I'm converting a reed organ to be MIDI controlled, and I'm using solenoids to pull down on the keys. There's 88 solenoids total, and I'm going to be powering it with 2x 40A 12V power supplies.

The solenoid control circuit I'm using is this:

My problem is that each solenoid draws around 1.3A @ 12V. This results in very hot solenoids after less than a minute of being held down.

There are clever solenoid driver circuits that use a capacitor(http://www.kynix.com/Product/36.html) to give the solenoid the full power at first, then once the capacitor is charged (therefore no longer conducting DC power), power flows through a resistor that limits the current. However, this system needs time for the capacitor to discharge before being triggered again, and the capacitor has to be of a large value which would not work in this application well (my solenoid driver board design is dense enough at the moment... I want my component count to stay as low as possible). There are more complex circuits that get around the capacitor discharge problem using transistors and even more components (how they work is besides the point here) but again, I don't want many more components since every component gets multiplied by 88 circuits.

However, I think I have an idea for a solution, but I don't know if it would work. My idea is to PWM the raw power going into (all of) the existing circuits and then adding a single capacitor in parallel with my solenoid, like this:

Thus, when the MOSFET was triggered, this capacitor would get charged to 12V before discharging into the solenoid, making it fire. Now that the solenoid is down, the PWM power (lets say its 50% duty cycle, therefore equal to 6V) can keep it held down.

Is this idea completely flawed? If it is, any ideas? Thanks.

@Pemega Sounds reasonable. But if you were to drive the solenoid both ways... on and off..... and add some mechanical hysteresis.... then you can bypass the problem completely. There was a patent in 1972 http://www.google.com/patents/US3683239 for this.
If they do not exist then all you would need to do (I know, it is never "that" easy) is to glue a small permanent magnet to the "on".... "down" side of the solenoid.
But they do exist...... try Googling "latching solenoid".......
If 1Kgf holding force and 10mm stroke will suffice........ https://www.ebay.co.uk/itm/BLP-Latching-Pull-Action-DC-D-Frame-Solenoid-10mm-stroke-12W-12Vdc-A1-3002500798/322919540231?hash=item4b2f812607:g:GLoAAOSwTf9ZWmss
The magnet needs to be powerful enough to "hold" the key return spring, and you might
need a sound damping buffer at the "stop"...... so a little more powerful again.
Repeat attempts at demagnetisation by the solenoid will mean that you need a permanent (nickel?) magnet..... something like what is used in a loudspeaker......
David

@whale-av Damn, son! That sounds like one meaty ass project! Your idea does not seem flawed to me, however that whole setup x88 is going to be a really intense build! (you know that already though, i'd bet.) One thing I find is helpful when working with gear that eats electricity like it's going extinct is to add a large battery to the mix. What I mean, is hook up your power supply so that it has a powerful battery helping the power along with your wall plug. After so long, the battery will exhaust itself and you'll be back to similar problems, but that will at least give it quite a bit of run time with easier amperage draw. When it wears out, turn it off, give it some time to recharge and then your up and running again. Ample amperage will help improve your hot solenoid problem as well as charge capacitors a bit faster. It's not a fix-all solution, but in a project like this one, every little bit helps i'd imagine.
Cheers.