUST display no measure (normally 'OL' = open loop or '1 .' = measure too high). Second, measure as the first but applying the 5V through the 220 ohm resistor to pin 1 and connect pin 2 to ground (pins 3 and 4 unconnected). Multimeter MUST display 0.1V to 0.4 (typically 0.25 according the vishay datasheet). If this is correct then step 2.

  1. Test the wiring: It's very simple. If step 2 works then the input wiring is ok. Test the output wiring with a short-circuit between pins 3 and 4 with everything connected (no multimeter needed) and the servo drive MUST react. If no, there is your problem.

Hope it helps

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Hi Cesar, thank you, I have used the multimeter and perform the tests you outlined, thankfully the opto is still working. I will be rechecking everything I have done but it is very good to know the device is ok, thank you. – Newbie Feb 6 '14 at 16:30

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