Opto Isolator Circuit

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labview1958
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Opto Isolator Circuit

Post by labview1958 » Thu Aug 11, 2005 1:14 am

I have made an opto Isolator circuit to isolate my PC from the rest of the circuit. Here it is:
[img][img]http://img299.imageshack.us/img299/7402 ... 8rd.th.png[/img][/IMG]<p>However it does not work. A friend ask me to connect the input of the opt-isolator to the ground of the PC. Would I fry my PC? Actually there is a NI-6025E DAQ card. A digital signal is being generated by a labview program to control something.

dyarker
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Re: Opto Isolator Circuit

Post by dyarker » Thu Aug 11, 2005 3:32 am

There is no resistor between the collector and power supply. The circuit in the drawing is known as a silicon to carbon converter or a one-shot fuse tester :) <p>Get a new opto-isolator, add a 2K to 10K resistor, take the output from the collector.<p>cheers,
Dale Y

peter-f
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Re: Opto Isolator Circuit

Post by peter-f » Thu Aug 11, 2005 9:21 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Dale Y:
... a silicon to carbon converter ...
<hr></blockquote><p>Good observation, but your physics are derived from some sorcerer!
Silcon = silicon - convert to carbon only via sub-atomic reaction, which I hope this device cannot do!
;-)

Gorgon
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Re: Opto Isolator Circuit

Post by Gorgon » Thu Aug 11, 2005 12:21 pm

Hi Labview1958,
You might be lucky, not frying the circuit, what are the values of the resistors connected to the LED?<p>Do as Dale told you, connect a resistor(10K) between the collector and the +12V, and try again. You will not do any more harm by this!<p>If the output is 0V with no input signal, you go and buy a new optocoupler! But, before we can tell anything more we need the values of the resistors, and what type of optocoupler do you use?<p>TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

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philba
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Re: Opto Isolator Circuit

Post by philba » Thu Aug 11, 2005 1:45 pm

Here is a correct way to hook up a device like that.
Image<p>Regardless of conversion of silicon to carbon or any other element, not having a resistor between the phototransistor and +12V WILL destroy the device if the LED lights up. You may have been fortunate in that you didn't connect the LED gnd to the PC gnd and did not complete the circuit, thus never actually lighting up the LED.<p>I would use this circuit first with out connecting it to your PC/labview card. With the LED connected, you should read 0V, with the LED disconnected, you should see 12V. When you connect the LED to the parallel port of the PC, you will need a lower R1. You'll need to determine the value from your opto's data sheet - get the Vf and If values (LED voltage drop and LED forward current, respectively). Then use R = (Vs - Vf)/If to get the resistor value. Vs is the output port's voltage, 5V for a desktop, 3.3V (probably) for a notebook. Err on the high side for R.<p>As long as you are looking at the data sheet, check to make sure your opto can handle 12V.<p>To answer your final question, yes, the LED needs to share ground with your PC.<p>Phil<p>[ August 11, 2005: Message edited by: philba ]</p>

dyarker
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Re: Opto Isolator Circuit

Post by dyarker » Thu Aug 11, 2005 10:13 pm

Silicon becomes carbon when the magic smoke comes out. hee hee.<p>By having the LED anode to +5V, and pulling down the LED cathode, as in the original drawing; there is no logical inversion. But hard to tell object of this exercise.
Dale Y

labview1958
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Re: Opto Isolator Circuit

Post by labview1958 » Fri Aug 12, 2005 1:37 am

Here it is!<p>Description Opto Coupler, Transistor, Approved, ISQ74
RS Stock No. 307-064
Manufacturer ISOCOM COMPONENTS
Part No. ISQ74<p>The resistor attached to the 5V power supply is 100 ohm and that attached to the ground is 1kilohm. The resistor attached to the 12V power supply is 1 kilohm.<p>"To answer your final question, yes, the LED needs to share ground with your PC."<p>How about the 5V power supply negative terminal? Where does it go? The +5V is connected to the input diode, what happens to the negative 5 V power supply terminal. Is it also connected to the PC ground?

dyarker
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Re: Opto Isolator Circuit

Post by dyarker » Fri Aug 12, 2005 2:48 am

Where is the +5V supply?<p>If it (the +5V) comes from the same board in the PC as "Digital signal from computer", it'll work. If not, then the drawing is only about 25% complete.
Dale Y

