Phil Dayson writes:

> In previous postings I have described a MIDI-controlled solenoid
> operated harpsichord.  I have run into a problem and am looking for
> some advice.  The solenoids (from QRS) draw about one ampere.  They
> are controlled by one of John Wale's MIDI units through a homemade
> driver board.

Is this the "Harpsitron", on the same Web page as the "Xylotron"
xylophone?  Those are very good for home-built instruments.

> Recently one of the solenoids burnt out and in the process went short
> circuit.  This in turn caused a huge current in the driver which
> subsequently charred and sent a plume of smoke into the air.

Often the reverse occurs, where the driver transistor shorts out and
lets the smoke out of the solenoid!  Either way is bad.

> I know that the commercial solenoid pianos have protection circuits.
> Are there any suggestions on how to prevent this from occurring again?
> I guess I could put a fuse in-line with each solenoid but I would hope
> there is a better solution.

At this stage, a fuse per line might be easiest, although it will not
protect against a shorted driver transistor's destroying the solenoid.

If you're willing to rebuild your driver board, you can use either or
both of two approaches.  First, the current limiting:  In the output
circuit, you add a resistor in series with the coil current, so it
develops a voltage dependent on the current flowing.  This voltage is
fed back to the drive circuits such that if it exceeds a certain
amount, the output transistor is shut off or at least throttled back.

Another feature, that should be used in any solenoid driven percussion
or plucked instrument, is to feed the full voltage and current for the
first few tenths of a second, then to back off the current to a
fraction of that just to hold the solenoid closed.  This requires a
capacitor, a diode, a few resistors, and maybe more transistors in
between the John Wale board and the output transistor.  In fact, so
does current limiting.

The second idea helps the first -- if the coil shorts out, the
electronics will run the full current through it only for a fraction
of a second.

There are probably simple, elegant designs that accomplish both of
these with a minimum of extra parts.  Any half-baked engineer can
design a working circuit with two IC's and a half-dozen transistors per
note, but to do it small and cheap takes real ingenuity.   Let's see
what people come up with.

Mike Knudsen