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Optocoupler 4N33 Answered

Hey all, I am building an arduino firework controller and have a question about optocouplers vs relays. I was planning on using 2 9volt batteries to power a nichrome igintor. Instead of using a relay I was planning on using a 4n33 optocouplers to save money. Is this ok? Also what is the diffrence between a 4n33 and 4n35? Thanks in advance!


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1 year ago

I highly doubt the opotcoupler will be capable of switching your wire as they can only tolerate a few mA.
You would need a suitable transistor to switch on and off the wire.
Considering the on time I don't think you will have any problems with a small relay.

Jack A Lopez
Jack A Lopez

1 year ago

You have given me an outline so general, that there is no way for me to give you a yes-or-no answer to questions like, "Will it work?" Or, "Is this ok?"

Although I can kind of guess at some of the things you're probably doing wrong.

I am guessing one of your errors is thinking an optocoupler is a kind of drop-in replacement for a relay. Optocoupler and relay are not exactly the same thing.

An optocoupler is a LED and a phototransistor, together in the same package. Note that the input side is a LED. Sane designs will have something in series with that LED, usually a resistor, to limit the current to it.

Regarding the output side, the phototransistor, there will be some limit to how much current that can withstand. The answer to that question, "How much?", can be found in the data sheet.

There are places like alldatasheet.com, and others, who collect data sheets for ICs, and share them with anyone who visits.



In response to your question, "what is the diffrence between a 4n33 and 4n35?"

The 4n33 has a phototransistor plus an ordinary transistor, together as Darlington pair, as its output stage, while the 4n35 just has a single phototransistor as its output stage.

The consequence of this is probably that the 4n33 has more gain, which is good for a switch.

Also the 4n33 has slightly higher max collector current rating, 125 mA, versus 100 mA for the 4n35.

So the 4n33 is probably superior to the 4n35 for a switching application.

Although it is not going to work at all if you need significantly more than, 125 mA = 1/8 ampere, of current to make your nichrome wire get hot.

Another thing you're probably doing wrong is using those little boxy "9-volt" batteries. This guess on my part is based on a guess that you want a few amperes of current to make the ignitor work. I am guessing a stack of AA cells, would be capable of supplying more current, for longer time, and cheaper to replace.

Although, if the ignitor works at around 1/10 A = 100 mA, or less, then maybe it is a good match for those weak little 9-volt batteries, as well as these optocouplers which have similar max current ratings, around 100 mA, or so.

I mean that is one of the big mysteries, to me, of your design. I have no idea how much current*time product, or power*time product, is needed to make your ignitor get hot.

Seems like if we knew that, then the question of what kind of switch to use; i.e. relay or transistor, or whatever, would be easier to answer.