> I work for Agilent (used to be HP) and they used a LOT of
> reed
> relays. They come in form A (SPST, NO), form B (SPST, NC)
> and
> form C (SPDT). The form C relays need a permanent magnet
> to hold
> the NC contact when not energized. IIRC, the energizing
> current
> is the same between forms A, B, and C, (being limited by
> heat dissipation)
> and the result is that the contact pressure is half as
> much for form C.
> Back when we had component engineers and a reliability lab
> (before
> they were all fired to save money) the expects told us
> that form
> C was inherently less reliable than A and B, presumably
> due to the
> reduced pressure. We were instructed not to use form C
> unless
> absolutely necessary.
After some thought and a look at this last night, I'm going
to strongly disagree with the idea that reduced holding
voltage reduces current rating in most, if not all, relays.
My reason does not conflict with Rick's data at all.
While I haven't seen every relay in the world, I've looked
at a lot of relays. I have open samples of dozens of
different relays collected over the years because, when
picking a relay, I always look at internal construction. I
also have glass and ceramic vacuum relays that are open, as
well as open frame relays.
In every single relay I've looked at there is some spring or
spring action that controls energized relay contact
pressure. I can't find a single relay in all my relays that
has a floating armature connected to a rigid contact. Even
in very high current form X relays or double make double
break relays, the contacts are buffered by something "spring
like" or an actual spring that flexes.
Now think about how the electromagnet works. When resting,
the armature is either out of the coil or up away from the
pole piece some distance. The flux path has an air gap. As
coil current increases the armature moves towards the other
pole or into the coil, and flux density through that
magnetic circuit increases rapidly just from the distance
reduction alone. If you have enough coil current to overcome
the return spring that holds the relay in the normally open
position and open the normally closed contacts, you can be
assured the relay armature will fully move until the plunger
or armature bottoms out. I wasn't able to find a single
relay that didn't behave this way.
So now we have exactly the same mechanical travel regardless
of applied holding current as long as the armature is
seated. ALL of the contact pressure is determined only by
the spring tension against the contacts when actuated, just
as it is determined by return spring tension while resting.
In small open frame 10-20 amp contact relays like used in
antenna switches, the spring is actually the relay contact
bars. Those bars, beryllium copper in some open frame low
and medium current relays, are the springs that limit
contact pressure when the relay is energized. If you have a
relay like that push the relay closed and watch how that
spring bends just before the relay armature bottoms on the
magnet pole. No matter how hard you push on the armature the
armature won't travel further, so the contact pressure never
changes once the relay armature has bottomed.
You'll see the same thing is true with very high current
open frame relays, like power contactors. The contacts are
on rigid carriers, but the rigid carrier is buffered by a
spring on each contact bar. Again this lets the contact seat
just before the pole piece and armature bottom out. Once
again, no matter how much coil current, the contact pressure
is the same.
All of the vacuum relays I have use a similar method to one
of the two above. They either use a separate spring or a
"spring material" that flexes in the contact support bar.
There are several very good reasons relays do this. First,
the contacts or armature might be physically damaged or
deformed if pressure was simply limited by coil current and
not a controlled source like a spring. Second, the
manufacturer would have an impossible job adjusting the
relay for correct holding pressure at a given voltage if he
relied on flux density to control contact pressure. He could
never get it right! The reason is temperature would greatly
affect holding pressure. Not only does the coil change
resistance a great deal with temperature, the materials in
the relay will change size. Even at a constant temperature
the contact pressure would vary as the coil heated. I
measured 30-50 degrees F temperature rise over ambient in a
small 1.2 watt relay coil in 15-20 minutes of operating in a
0 degree F test chamber. It is impossible to have anything
close to steady pressure without a buffering spring.
This perfectly fits with my observations of how relays work.
A few weeks ago I spent a week or two trying to find a way
to force a better connection in a transfer relay. I had a 12
volt relay with higher than desirable contact resistance and
I tried to activate and hold the relay with 35-30 volts to
see if reliability on closure would improve. It did not. The
only thing higher voltage did was reduce transfer time.
Amplifier manufacturers like Heath and the rest commonly
used open frame relays for antenna switching. The big
problem with those relays was receiving through the relay
when they were out in service for a while. This was caused
by a lack of current through the NC contacts allowing a film
to build up, and the fact two contacts were effectively in
series in the receive path. I initiated a change in relay
design that removed the unused center NC contact from the
relay. This allowed more bending moment on the contact
carrier bars (which are springs) from the return spring
pressure, and this increased wiping action. If you look at
some Heath, Ameritron, and other manufacturers you will see
the unused NC contact bent up out of the way. This was a
first step until custom relays could be produced.
This actually all fits with what Rick said. The form X, or
double make double break relay is less reliable for contact
resistance when compared to normal single contact point
relays. That arrangement has twice the number of contacts in
series with the signal path. This means it is much more
likely to have resistance problems at low contact (NOT coil)
currents. You have to be sure two contacts stay clean, not
just one. The second issue is if you use a SPST relay
instead of a form X, all of the spring pressure goes to ONE
contact. This doubles contact return spring pressure
available to bend the flexible contact carrier and increases
wiping action. This is for a constant coil size.
If I would have read this thread four weeks ago... the two
weeks I recently spent looking at relays would have been
more productive. Now, with a few hours work last night, I
answered my own question why increasing coil voltage did not
increase pull in reliability the slightest amount!! I hope
this saves someone else work and worry.
73 Tom
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