Solenoid coil controller

[CoilKid]

I am working on a project that requires me to use a solenoid. I came up with this schematic, based off of this. I use an FET, a photo-interrupter, several resistors, a choice of two diodes, a CVCC, an 11.4V battery, and of course, my solenoid.



641-1419-2-ND (Diode 1)
1N4148-TAPCT-ND (Diode 2)

With the inductor as the load (solenoid) will this circuit work as I expect it to? Or, would it turn the inductor on when an object passes through the interrupter?

[Dave]

I'm guessing you want to make a "magic" levitation stand?

When the IR beam will be broken, the NPN phototransistor will turn off, allowing the gate-source voltage of the FET to rise and turn it on. So this won't work for you.

You can easily fix this by switching the position of R2 and that phototransistor.

[CoilKid]

Actually, if the FET turns on when the photo-interrupter has an object in it, its perfect.
I am actually building a circular accelerator as a physics project. Kind of a scale, simplified model of CERN or FermiLab's Tevotron. To demonstrate how they work, in broad terms. This is the schematic for the electromagnet power supply.

I thought it should work, but I also thought I should check because I know when an inductor (the electromagnet) turns off, it dumps a voltage spike into the system. That's why I have a freewheeling diode next to the inductor. One of the choices I posted for said freewheeling diode is a Shottky, which is why I used the schematic symbol for one.

Do you think it would work for it's intended purpose? (That is, would it switch the FET on when something occluded the photo-transistor?)

*I meant to ask "Will it work as intended without burning anything out?"

[tautech]

Your use of an external diode for protection of the Mosfet is correct.
Without knowledge of the solenoid specs, I would use at least a 1A rated diode.
The back EMF can be powerful, but it is a single non-repetitive event.
Study datasheets.

[CoilKid]

What specs would you need? I know a lot about the coils. The one thing I don't know, is the inductance. Anyway, aren't the diodes listed rated for 1A?

The Shottky is rated for 40V @ 5A, and the other one is rated for 75V @ 10mA. (I think)

Thanks

[tautech]

Definitely use the 5A diode.
"single non-repetitive event" is the info needed from datasheets for semiconductor usage of this type.

[gxti]

The current through the freewheeling diode will start at the same value as whatever was flowing through the coil before the switch opened, and decay from there. So your diode only has to be beefy enough to take the coil's nominal current, and only for a brief time.

One thing to watch out for is that the lower the forward voltage of your diode, the slower the energy is dissipated. That energy is stored in the magnetic field around the coil and if it collapses slowly it can cause the solenoid itself to release more slowly than it otherwise would. This phenomenon can for example shorten a relay's life by preventing it from opening as fast as it was designed to.

[CoilKid]

Hmm. So if I use the one that can handle 5A, this circuit should work as intended? I want to nail down the design before I order the parts.