SS Relays - Revisited

Jon, someone, anyone please shed some more light...

I ordered one of these
http://www.crydom.com/pdf/dc60.pdf
I hook it up, power up the input.
I'm no electrical engineer, so I don't know the proper
terms/techniques or anything here, but I set my voltmeter to ohms and
touch the output poles and it shows a reading... i shut the input
off, take another reading and nothing. So, from my limited knowledge,
everything looks good. I then hookup the output sides to my plasma
trigger (that's what I am using it for, to fire my plasma cutter) I
turn on my plasma cutter, and it shows a fault... so I'm
thinking 'ok, I did something really dumb here and cooked my plasma
cutter'. I then proceed to disconnect one side of the relay... the
relay input is powered off this whole time, I turn the plasma cutter
back on, and of course the fault is gone. The relay input is still
off, so I reconnect the output and 'IT FIRES THE TORCH!!! (and scares
the bajeezers outa me)'.

So, my question is why would a closed (off) relay act as though it
were on? The torch trigger shows about 18VDC. I have the positive
18VDC connected to the + side of the relay output and the relay input
is turned off. The only thing I didn't check is what the amperage is
on that 18VDC on the torch trigger. Could that possibly have anything
to do with it, if the amperage on that 18VDC were higher than the
rated load current range of the relay?

Please someone help!
Chris

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> mrpootoo wrote:
>
> >Ah! I ran into this problem and couldn't figure out what was going
on
> >for the life of me.
> >
> >What type of relay do you use for switching DC?
> >
> >
> There are solid state relays for DC use. Crydom, Omron, etc. make
> these. If you want a mechanical contact relay, those are
available, too.
> The problem with DC is there is no zero crossing to allow the arc to
> break. Some DC contactors have magnets to bend the arc in a loop,
> others use insulating barriers that pass between the opening
contacts.
> And, of course, if space permits, they just open the contacts wider.
>
> Jon