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Resistor size for optoisolator

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JohnnyB60

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Hi all, since I’m a newbie, I thought I would let you know where I’m at as far as electronics. I’m not an electronics tech so I don’t know a whole lot, but I have had some classes in collage about 30 years ago. I only remember or know enough to be dangerous. :D

I’m trying to use optocouplers to connect board that controls LEDs to provide open/short inputs to another board. The LED board has a 5VDC low current output with a common negative ground. I bought PS2501-1 Optically Coupled Isolators from Jameco but I cannot figure out what size resistors to use between the anode and 5V power. I used an online resistor calculator and I tried to test it with a 220 Ohm resistors and a 4.5V power supply, but I don’t know if I burned it up or if I maybe got it to hot with a 15 watt soldering iron. Anyway the coupler is done and there is no more life to it.:(

I’m giving up on trying soldering it and I’m placing an order for some sockets and replacements for the bad ones, but I also want to order the correct size resistors. So can someone tell me what to order?
 
The IR LED has a forward voltage of typically 1.17V as listed on its datasheet. so with a 4.5V supply the 220 ohm resistor has 4.5 - 1.17= 3.33V across it and then its current is 3.33/220= 15.1mA. Its max allowed current is 80mA when it will be very hot.
Its IR LED could have a forward voltage as high as 1.4V when the current will be 14.1mA.

The PS2501-1 has 8 ranks for sensitivity of 80% to 600%.

A good solder joint takes 1 second which is not long enough to damage an IC. If the soldering iron is too cool, too hot or is corroded then it cannot solder properly.
 
If we look at this data sheet it tells us a forward current of about 10 mA with a forward voltage of about 1.17 volts so we have 5 volts supply - 1.17 volts diode / 10 mA = about 383 Ohms. However you will never find a 383 ohm resistor so maybe use a 390Ω resistor value. That or 360Ω since we used typical and not maximum values for the diode in the opto coupler.

Hope that helps.

<EDIT> Beat by AG. :) </EDIT>

Ron
 
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Thanks guys, but I’m still confused. If I understand what you’re saying by your calculations it should have worked. I actually hooked up 8 of these and all 8 are showing up bad. One is completely shorted and the other 7 are open so I’m confused as to what destroyed them.

The only other thing besides the soldering would be my handling of them. I had a difficult time installing them on my board because they kept popping out of my tweezers and landing in the rug. My eyes aren’t so good anymore and I had a hard time finding them in the rug, but could static electricity be the cause of their failure?
**broken link removed**
 
They look to be wired fine. All the pin 2 cathodes to common and the 220 Ohm resistors in series with the anodes to limit the LED current. I doubt they were victims of ESD. Also it would take considerable heat for considerable time to kill them. However, the soldering would be the last possible guess. You may want to do a little practicing with the soldering for the next batch. I assume when you say shorted or open you are checking between pins 1 & 2 using a meter with a diode test function? What size soldering pencil (wattage) are you using? Also what diameter and composition of solder?

Ron
 
They look to be wired fine. All the pin 2 cathodes to common and the 220 Ohm resistors in series with the anodes to limit the LED current. I doubt they were victims of ESD. Also it would take considerable heat for considerable time to kill them. However, the soldering would be the last possible guess. You may want to do a little practicing with the soldering for the next batch. I assume when you say shorted or open you are checking between pins 1 & 2 using a meter with a diode test function? What size soldering pencil (wattage) are you using? Also what diameter and composition of solder?

Ron
Yes, I am measuring across pins one and two. I may not be saying it right but the second one from the right #7 is showing 823K ohms in both directions and I can’t read anything in either direction for the rest.
As far as the solder I can’t read label on Kester spool. I only know that it’s fairly small in diameter, very shiny, rosin core and does not contain lead. I thought I was pretty fast in soldering it. I had a couple of breaks in the wire because I didn’t have the proper stripper and I may have used a little more heat to repair those, I don’t think it was enough to travel all the way back to the IC. Oh and I was using a 15 watt pencil that I just bought from RadioShack.
 
I almost forgot that when I used the diode test function, Igot nothing on any of them in any direction. could I have been doing that wrong?
 
May be you should measure current flow across the transmitter diode.

A current test tells you more than a diode test.

Boncuk
 
I’m trying to use optocouplers to connect board that controls LEDs to provide open/short inputs to another board.

