> From: "Mark A. Corio" <Mcorio@aol.com>
> I have some sensitive analog circuitry that I need to do some relay switching
> in for varying some filter bandwidths and other things.  I am considering
> using reed relays.  Does anyone have any comments about this?  I would like
> to drive the relay coils direct from a PIC I/O pin (16C74A or 16C84.....not
> sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
> coil suppression or do I need external diodes as with other relay types?  I
> have no experience with reed types but believe the inductive kick should be
> smaller from reeds.

One trick for suppressing inductive flyback is to place a 'shorted turn' around
the coil.  This could be in the form of a cylinder made of copper foil,
soldered to form a tube.  If it fits inside the coil (and around the reed)
then so much the better.  This technique turns an inductor into a resistor
although, depending on coil geometry, there may still be sufficient
leakage inductance to cause some flyback.  Best put a zener in parallel.

>
> In the past, for this type of application I have used low signal level
> latching relays so as not to have a constant current flowing through the
> coils......I have had problems with noise on the coil drive signals being
> imparted on the signal in standard type relays.  This approach also let me
> use a common ground plane without the coil current causing problems in the
> return path.  Remember, I am very noise sensitive here.  In my current
> application the relay (reed or otherwise) will have the coil driven by a
> signal from another board (both ends of coil) without electrical connection
> the board with the signal.  Are reed relays more sensitive to coupling of
> noise to the signal from the coil since the contacts are surrounded by the
> actuating magnetic fields?  Any experiences with this are of most interest.

Again, the copper sleeve may be grounded (to signal ground) which will give
additional protection against capacitively coupled noise.  There may be a very
small magnetic coupling to the signal if the reed is slightly helical: very
small indeed, considering the 'turns ratio'.

Regards,
SJH
Canberra, Australia