I need to interface a relay to an old CNC machine. The machine sends a
24 volt pulse as a signal to operate turrets, indexers, etc. I need to
keep a relay on for about 1 second after being actuated by the pulse.
To do this I connected a capacitor across the relay coil. When the
relay recieves the pulse it now stays on for the time I need. But I
have only bench tested the setup. Is there any danger of sending a
voltage spike back to the machine with the above setup?
Thanks,
Eric
You "bench tested it?" the spike aside for a moment, how does one
bench test it?
A "pulse" may be any pulse width - needs to stay on long enough to
charge the cap to hold the armature down when the pulse is gone - did
you consider that? One assumes this is a DC pulse - since it wouldn't
work with AC . . .
The output impedance of the pulse generator may be low or high - if
high you have a time constant between the impedance and capacitor that
may limit charging.
Two of several failure modes is that a spike goes back to the pulse
generator and eats it - unlikely because the cap would absorb the
energy - there is no spike to speak of, the cap just peters out, it
doesn't switch off abruptly. The cap would appear to be a short
circuit to the pulse when initially charging and that may eat the
pulse generator.
So, without knowing more about the device generating the pulse there's
no good way to tell.
If you are controlling an AC device with the relay, you may want to
consider a solid state relay. Many take an input of 3-32 volts for
the coil at 10 milliamps and will switch the output down to 24 VAC or
up to 240 VAC.
The 10 milliamp spec means it wouldn't load the generator as much as
any coil you're likely to find, 3 volt spec would take a smaller cap
to stay on, and wouldn't generate any reverse EMF when the circuit
opens.
One good idea might be to use a solid state relay.
The one second will be highly variable with this jury rig, one assumes
that would be tolerable?