Re: [CAD_CAM_EDM_DRO] Spindle Relay Question

In a message dated 5/25/2005 10:01:01 P.M. Central Daylight Time,
Dave@... writes:

I guess I should have been more specific, in that I would suggest a good
old fashioned electromechanical DPDT relay to do the switching of the
power leads to the motor, and driving the coil of the DPDT power relay
with a simple solid state relay as an amplifier for the TTL output of
the computer.<<
OK, as Emily Latella used to say, "Well. That's completely different!
Nevermind!" The DIP ULN2003 provides SEVEN separate Darlington transistors with
built-in base-resistors of value specifically for TTL input. The ULN2004 is
"same" except it takes up to 12 VDC input. Also, these ULN parts have seven
"reverse-EMI diodes" which connect from +Vcc to collector-out, so the stored
EMI in the relay-coils don't blow the transistors. That is, you don't have
to ADD your own cross-the-coil diodes. If SEVEN outputs is too many, take a
look at the 8-pin DIP 75477. It has TWO "coil drivers" and diodes, as well,
and is "TTL-input" specified.

Nearly all solid state relays use triacs or back to back discrete SCRs
which remain conducting once triggered until the next zero crossing of
the current passing through them. They can be triggered on by a very
brief spike of noise that would never even be noticed by an
electromechanical relay. If you get a spike at the wrong instant, it
can trigger an SSR on at the wrong time and create the short circuit you
described, and let the magic smoke out :o) That's probably what
happened to your original system. <<
Yes, I had assumed that. Jan Rowland





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