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Topic: Opto-isolator, appropriate use (Read 1 time) previous topic - next topic

Hello Arduino people  :smiley-mr-green:

I have been looking for a device to switch a ignition coil, a 12V, 2.3k ohm one. Because this is a inductive device, I cannot simply connect it through a mosfet, because the mosfet at some point will get "disturbance". Now I could connect all kinds of diodes, caps, etc. But I am looking for a component that already has these things.

Someone in the #arduino channel adviced this component:
http://www.sparkfun.com/products/9118

Now what I wonder about is, will it be able to switch at the speed I need it to, which is turn on within 0.01 milliseconds, and turn off the same. I also wonder, will it be able to take the battery to coil load on the "high voltage" side ?

I hope someone can help with this.

Kind regards,
Sean.

majenko

An opto-isolator is basically a Bipolar Junction Transistor (BJT) where the base is replaced by an optical sensor.  That sensor is triggered by an LED inside the package.

The LED side is just like running any LED - add a current limiting resistor and connect it direct to the Arduino.

The other side is just like any transistor.  Yes, it can switch fast - much faster than your 0.01ms.  Whether it will handle the current demands - well, that's another matter.  Looking at the data sheet, I would say not.

However, all is not lost.

The ideal drive system for this would be what is known as a "Darlington Pair" - a pair of transistors configured so that one controls the other.  This enables the switching of much much larger currents than a single transistor alone.  And the "low" side of the pair could easily be your optocoupler.

On a side note, you should ALWAYS have a diode connected in reverse across ANY inductive load, regardless of how good the switching system is at handling "noise".  The induced back-EMF in an inductive system can be HUGE and is likely to nuke whatever you connect it to without a diode.

Hello Majenko,

Thanks for the reply !

The problem I have with the reverse diode across my inductive load, is that whenever I do connect a diode, there will be no more spark. Because the ignition coil will only spark when the current in the primary coil "collapses", I don't think its possible to work with a diode, because the diode would let the collapsing current spike "escape". Which would not allow the current to reach high enough voltages to create a spark in the secondary coil.

Please, please, correct me if I'm wrong, I have been struggling with that one for a while.

Kind regards,
Sean

SurferTim

Quote
On a side note, you should ALWAYS have a diode connected in reverse across ANY inductive load, regardless of how good the switching system is at handling "noise".  The induced back-EMF in an inductive system can be HUGE and is likely to nuke whatever you connect it to without a diode.

The back emf is what makes the coil spark.

Quote
The problem I have with the reverse diode across my inductive load, is that whenever I do connect a diode, there will be no more spark.

The back emf is what makes the coil spark.

I use a transistor between the coil negative and ground, basically replacing the old points. That should indicate how long ago that was, but the theory remains the same. I used a transistor with a 400v rating.


certainly use an opto to isolate your controller from the coil, the spikes will wreck it

recheck the resistance of the primary, i suspect 2.3 ohm not 2.3kohm. most modern
car/bike coils are 1-1.6 ohm with the switch (MOSFET, BJT whatever) limiting on state
primary current to 4-8 A.

in Kettering ignition systems the negative going spike is normal, dont panic. if you attempt
to clamp it down or otherwise damp it you wont get a spark. in fact it is that same spike
that gets transformed into the secondary and thence to your plug.

MOSFET's arn't very good in the reverse bias region and can be expensive and time consuming
to get right. Horozontal output trannies from old TV's work. they are just a highvoltage BJT,
old school stuff. they have poor forward current gain though, commonly about 5. be prepared to
amplify big time. high voltage darlington pairs could be the ticket tho...

majenko

Hmmm... Tricky one.

Scrap the diode then.  Whatever you use will have to be able to handle a massive reverse voltage spike gracefully.

A MOSFET is out - the parasitic diode will just absorb the spark.

It has to be a BJT of some form, but one with an exceptionally large Emitter-Collector voltage.  The drive transistors on that board would no way be able to cope with that.  Something like a TIP50, or a BUL216 might possibly do the job - the BUL216 is specifically designed for large inductive loads like fluorescent lighting and switched mode power supplies.

Also, you'll be looking at a forward current of around half an amp or so.  That's quite a lot - yes, the on-board drive transistors (rated at 600mA) will just be able to cope with that - but without much leeway.

Personally I'd recommend asking about BJT based switching of an ignition coil on an Automotive Electronics specific forum.  I'm not an expert in making big sparks ;)

Could this one do the trick ?

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000023.pdf

Kind regards,
Sean.

majenko

That looks beautiful.

Tailor made for ignition systems.

I'd still be inclined to keep it separated from the Arduino with an opto-isolator - just to ensure there can be no interference between the two circuits.

interesting topic

with the switch, whatever it may be, in the negative (earth) side of the coil the spike
will be negative, yes negative with respect to ground. MOSFET's dont like it! you can put
a fast doide in series with the MOSFET to bear the brunt of the reverse bias but it must turn
off faster than the MOSFET. parasitc capacitance becomes critical in the sharing of the reverse
bias.

easier to use highvoltage BJT's. a 1000-1500V BJT will probably have a reverse bias tolerance of
400V.

Oke, well thats going to be it then. MCU -> Opto-isolator -> IC Driver -> Coil.

Once again, thanks for all the ideas and brainstorming everyone !

Kind regards,
Sean.

just had a look at the link you posted, ideal!!!

to think of the time and money i spent those years ago to make something like
this unit!!!

Haha, well, all credit go's to "neverther" from the arduino channel, he found out they use this IC in the megasquirt project.

Kind regards,
Sean.

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