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

blipinthedata

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.
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blipinthedata

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.


joeaverage

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...

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