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

Paul Davis wrote:

>I've had it happen in circuits before. Maybe, though, it was the back emf spike which caused the circuit to re energize the coil? In any case it's a good idea to have the diode.
>
>Paul Davis
>
>
>On Sun, 11 Jan 2004 16:13:56 -0600, Jon Elson wrote:
>
>
>> Paul Davis wrote:
>>
>>
>>
>>
>>> Diodes are a good idea for DC even if you don't care about the
>>> spike caused by the back emf. The relay will remain latched
>>> until it dissipates it's magnetic field. Assuming it doesn't
>>> destroy your driving electronics or short it's coils it can cause
>>> the relay to remain latched for a few seconds after the input is
>>> removed. So for the sake of speed it's a good idea to have the
>>> diode.
>>>
>>>
>>>
>>>
>> Huh? With the diode, the field dissipates slowly, but it is
>> usually done in tens of
>> milliseconds. Without the diode, the field dissipates VERY
>> rapidly, unless it
>> arcs over in the coil, and the field collapses in microseconds.
>> So, the diode slows it down, rather than speeding it up.
>>
>>

Oh, yes. Uncontrolled switching spikes are FAMOUS for causing totally
unpredictable malfunctions in digital circuits. I have seen amazing
interactions,
where a relay being turned off in one unit triggered bizarre malfunctions
in equipment that seemed quite remote to the circuits that were connected to
the relay. And, even experts have sometimes been stymied in finding the
cause of the problem.

Jon