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philba
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Re: Opto Isolator Circuit

Post by philba » Fri Aug 12, 2005 8:54 am

I don't understand, where did the +5V come from? I think most optos can be used with out a supply for the LED - given the resistors you mention. This assumes several mA max from the PC interface.<p>Hmmm, your problems are like onions, we keep peeling to find new layers...<p>[ August 12, 2005: Message edited by: philba ]</p>

labview1958
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Re: Opto Isolator Circuit

Post by labview1958 » Fri Aug 12, 2005 7:39 pm

Please look at the diagram in my first post. The +5V is from the power supply to the input of the opto isolator . It is separate from the PC. The PC together with the NI-DAQ 6025E card supply the digital signal. There is my problem. Where does the negative terminal of the 5V supply connects to? I understand the external 5V supply is needed to power the input of the opto-isolator and to amplify the digital signal. Is my understanding correct?

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philba
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Re: Opto Isolator Circuit

Post by philba » Fri Aug 12, 2005 8:49 pm

no. you have it wrong if all you are trying to do is isolate the PC signal. Hook up the resistor on + side of the LED (anode) to the signal output of the PC, hook up the resistor on the - side (cathode) of the LED to ground. You can actually get by with a single resistor. The signal is going to drive the LED. <p>You have to check the datasheet of the opto to see what kind of current the LED will require. Then you need to see what the PC side can supply. If its the parallel port of the PC, it can supply several mA no problem. If it's your daq card then look at the manual/datasheet/whatever. If the LED can get by with that much, then you are good. <p>Otherwise, you'll need a transistor to boost the current but I'm not going there...

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Re: Opto Isolator Circuit

Post by Gorgon » Sat Aug 13, 2005 8:57 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by labview1958:
Here it is!
Part No. ISQ74<p>The resistor attached to the 5V power supply is 100 ohm and that attached to the ground is 1kilohm. The resistor attached to the 12V power supply is 1 kilohm.
<hr></blockquote><p>Hi Labview1958,
The value of the resistor on the drive or LED side should only be a total of 120 ohms. You only need one resistor. This result in a current of about 15-20mA depending on the capacity of the PC-output driving this. <p>The ISQ74 is a quad optoisolator with a low current transfer ratio, CTR of min 12,5%. This give you the output resistor. 16mA*0.125 = 2mA drive capacity of the output transistor. At 12V your minimum load resistance is 12/2=6kohm. With a safety margin you should select an 8k2 or 10k resistor for this.<p>An optocoupler is not easy to specify and this computation will do for relative slow changing or stable on/off signals. <p>TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

dyarker
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Re: Opto Isolator Circuit

Post by dyarker » Sun Aug 14, 2005 10:18 pm

The negative side of the separate +5V power supply should connect to common output pin of NI-6025E.<p>To simplify even more, use +5V output of NI-6025E card, and forget separate supply.<p>[ August 15, 2005: Message edited by: Dale Y ]</p>
Dale Y

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Re: Opto Isolator Circuit

Post by cato » Mon Aug 15, 2005 5:04 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Dale Y:
Silicon becomes carbon when the magic smoke comes out. hee hee.<p>By having the LED anode to +5V, and pulling down the LED cathode, as in the original drawing; there is no logical inversion. But hard to tell object of this exercise.<hr></blockquote><p>That would imply that the magic smoke is Oxygen, which is an invisible gas oderless. However, in my experiance, the majic smoke is quite visible and is quite far from oderless.

dyarker
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Re: Opto Isolator Circuit

Post by dyarker » Mon Aug 15, 2005 5:43 am

peter-f and cato,<p>You are now pulling my leg now, aren't you? Right?<p>Of course silcon doesn't really change to carbon, and there is no magic smoke. Those are just less blunt ways of saying a mistake resulting in ruined electronic components has been made. Everybody that works in electronics has "smoked" some transistors and diodes.<p>---------------------
The way labview1958 had the LED connected, Gorgon is probably right that the opto-coupler is still okay. The LED probably NEVER turned on, so the transistor also never turned on and had an over current condition.<p>Cheers,<p>[ August 15, 2005: Message edited by: Dale Y ]</p>
Dale Y

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