Alan B. Pearce wrote:

> My first thought on seeing the description of the symptoms is that the power
> supply has a lot of impedance in it, and when the relay tries to pull in the
> supply voltage drops to a point where there is not enough volts for the
> relay to hold in, and so drops out. this will create an oscillatory state,
> which IIRC was the original problem. If this is correct, then no capacitor
> anywhere but on the power supply (along with some heavier wires) is going to
> fix the situation.


No, because the cap B-E is fed through resistors
of tens of kohms. Coupled with the large hysteresis
of the relay itself all that is required is to
slow changes at the base. The difference between
typical pull-in and drop-out voltages are around
2:1 or 3:1, which is way outside the expected
PSU variations.

He said the vibration problem occured at relay
turn-off, and then adding a cap C-B as Peter
suggested didn't fix it. I'm not surprised.
-Roman

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.