Could you explain this in a bit more detail?
How are you turning the opto on & off... switch, PIC, etc.?
What is the opto's load?
 
Could you explain this in a bit more detail?
How are you turning the opto on & off... switch, PIC, etc.?
What is the opto's load?

I have another board that controls LEDs. It basically is a programmable board with a 5V outputs that that turns on LEDs in any combination. I haven’t even connected it up yet because I want to know for sure if the opto works first because any short will damage the other board. So far I have only connected a RadioShack adjustable DC Power Supply set at 4.5V.

I’m then using the Opto pins 3&4 to provide on/off 5V DC input control for another board. The other board does work if I short out pins 3&4.

I wonder if that 5V from the other board in combination with the 4.5 V on Pin 1 is causing the problem. Maybe I need another resistor on that side. The manual for the other board gives 5 or 6 examples the types of input to use and besides the standard toggle switches and relays, they show a logic input using a 1K resistor in series with the +5Volt side.

I would much rather prefer to use a STDP relay instead of the Opto but I’m limited to 20mA by the programmable board.
 
OK… so the way I understand it, your using the 5V signal that controls the FET’s on board “A” to drive the opto. The output of the opto is to provide an “on/off” signal of 5V to board “B”. Is that correct?

... they show a logic input using a 1K resistor in series with the +5Volt side.

Do you mean a “pull-up” resistor? On board "B" is what is the "logic" of the on/off... 5V= on, or gnd = on?

If you don’t need galvanic isolation, you can do this with transistors.
 
OK… so the way I understand it, your using the 5V signal that controls the FET’s on board “A” to drive the opto. The output of the opto is to provide an “on/off” signal of 5V to board “B”. Is that correct?
Do you mean a “pull-up” resistor? On board "B" is what is the "logic" of the on/off... 5V= on, or gnd = on?

If you don’t need galvanic isolation, you can do this with transistors.
Yes, that is what I got. Instead of controlling LEDs, I want to use the program board to control a servo driver board.
I originally was just going to use the logic example to control the server driver, but after talking to the manufacture of the servo driver they said that I could not have the negatives of the two boards A&B connecting because the driver board was a common chassis ground and the LED program board had an isolated ground. This is one of the reasons why I have not tried to connect up the program board.

The term “pull-up resistor” has been tossed around with some Electronic Techs during my original design phase, but I’m not familiar with what that was exactly. The manufacture of the driver board recommended the Optoisolator without the resistor, but my coworkers are questioning that. So I just don’t know

The inputs on the Driver Board are active low. they change when the line is pulled to ground.
 
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How much current do you have to "sink"?

I have no idea. I don’t think it’s very much, because the driver board operates on its own power to run a SX28AC microprocessor.
I was going back through my emails from the manufacture of the driver board and I can’t find the part of not using that 1K resistor on the output side of the Opto, so I might have misunderstood that.
 
The amount of current you can (have to) "sink" relates to the CTR and the diode series resitor. Just a guess, but could the 1K be the pull-up resitor to the "on/off" pin on board "B"?

Do you need galvanic isolation?
 
The amount of current you can (have to) "sink" relates to the CTR and the diode series resitor. Just a guess, but could the 1K be the pull-up resitor to the "on/off" pin on board "B"?

Do you need galvanic isolation?

I don't know what a galvanic isolation means, I'll have to look that up. According to the manual the 1k resistor is only used for logic inputs.
 
I got it working, thanks everybody. Yea!

I figured I had nothing to lose and a lot of Photo couplers to burn up, so I experimented with a few configurations and discovered that I had the +5V connecting to pin 3.

I also needed a 1K ohm resistor in series from Pin 4 to +5V. :D
 
as i know the out puts would be a photo transistor, so you can not expect dry contact out puts from it. you should know the termination of the circuit connected to it to make it to work correct.

also you are using opto as a logic coupler so you may not need full current to operate it, you stil can use higher resistor value.

if you are connecting a LED to the output then you have to use a current limiting ressitor in the output too. may be this is the reason for the damage of the couplers
 
you are using opto as a logic coupler

I think this was where I went wrong, because the Manufacture did have a reference in the manual about using a 1K resistor for a logic input to board B, but people kept telling me that it would be just like a relay. I don’t know enough about electronics to know any better, so I missunderstood..
 
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