Em 28/08/2013 23:57, IVP escreveu:
>> 1N5818 which is small, cheap, and readily available
> We had a lengthy discussion about this some years ago
>
> The outcome, ISTR, was that a 1N4001 or 1N4148 is better
> than a Schottky
>
> I might be inclined to have a diode for each coil and perhaps look
> at something at the driver end too, considering the length of cable
>
> Maybe a reverse-biased Schottky to catch sub-0V transitions,
> and a zener to clamp > Vcc transitions.
>
> I've found from experience that relay coils will quite readily induce
> undesirables onto wiring
>
> Joe


Just a reverse biased diode in parallel with the relay coil (or any
other inductive device) is enough.
This diode is for recirculating the BEMF of the coil when the driver
turns off.

The diode must be rated for the same current that flows through the coil
or coils, because in an inductor there can't be a step in current, the
current will decay with time, being the shorter the time as the voltage
is allowed to raise further. That is, without the diode, the coil will
produce a very high voltage pulse trying to keep at first the same
current flowing. This pulse will be high but short because the available
energy is finite. With the diode in the circuit, the voltage can't raise
above the Vf of the diode, but the current will decay much slower.

The ideal location for the diode is the closest to the coil's terminals,
to minimize the inductive loop. Placing the diode near the PLC will make
all the wiring from the PLC to the coil to act as a transmitter antenna.
The protection to the PLC is the same though.
If various relays are used but are close to each other, then just one
diode may suffice.


Isaac

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.