piclist 2002\10\29\164234a >
Thread: Speeding up a relay - zero crossing detection
www.piclist.com/techref/io/motors.htm?key=relay
BY : Russell McMahon email (remove spam text)

> A 5.6Kohm resistor in series with the catch diode gave me a response about
> like the 33V zener.  I am going to try a 51V zener I just happen to have
> laying around and see how that works.

A resistor in parallel with a zener may give you a better result.
Roman questioned my comment about a higher voltage zener giving a better
result. His real world observations obviously make sense. In an ideal world
where there is no stray capacitance I think what I said holds as the voltage
will rise to whatever value it needs to to dissipate the energy. But, as I
noted, the stray capacitance will  store energy as the voltage rises and you
then get an oscillatory decaying waveform once the peak voltage reached is
less than the zener voltage. If you have say a 27v zener and a resistor in
parallel with it that is HIGHER in resistance than one that would drop 27v
initially then the zener will initially dissipate more energy than the
resistor alone would but the resistor will continue once the voltage drops
to below Vzener.
example to make this less clear :-)
100 mA initial current.
20v zener
R effective = V/i = 20/100mA = 200R.
Place say a 1000r in parallel with zener.
When zener conducts the 1k will conduct 20/1000 = 20 mA or only 20% of the
initial 100 mA so the zener is taking most of the energy BUT the resistor is
not allowing the voltage to rise as high as it otherwise would. Energy in
core is L x I^2. When I drops to 20 mA resistor will take it all at 20v so
below this zener will drop out. But by then current is 0.2 of initial and
energy is down to 4% of initial. Hopefully (what's the bet) relay will have
dropped out by now. The resistor damps the otherwise long term oscillatory
tail.

Passing thought:     Custom relays can be built with eg brass or copper head
and/or tail end slugs to tailor operate and release response times and
shapes but that's a whole new arcane world.

Russell McMahon

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