TYCO has a variety of models now.   Some have ultra-low hold current and include the diode protection.   Others omit the fancy stuff.   They clearly specify their maximum "break" current which is what you want a lot of it you are trying to interrupt an errant high current circuit.   e.g. in Bill's example, if you had a stuck pressure switch, you could still shut down the power by pulling the breaker that powers the relay.

On Mar 28, 2013, at 2:12 PM, Sky2high@aol.com wrote:

Bill, et al,

 

Relays should have a diode reverse connected across the coil to stop the voltage surge generated by the field collapse when power is removed from the coil.  This will help protect the pressure switch contacts.

 

Grayhawk

 

In a message dated 3/28/2013 12:58:55 P.M. Central Daylight Time, super_chipmunk@roadrunner.com writes:

  Would there be any drawback to using a higher- rated contactor? I found the Lancair part to be a P.O.S.- it was marked continuous duty but when put on a power supply it rapidly overheated and shorted to the case.

 

Instead I’m using an antique 50A Leach B-5B aircraft style that easily operated for hours. The pump supplied with my IV-P draws a maximum of 50A, so the Leach might be marginal but I think I’ll try it initially.

 

On Colin’s recommendation I also purchased a TYCO LEV100A5ANH , rated 100A that I could use instead, or elsewhere. The Leach draws 0.16A, the TYCO draws 0.25A at 28V. The Lancair- supplied Suco pressure switch is rated for 4A as best I can determine so either unit should be compatible.

 

  I don’t know what other models might use but consider the contacts inside the pressure switches. If they should get overloaded and weld closed the effect would be the same as welding the relay points.  -Bill Wade

 

From: Gary Casey

Sent: Thursday, March 28, 2013 9:36 AM

To: lml@lancaironline.net

Subject: [Norton AntiSpam][LML] Re: Hydraulic gear pump failure - Relays

 

Pardon me if I'm jumping into a discussion a little late (and a dollar short), but there might be some confusion on how and why relays are designed for intermittent duty.  As an example, take a standard master relay.  It is rated for continuous duty and something like 80 amps.  That means the coil won't overheat if left on forever and that the contacts also won't overheat when passing 80 amps and that the contacts won't weld when interrupting 80 amps.  It is often the limitations when opening the circuit that determines the rating.  Current will try to keep flowing and if the contacts don't separate rapidly and with enough force the inevitable arc could create enough heat to weld the contacts and prevent opening.  If it is desired to design a relay in the same package (for cost, size and weight reasons) that can be used intermittently to pass a much higher current the design parameters are different.  More force is required to increase the rate at which the contacts are opened so a higher force spring is installed.  Then more force is required to close the contacts, so fewer turns of heavier wire are used in the coil (that's right, FEWER turns, but that's another discussion).  The result is that the relay can now successfully switch maybe 300 amps, but the coil will overheat after a few minutes of operation.  That's the difference between a master and a starter relay.  And I have never, ever heard of using a relay as the "weak link" in the system.

Gary

ps:  My Lancair list emails now come all messed up (see below).  Why is that?  Am I doing something wrong?

 

The only caveat with that fix, Fred, is that intermittent relays are ther=
e
for a purpose.  They are supposed to be the weak link in a circuit where =
a
continuous stuck relay could result in greater problems than a failure.=20
Does that protective function matter in this circuit?=0D
Should be  considered.=0D
John=0D

=