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Forums
4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Coilgun Triggering

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currentkills91
Thu May 05 2011, 03:46AM Print
currentkills91 Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265
Hi again,

I was wondering, could i use a comparator in place of an op amp in a typical optical switch for a coilgun. I've started making a 2 stage rifle and I have a few comparators laying around, and if i could save a few cents why not do it? So if I i can, what would i need to do to make it work?
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Mycologist
Thu May 05 2011, 12:26PM
Mycologist Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22
If you're switching, surely a comparator would be far better than an op-amp anyway? You'd just connect it up as you would the op-amp, seeing that it does the same thing, but with more open loop gain.
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James
Thu May 05 2011, 04:59PM
James Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Op-amps and comparators are nearly the same. One is optimized for comparing and one is designed for amplifying with controlled gain, but there are many applications in which the two are more or less interchangeable.

The output stage and drive capabilities can differ a bit, and a comparator will generally have a faster slew rate at the expense of less than stellar performance as an amplifier, when in doubt consult the datasheet.
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currentkills91
Thu May 05 2011, 06:14PM
currentkills91 Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265
Okay, so if i were to use an lm393 comparator for the switching i wouldn't have to change anything in the circuit except the gain? (sorry im kind of a noob when it comes to electronics)
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Mycologist
Fri May 06 2011, 11:15AM
Mycologist Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22
Yes if you just plug in the lm393 where the opamp is in the schematic, it would probably work, unless for some reason there was a negative feedback resistor (although I can't see why you would want one even with the opamp) which you would need to remove.

Bear in mind you may need a pullup resistor of somewhere between 1k and 10k to get the output of the lm393 to turn on properly; at least that has been my experience with them.

Also, you should probably note that the lm393 is a dual comparator chip, it might benefit you to parallel up the two comparators so they can source more current; particularly if you're driving a mosfet or IGBT, or else the gate capacitance will eat into your switching time, leading to unwanted power dissipation in your transistor.

(If you have any other questions it would probably be helpful if you posted a schematic ^^)
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currentkills91
Fri May 06 2011, 12:47PM
currentkills91 Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265
Okay, so i was searching around some more and i went on anothercoilgunsite.com and found an optical switch that runs off an lm393 comparator :D well i built it and it works amazingly (: that means i have the coil forms made and the switching circuitry done... :DDD
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