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The Ghost in The Machine

On Sep 9, 7:44*pm, John S wrote:
On 9/9/2011 6:53 PM, Jamie wrote:

John S wrote:

On 9/9/2011 6:30 PM, Jamie wrote:

ehsjr wrote:

I don't know how to make it clear enough for you to understand.
While I do appreciate your attempt to help, I must say it misses
the point.

I want a *specific* link to an *AC/DC* relay of the type *NT*
had in mind, or to an ap note or reference that discusses a relay
coil that is rated for *both* AC and DC as he indicated.

Do you have a link to what NT was talking about? Here's what
he said:
"The various ones I've seen that have coil 2 ratings, one for ac one
for dc, have consistently had the dc coil rating be half the ac
voltage rating."

I _do not_ need generalizations, hand waving and condescending
responses. You mentioned a relay that will "do" DC because it
has an imbedded (sic) diode and say "The voltage ratings are the
same." That is *NOT* what NT said. He talks about the dc
coil rating being half the ac rating.

Ed

Is this what you're looking for ?

http://relays.te.com/appnotes/app_pdfs/13c3250.pdf

Geeeez. I posted that link in my _first_ post in the thread, and
you replied to that post. I guess you either didn't read the post
properly, or didn't understand it. The ap note does not agree with
the DC at ~50% of AC rating idea - details are given in that post.

Sorry, I didn't see that.

It explains operating AC coils on DC and AC coils using
the internal DIODES for shading rings design.

No, that wasn't a hack as far as the diodes goes, that is a practical
design in a family of relays, We use large contactors with the dual
coil and diodes in them. You don't see this from the out side world but
they are incased in the encapsulation. Those particular types can have
DC going directly to them. It's just the way they are with obvious
reasons.

Yup. To operate an AC relay on DC, (the Tyco ap note says
doing so is impractical, but that you can do it in an
emergency) you should install a residual and you must lower
the voltage below the AC rating. An AC rated coil needs DC
in excess of 61% of the ac rating on the minimum side, and
less than 82% of the ac rating on the maximum side, per the
example given in the ap note.

The app note is pointing equivalent power dissipation, as you
know relays will pull in at a lower current So I don't see a problem
here.

Why don't you put a few AC types on the bench and test them yourself?

I think you'd be surprised in what you find. Lab testing tells a lot
about what you can do.

The only problem I have found with this practice of using AC on DC is
that some AC units use material that has a high hysteresis and the
AC will help keep this down. They do this in cases where they don't use
the dual coil system. In DC operation, I've seen it over magnetize the
core and cause a little slow release on the contacts. But that may not
be such a big deal in most cases.

Have a good day.

Jamie

I have an Essex contactor which was removed from my old A/C outside
unit. It has a 24VAC 50/60 Hz coil. I can make some measurements on it
if anyone is interested.

John S
If you have IR gun/camera, you should also do some comparisons.

Jamie

Sorry, I do not.

John S

CAN IT BUDDY!!!