# Best Opto Isolator ever?? Strange goings on.

I bought some MOC3010M to use with AC control Triac.

White Papers:

http://www.mouser.com/ds/2/149/MOC3010M-196146.pdf

Far as I can tell the emitter led between pins 1 and 2 is supposed to drop a max of 1.5v and is supposed to trigger the Triac at a minimum of 10ma?

I am using Arduino output pin to send 5v to it through a 1K resistor. As far as i can tell what is realy happening is that is is dropping 2.44v (2.56v measured across 1K resistor and subtracted out of 5v) and is still able to trigger the Triac with a current of only 2.44ma (calculated with ohms law)??

Is the white paper way off? Is my methodology way off? Anyone know why this is happening please?

BTW I do not have the AC and Triac hooked up yet. I am just using a led/resistor powered by 5v as a stand in on the bread board.

Thank You.

There is something strange in your mesurement. Leds have normally very stable forward drops. And the datasheet says typical 1.15V @ 10mA. To calculate the correct resistor value you have to use a combination of kirchhofs voltage law and ohms law. Kirhoffs voltage law states that the sum of voltage drops equals the applied voltage. So Uf+Ur=U. Assuming that you have U=5V and the led drops 1.15 gives you Ur=U-Uf->Ur=5-1,15=3,85V. Since the resistor and the led are connected in series the same current is flowing through both. So we can use Ohms law to calculate the value of the resistor R=Ur/I-> R=3,85/10e-3=385Ohm. This value is not critical at all since we are talking about a on/off situation. The datasheet states that a current of max 15mA will always trigger the triac, as long as the trigger pulse i longer than 100us. If you want to use a shorter pulse you have to increase the current (figure6). Presumably the triac will trigger at a much lower led current but the manufacturer does not guarantee that.

All I can say is I defiantly have a 1K resistor in series with the Transmitter LED inside the opto and it's defiantly working. I just hooked it up to the BT137 and it turns it on and off when it should. So strange. Maybe I will try a different opto just in case I have some wierd fluke.

The probability of a multimeter showing the wrong value is small (as long as the batteries are ok) is small compared to other error sources. Like having additional resistances in the circuit. Choosing the incorrect measurement points. Having a diffrenet voltage than expected etc

Ok, I measured right across the opto. It's only dropping 1.11v and the 1K is dropping about 2.5v. So I'm at 3.5volts when I should be at 5v. Something is pulling my 5vdc way down. Using Arduino to power it. It is just driving an Atmega, 2 regular LED's an opto and the gate on a Triac. Hmmm Also an LCD with back light. I wonder if I am just asking too much. I'm going to turn down my back light and see what happens.

How are you powering you arduino? By usb? Take a measurement of the supply voltage as well. But your values make a lot more sense now

Also running a 16mhz crystal. I unplugged Arduino from the load and only getting 4.5V out of it's +5. Perhaps the battery is dying. Will look into it tomorrow.

However, 2.5vdc drop across 1k resistor still only = 2.5ma through the LED Emmiter. Should need 10 per white paper? I still don't understand that. My meter is off by only 20mv according to my Tek 1002B O Scope:)

Yes you are getting 2,5 mA. It is the resistor that determines the current, not the datasheet. But as i said before, the triac will probably trigger @ 2,5ma or even lower but the manufacturer does not guarantee it.

The figures in a datasheet are one's that the manufacturer are guaranteeing will work for all devices at all temperatures and all specified output loads. If a particular device performs better then that's great but if some perform worse the manufacturer could be in for an expensive time dealing with dissatisfied customers and fallout therefrom.

You sometimes see a parameter in a datasheet being given both as "typical" and worst-case values. You always ignore the typical, go with the worst case if you want your circuit to be reliable. Some manufacturers like to put the typical figures at the top of the datasheet to make the device seem better than it is.

late to the party and confused.

the LED needs 20ma. Based on 1.2volt drop:

the power supply is 5 volts

the calculated resistor is 220 ohms

• or -

the 1k ohm should give you about 4mA

The LED in the opto is treated like any other LED.

what values for the voltages do you get when you plug in these numbers ?

Far as I can tell the emitter led between pins 1 and 2 is supposed to drop a max of 1.5v and is supposed to trigger the Triac at a minimum of 10ma?

From Figure 6 in the datasheet, it looks like only 1 mA is required to trigger when the LED pulse width is >50µs.

