Homebrewed Electricity > Controls
Using Mosfets
OperaHouse:
Years ago, I had to make dots of glue, 3ms on and 3ms off, which is pretty fast for a solenoid. While I never used this chip, I still remember it LM1949. They probably don't even make it anymore, but it had a nice application sheet explaining the dynamics. Using a scope you can easily see how mechanically fast a relay is by watching the slope of the current change as the inductance varies. It is a lot of fun driving mechanical stuff. Just wish the FET you buy is what you expected. I feel sorry for anyone starting out in electronics today. With all the fake parts it is hard for someone to build something that has a possibility of working.
Mary B:
--- Quote from: OperaHouse on April 23, 2022, 04:53:24 PM ---Years ago, I had to make dots of glue, 3ms on and 3ms off, which is pretty fast for a solenoid. While I never used this chip, I still remember it LM1949. They probably don't even make it anymore, but it had a nice application sheet explaining the dynamics. Using a scope you can easily see how mechanically fast a relay is by watching the slope of the current change as the inductance varies. It is a lot of fun driving mechanical stuff. Just wish the FET you buy is what you expected. I feel sorry for anyone starting out in electronics today. With all the fake parts it is hard for someone to build something that has a possibility of working.
--- End quote ---
Why I point people with questions on where to buy parts towards Digi Key, Mouser, Allied Electronics... all 3 supply the electronics industry where a counterfeit part could run a multi million dollar production run so the chances of a counterfeit are a lot lower. I prefer Digi Key but they are located in MN so it is overnight shipping to me 99% of the time.
Warpspeed:
Driving fuel injectors and stepper motors really fast is all pretty standard stuff these days.
There is no mystery, no secret science involved.
Regardless of scale, an inductor is still just an inductor.
joestue:
i had heard somewhere that the irfz44 was actually used in full avalanche mode driving certain car fuel injectors.
thing is you need the reverse voltage across the coil to go to some significantly high value if you want the solenoid to close as fast as theoretically possible.
shorting the coil with a diode will hold it open as long as possible, it will still close of course because most solenoids are inefficient pieces of S, but, its a lot faster if you let the energy dump itself into the mosfet at 4-5 times the supply voltage. how long it lasts... well.. if you need a tvs diode across the fet then you're doing it wrong.
Warpspeed:
--- Quote from: joestue on April 24, 2022, 11:43:37 PM ---
Thing is, you need the reverse voltage across the coil to go to some significantly high value if you want the solenoid to close as fast as theoretically possible.
Shorting the coil with a diode will hold it open as long as possible, it will still close of course because most solenoids are inefficient pieces of S, but, its a lot faster if you let the energy dump itself into the mosfet at 4-5 times the supply voltage. how long it lasts... well.. if you need a tvs diode across the fet then you're doing it wrong.
--- End quote ---
All very true Joe.
Different applications require different solutions, but a big mechanical drive solenoid, or a linear motor, probably do not need to be made any faster ramping down, as ramping up the current.
So the reverse applied voltage need be no higher than the forward voltage.
That requirement is very easily and neatly met with the classic diagonal half bridge topology I suggested above.
Which is precisely why I suggested it.
And I fully agree that a TVS diode is a band aid solution that is just not required, unless its all being done wrong.
But its all a complete waste of time pointing any of this out.
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