Opto & MOSFET - Can someone double-check this for me?

Hi! Just a quickie, I’m about to put the following circuit together. I’m a bit short on the required components having only a couple of each in the parts box, so I just want to check I’ve got things right as I’d rather not blow anything. Seems simple enough but I’ve been known to miss the obvious before…

Thanks :slight_smile:

It's okay. Allthough it's a bit unusual to use the transistor of the optocoupler that way, but it is okay to do so.

I don't know the type of optocoupler, so I don't know if the 220 Ohm is right. The 100K is large, a resistor of 1k to 10k would be better. The 100k could pickup noise, and the leak current of the transistor might be a problem with the 100k resistor. Is the load inductive ? In that case you need a proctection diode over the load (flyback diode). Is the +5V the same as the +5V of the Arduino ? In that case whay use the optocoupler ? A serial safety resistor of 1k from the Arduino output to the gate of the mosfet is safe enough.

Thanks for the feedback, change in resistor value has been noted! :slight_smile: The Opto in question is a Sharp PC120, and according to the datasheet the rated input If is 50mA absolute maximum. I’m giving it 22mA, should be fine I think, as long as it fires!

As for the load, it’ll be resistive (a hot wire ignitor), so I’ll be drawing quite a lot of current but for a very short amount of time. My MOSFETs are over-spec’ed for the application, 60V/30A max, but I’d rather have over than under.

I might put a flyback diode on there anyway… I guess you’re right that I don’t REALLY need the opto, but I thought it might be handy to make some little “breakout boards” that could drive most loads safely with no danger to whatever arduino board I happen to be using. A sort of “one size fits all” drop-in module. So as I had a couple, made sense to experiment!

You say it’s unusual to use it that way? Out of curiosity, what would be the “usual” way of doing it?

He probably means like "common emitter", but your way is OK, too. In fact, your way, "common collector", the FET is On when the Arduino output is HIGH.

[quote author=Runaway Pancake link=topic=104813.msg786017#msg786017 date=1336395124] In fact, your way, "common collector", the FET is On when the Arduino output is HIGH. [/quote]

Good! :D That's what I was aiming for. Last question before I get off my lazy bee-hind and go breadboard it, if I wanted to put an indicator LED on there to show what the output's doing, can I put it, and it's resistor, between +5V and the collector on the opto? I only have two... so popping them is not ideal.

http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm

download this for schematic diagram software its very easy to use once you get the hang of it

Haha, not a fan of my hand-drawn scribblematics, I take it? =( Sorry, but I find it easier for doing things quickly... using schematic editors with a touchpad is too much like hard work. Thanks for the link though, I'll definitely bookmark it as I can see it being useful when I get around to buying an actual mouse.

Just breadboarded everything, works fine, found a better place to put my indicator LED and all is well! Might do some soldering later on to celebrate.

The normal current for the optocoupler led is 20mA. This 20mA for the led will put the transtor into saturation. But in your schematic, 10mA for the led is also enough. At 20mA the forware voltage over the led is 1.1V (see the datasheet). You have a current of (5-1.1)/220 = 17mA -> perfect.

The MOSFET is logic-level?

MarkT: The MOSFET is logic-level?

Indeed :)