It is the resistor that determines the current, not the datasheet.

Real nice Nilton :)

Dave.

As per Ohms Law, 5v / 1000 Ohms = 5mA current. Does it not?

dlloyd

That Explains it :) 10 points for Gryffindor :)

Thank You All.

All this discussion of an unimportant characteristic. If the data sheet says maximum guaranteed turnon is at 15mA why wouldn't you give it that? I would give it 20mA like any LED. Are you out of pins?

The diode voltage drop is spec'd at 1.5V max drop. 5V - 1.5V = 3.5V. 3.5V / .02A = 175 ohms. Closest value is 180 ohms.

Show us what else you are driving with that same pin and how, we might be able to suggest a simple work-around if you can't give it 15mA because of other devices you are connecting in parallel.

I have no problem giving it that. I have just never done this before and so I was shocked to find it actually worked at a much lower current. I did not know that was normal but I realize now that it is. I don't think the amount of current needed to trigger the Opto is unimportant :) If you don;t have enough, you don't get the trigger, if you get too much...maybe you blow your LED? It's going to be on a lot :) I have a lot of time working with electricity but it's such a vast field there is so much to learn all the time. I'm new to some of these components and so I need a little help sorting through the volumes of info in the data sheets etc. All help is appreciated.

When the object of your circuit is to turn it ON, you should pay attention to the data sheet and not just turn it halfway ON or operate it in a linear mode. If the manufacturer says it needs 15mA, then give it at least that. It also says your MAX current is 60mA so it stands to reason you want between 15 and 60mA. As long as you stay inside those parameters you should be fine. Even at 60mA x 1.5V you are dissipating only 90mW.

jarrod0987:

It is the resistor that determines the current, not the datasheet.

Dave.

As per Ohms Law, 5v / 1000 Ohms = 5mA current. Does it not?

yes, all the way up until you add anything else to the circuit., like an LED.

5 volt power less 1.2 V for the LED = 3.8 volt

3.8 / 1000 = 3.8mA

the calculation is (supply voltage - LED voltage drop ) / limiting resistance.

OOps, your right I forgot to subtract the LED Voltage from the R :)

am using Arduino output pin to send 5v to it through a 1K resistor. As far as i can tell what is realy happening is that is is dropping 2.44v (2.56v measured across 1K resistor and subtracted out of 5v) and is still able to trigger the Triac with a current of only 2.44ma (calculated with ohms law)??

mackyy20: am using Arduino output pin to send 5v to it through a 1K resistor. As far as i can tell what is realy happening is that is is dropping 2.44v (2.56v measured across 1K resistor and subtracted out of 5v) and is still able to trigger the Triac with a current of only 2.44ma (calculated with ohms law)??

the origional post said teh device is the MOC3010M the data sheets lists Ift for this as 15mA with a maximum of 60mA

the Application Notes sheet : https://www.fairchildsemi.com/application-notes/AN/AN-3003.pdf says : Basic Electrical Description The AlGaAs LED has a nominal 1.3 V forward drop at 10 mA and a reverse breakdown voltage greater than 3 V. The maximum current to be passed through the LED is 60 mA.

and list the MOCxx10x as 15mA

I would offer that an LED will have output degradation over the life of the unit. so the initial LED output will far exceed the ratings, but after x hours, the degradation will reduce the output. so your initial testing will prove proper operation, on the bench, with high level control of temperature and power, with a new LED that has had near zero burn-in time.

the manufacturer has to allow that some LED's will outshine others in any batch while also allowing that some receivers will be more sensitive. conversely, the worst LED of the batch coupled with the worst receiver of the batch will or should still work.

since we often speculate on these things, the production line for both devices and raw materials controls and manufacturing tolerances, etc, etc may have been delivering a superior product.

more speculation is that they may have made a huge lot of the xx23 with their 5mA threshold and since they fit the specification for every one above that, just sold the better product with a different label.

more speculation is that the LED is a POC and will fade sooner.

the app notes list 1.5v for the voltage of the LED

my take is that the manufacturer offers a minimum. best to meet that. they do offer that going past that may degrade the LED prematurely,so there is no need to drive it any harder.

if you want to run it at a lower power, that is your choice. if you are making a board, I would offer that couple of vias, placed on either side of the resistor would allow for future modification.