ludnough said:
First, thanks to you and Charlie for your helpful responses.
The one I need to know about is being driven by 24VDC. For my own
personal information, could you also tell me what problems and fixes are
AC-specific?
The most common suppression for DC coils is to put a diode across
them, oriented such that the normal drive voltage reverse biases the
diode, turning it off. When the drive contact opens, the coil voltage
reverses, turning the diode on, so that the coil current transfers
from the switch to the diode, and the coil voltage does not rise more
than a volt or so, greatly reducing the voltage across the contacts
(or other switching device). This low reverse across the coil does
slow the decay of the magnetic field, and slows the release of relays,
so if you want fast release, you have to add a resistor or zener in
series with the diode, to allow the voltage to rise higher, while
still controlling it. A small capacitor across the coil also reduces
the rate of rise of voltage as the coil current charges up the
capacitor. This is the method used on ignition systems to protect the
points from arcing damage. It allows the contacts to get enough air
between them before the voltage rises high enough to strike an arc.
This is similar to what is done with switched AC inductive loads. The
diode cannot be used, because the normal drive voltage will forward
bias the diode and destroy it or something else with uncontrolled
current. So the capacitor method is used, often with a low value
series resistor to limit the inrush current when the AC is first
switched on (in case switching takes place at the peak of the cycle).
For instance, a 120 volt AC relay coil might have a .1 uf 400v film
cap in series with a 470 ohm 1/2 watt resistor, if the relay is small,
or a 1 uf cap and a 47 ohm resistor if it is a big contactor.
I like to also put an MOV (resistor that varies with voltage, high
resistance at low voltage and low resistance at high voltage) rated to
switch on at a little higher AC voltage than that used to drive the
relay. This limits how high the voltage can be pumped across the cap,
making sure the spikes don't eventually damage the cap. But an MOV
alone is not very good at eliminating the arc. It just reduces the
strength and duration of it.