Polarity on an optocoupler?

[morpha2]

Hi guys,
I'm working on a DOD-280 style compressor from GGG. Real quick, can you tell me how to identify the positive side of an optocoupler? I'm using an NSL-32. Thanks!

[Joe Viau]

Hi:

Do you have a multimeter? If so, it should be simple.  The LED is, after all, a diode.

I found this at http://www.kpsec.freeuk.com/multimtr.htm#diode

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Testing a diode with a multimeter

The techniques used for each type of meter are very different so they are treated separately:

Testing a diode with a DIGITAL multimeter
Digital multimeters have a special setting for testing a diode, usually labelled with the diode symbol.
Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V).
Reverse the connections. The diode should NOT conduct this way so the meter will display "off the scale" (usually blank except for a 1 on the left).

Testing a diode with an ANALOGUE multimeter
Set the analogue multimeter to a low value resistance range such as × 10.
It is essential to note that the polarity of analogue multimeter leads is reversed on the resistance ranges, so the black lead is positive (+) and the red lead is negative (-)! This is unfortunate, but it is due to the way the meter works.
Connect the black (+) lead to anode and the red (-) to the cathode. The diode should conduct and the meter will display a low resistance (the exact value is not relevant).
Reverse the connections. The diode should NOT conduct this way so the meter will show infinite resistance (on the left of the scale).
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So, for a DMM, if it displays "off the scale" the red probe is on the cathode and the black lead is on the anode.  If you don't have a DMM, invest in one.  They are fairly cheap and will help you immensely if you're halfway serious about electronics at all.

HTH,
Joe

[petemoore]

  LED = diode and has cathode and anode, is polarized.
  take an LED and a little resitor 4k7 or so.
  You'll need the resistor to limit the current through the LED.
  Wrap the 4k7 around the cathode of the LED [flat spot hard to see on side of regular sized LED].
  Now run the circuit [LED and 4k7 resistor, 10k 'd work] on 9v.
  If the LED doesn't light you need to reverse polarity.
  The LED lights, and you can now note the Cathode / Anode polarity as pertaining to lit LED and..battery polarity, taking notes now makes it easier to remember what pole to hook the cathode [flat mark] to for lighting an LED. cathode and anode also have different length leads, see package or data sheet.
  Ok...now you know how to light the LED without burning it up...time to try that on the 'precious' opto.
  Get the meter reading resistance [set to high range], it's leads across the LDR side of the opto-cell [this variable resistor has two leads and like other resistors, is non-polarized]...now when you get the LED to light inside the opto-cell [where you can't see it], you should see the resistance across the LDR lead rise and fall, these will be the 'on/off' state resistances, off of couse is set by darkness, on 'can' be brightened, but you must not over-current the LED in the opto cell.
  Now you know your optocell works and which side of the LED gets negative polarization.