Re: [CAD_CAM_EDM_DRO] Relay question about diodes across the coil

washcomp wrote:

>Back when I was a kid learning about relay circuits on the job
>(1960's), most of the control circuits I got involved in used low
>voltage A.C. and I don't ever remember seeing a diode. I just found
>out that it's good practice to put a diode across the coil of relays
>to keep them from chattering due to the current stored in the coil
>inductance. Question is: Only on D.C. relays or is this supposed
>to be done on A.C. relays as well?
>
>

On relays energized by DC, you use a diode connected so it will not
conduct when the normal polarity is applied to the coil. When a
presumably transistor circuit turns the supply voltage off, the coil,
by its inductive nature, wants to continue conducting current. This can
destroy the transistor that drives the relay due to the large voltage that
can be produced when you try to abruptly stop the current in the coil.
The diode provides a safe path for the current to circulate in the coil,
and the coil's own resistance dissipates the energy in milliseconds.

The same problem exists for AC, and can cause sparking of contacts
when one relay's contacts break the path to another relay's coil.
But, since the AC supply provides both polarities, a diode would
short out the supply every half cycle. There are devices called snubbers,
arc supressors, transient voltage supressors, etc. They generally use
a capacitor and a resistor in series. You can make your own with
these two parts. For a 120 V AC coil, something like 22 Ohms in
series with a .01 uF capacitor will work in most cases. These values
can be tuned for the particular coil.

Jon