Trouble with Optoisolator

[Frosted]

HI all. I'm having trouble getting my optoisolator to work correctly. I'm using an NTE 3098 hooked up to an arduino to trigger a camera flash. I've hooked up the flash leads to the collector and emitter prongs on the optoisolator, and the cathode and anode are hooked up to ground and the arduino which triggers the flash.

The issue I'm having is that the circuit is complete between the emitter and the collector all the time, as I'm getting a 3.78V readout from my voltmeter. This means that I can't use the optoisolator to trigger the flash. Why is this happening? Shouldn't the emitter and collector circuit only be closed/complete when the anode and cathode have current running through them, thus lighting the led inside and completing the collector emitter circuit?

 

[crutschow]

If you read 3.78V then the opto isolator is off. When on it should read only a few tenths of a volt between collector and emitter.

 

[Frosted]

My understanding was that optoisolators would only complete the circuit if they were on. In my case, the circuit is complete regardless of whether the opto isolator is on or off. Or am I completely missing your point?

 

[ljcox]

The answer to your question is yes. The opto should only conduct when there is current through the LED.

So, as Carl said, if you are reading 3.78 V, then it is off.

Questions:-

What current does the camera flash circuit put through the Opto?

Do you have a resistor between the anode & the output of the arduino?

My point is that you may have blown the opto - either by too much current through the LED (ie. if there is no resistor to limit the current) or the camera current may have damaged the transistor.

 

[crutschow]

My understanding was that optoisolators would only complete the circuit if they were on. In my case, the circuit is complete regardless of whether the opto isolator is on or off. Or am I completely missing your point?

What is your understanding of "complete the circuit"?

A voltage indicates that the opto is not conducting current, so the circuit is not "complete". When the opto is on (like a switch), it will be conducting current and the voltage will be low, which completes the circuit.

 

[Frosted]

The answer to your question is yes. The opto should only conduct when there is current through the LED.

So, as Carl said, if you are reading 3.78 V, then it is off.

Questions:-

What current does the camera flash circuit put through the Opto?

Do you have a resistor between the anode & the output of the arduino?

My point is that you may have blown the opto - either by too much current through the LED (ie. if there is no resistor to limit the current) or the camera current may have damaged the transistor.

From the arduino to the anode of the opto, it is 5V and up to 40mA. The flash circuit, based on my voltmeter, has a positive lead connected to the collector on the opto, coming in at 3.78V. I'm unsure of the current.

I have a 4.7k resistor between the anode and the output.

What is your understanding of "complete the circuit"?

A voltage indicates that the opto is not conducting current, so the circuit is not "complete". When the opto is on (like a switch), it will be conducting current and the voltage will be low, which completes the circuit.

My understanding of "complete the circuit" is that there should be no current flowing between the collector and the emitter until the led in the opto is switched on via power from the arduino. Is this not correct? I'd like to understand how these devices work if I'm incorrect.

 

[Frosted]

After some research, I've determined that the type of opto I may actually have wanted to buy would have been an optocoupled solid state relay. This is the piece that the tutorial I'm following used; unfortunately, they didn't have that model at Fry's when I went so I tried to find a model that seemed to have the same specs as that one. Is there a difference between an optocoupled ssr and an optoisolator?

 

[crutschow]

An SSR is designed to carry much higher currents than an opto isolator. Also SSRs come in AC only versions which use TRIACS to switch the AC, and DC or AC/DC versions that use power MOSFETs to switch the current. Opto isolators generally only come in DC versions.

 

[ljcox]

From the arduino to the anode of the opto, it is 5V and up to 40mA. What does "up to 40 mA mean?

The flash circuit, based on my voltmeter, has a positive lead connected to the collector on the opto, coming in at 3.78V. I'm unsure of the current.
What voltage do you measure if you remove the opto?
I have a 4.7k resistor between the anode and the output.

This seems to conflict with your first sentence.

My understanding of "complete the circuit" is that there should be no current flowing between the collector and the emitter until the led in the opto is switched on via power from the arduino.
Correct.
Is this not correct? I'd like to understand how these devices work if I'm incorrect.

If you have 4.7k from the output to the anode of the LED, then rthere will not be enough current to turn the transistor on.

I don't know what opto you have, but normally, 470 Ohm would suffice.

 

[ljcox]

I have just downloaded the NTE 3098 spec.

It needs about 16 mA, so that means a 270 Ohm resistor in series with the LED.

 

[Diver300]

What is the voltage on the trigger lead of the flash gun with nothing connected? It may be too much for the opto-isolator.

 

[Frosted]

I have just downloaded the NTE 3098 spec.

It needs about 16 mA, so that means a 270 Ohm resistor in series with the LED.

Thank you so much!!!!!

I just had an epiphany. Originally I thought that there should be no current between the collector and the emitter. Rather, what I now believe is happening is that when the opto is turned on, that current either goes away, or becomes very low for an extremely brief second. When the current returns to the neccessary current to fire the flash (i.e., opto is off) then the flash fires.

Is this correct do you think? Either way it works now, but I'd still like to understand what's going on.

Here's a link to a page with a diagram of what I'm doing. Forget about all the stuff below the push button, as I'm not using those parts.

High Speed Photography - Let Arduino turn The Lights Off For You | DIYPhotography.net

 

[ljcox]

Thank you so much!!!!! You're welcome. I assume you mean that it works with the 270 Ohm resistor and your only problem now is the one outlined below. Is that correct?

I just had an epiphany. Originally I thought that there should be no current between the collector and the emitter. Rather, what I now believe is happening is that when the opto is turned on, that current either goes away, or becomes very low for an extremely brief second. When the current returns to the neccessary current to fire the flash (i.e., opto is off) then the flash fires. That does not make much sense to me.

Is this correct do you think? Either way it works now, but I'd still like to understand what's going on.

Here's a link to a page with a diagram of what I'm doing. Forget about all the stuff below the push button, as I'm not using those parts.

High Speed Photography - Let Arduino turn The Lights Off For You | DIYPhotography.net

I could not see a bypass capacitor in the diagram.

There should be a 100 nF ceramic & a 100 uF electrolytic across the arduino supply pins.

The lack of bypass caps could possibly explain your symptoms.