electric relay

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electric relay

Postby n3437d » Mon Aug 10, 2009 2:56 am

Relay gave up the ghost today - anyone have any good contacts for replacement?

Thanks,

Joel
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Re: electric relay

Postby lowNslow » Mon Aug 10, 2009 3:41 am

Which relay are you talking about? Starter or master.
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Re: electric relay

Postby gahorn » Mon Aug 10, 2009 6:03 pm

If the airplane is original it won't have a starter relay. The battery Master relay is available from Aircraft Spruce. PN-111-226, $18. 877-477-7823

Image

http://www.aircraftspruce.com/catalog/e ... rrelay.php
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Re: electric relay

Postby lowNslow » Mon Aug 10, 2009 8:09 pm

gahorn wrote:If the airplane is original it won't have a starter relay.

True, but how many of us have original airplanes. :roll:
However, if you do want to stay original, the cessna original relays are available for $400+. :wink:
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Re: electric relay

Postby gahorn » Mon Aug 10, 2009 8:27 pm

lowNslow wrote:
gahorn wrote:If the airplane is original it won't have a starter relay.

True, but how many of us have original airplanes. :roll:
However, if you do want to stay original, the cessna original relays are available for $400+. :wink:


I wasn't trying to "start" and originality argument with that comment. I'll bet fewer than 5% of all 170's have starter-relays. (I would consider the relay pictured above as an "original" relay...even tho' perhaps not "authentic". It's identical to the genuine Cessna replacement part.) :wink:
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Re: electric relay

Postby mongo2 » Sun Sep 20, 2009 11:43 pm

Looks like a nice update and I am planning to purchase one as well. I went the link on Spruce's site, and was just wondering if the
"not for certified aircraft" was anything I should really worry about? I have just re-installed my battery box and and also going to get a
solid state regulator as well.

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Re: electric relay

Postby gahorn » Mon Sep 21, 2009 5:46 pm

mongo2 wrote:...was just wondering if the
"not for certified aircraft" was anything I should really worry about? Rick


It does not state "not for certified aircraft". It says "applicable to all homebuilts". It also states "used on production aircraft".
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Re: electric relay

Postby lowNslow » Mon Sep 21, 2009 7:32 pm

gahorn wrote:It does not state "not for certified aircraft". It says "applicable to all homebuilts". It also states "used on production aircraft".

Welll....actually it does (large bold print next to the picture). However I think this is a recent change as it did used to say "used on production aircraft" which it is. Many Cessnas use this unit.
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Re: electric relay

Postby mongo2 » Mon Sep 21, 2009 11:03 pm

Thank you for the clarification. I'm sure it would be ok to use.
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Re: electric relay

Postby gahorn » Tue Sep 22, 2009 1:41 am

lowNslow wrote:
gahorn wrote:It does not state "not for certified aircraft". It says "applicable to all homebuilts". It also states "used on production aircraft".

Welll....actually it does (large bold print next to the picture). However I think this is a recent change as it did used to say "used on production aircraft" which it is. Many Cessnas use this unit.


A-HA! It IS a recent addition to their illustration ON LINE, but that note did not appear in the paper-catalog at which I was looking. In any case, it's the correct part to use.
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Re: electric relay

Postby lowNslow » Tue Sep 22, 2009 4:55 pm

mongo2 wrote:Thank you for the clarification. I'm sure it would be ok to use.

The only difference is that Cessna includes an external diode for surge protection that is not included with the Spruce unit (the original did not have a diode either). You can get one at Radio Shack or Spuce ( http://www.aircraftspruce.com/catalog/e ... idassy.php ) this one says it is for a starter solenoid but is the same one used for a master solenoid.

See this discussion on another thread:
viewtopic.php?f=6&t=6663&p=58840&hilit=diode#p58840
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Re: electric relay

Postby cessna170bdriver » Wed Sep 23, 2009 11:59 pm

Back in this thread referenced by lowNslow
gahorn wrote:Yes, that schematic leaves a little to be desired. That diode should probably be put upon every airplane in the fleet regardless of what equipment is/is not aboard. There is another Cessna Service Letter which adddresses that matter, but basically.....

When the master switch is turned off, the electrical "ground" is removed from the battery solenoid/relay, which subsequently relaxes as it's magnetic field collapses. When that occurs it is common for an electrical "spike" to hit the electrical system. That "spike" can be as much as 600 volts!

If the pilot forgets to turn off all his avionics before turning off the master switch then all the avionics receive that "spike" which may cause damage to some solid-state components. Installing a diode across the master (battery solenoid, not the actual cockpit switch) will provide an "escape" for that "spike" back to the battery. The diode does not have to be large, but a 1watt diode will cost you almost a whole dollar at Radio Shack and will do the job just fine. Connect it across the two large terminals of the battery relay with the diode symbol "pointing" towards the battery's connection with the relay. (Installing it incorrectly will let the smoke out of it immediately the next time you turn on your cockpit master. If you let the smoke out of any electrical device they will not properly work ever again. Don't believe? Try it.) :wink:


To clarify, the diode shown on the schematic in the Alternator Conversion Kit addresses a somewhat different issue than the one installed on many master and starter contactors. The one referenced in SK-172-22 suppresses spikes generated by the voltage regulator switching the field circuit on and off (a normal part of regulator operation). The one on the contactor suppresses the spike generated when the contactor is de-energized to shut down the aircraft electrical system. Both involve the cockpit master switch, but on different poles (circuits) in that switch.

Concerning connection of a diode on the master contactor (and starter contactor if you have one), "Connect it across the two large terminals of the battery relay" may or may not have been what George intended to say, but taken at face value, connecting the diode this way connects it across the CONTACTS of the relay. Connecting a diode in this manner will “protect” an already very robust set of contacts from the relatively small spike generated by the field collapsing in the aircraft wiring (essentially a 1-turn coil), and any small relays and motors in the system. However, the BIG spike is actually generated by the collapse of the magnetic field of the COIL in the contactor (many, many turns), so you would be better advised to connect the diode from the large terminal on the battery side of the contactor to the small terminal with the wire that goes to the master switch. ( :!: The admonition to place the diode band toward the battery connection still holds :!: ) This places the diode directly in parallel with coil, and limits the spike to usually 1 volt or less. Contactors that come with the diode already installed are wired this way.

Here’s the only illustration I could easily find. Contactors recommended for the 170 have the positive end of the coil connected to the battery terminal internally as opposed the external wire in the photo, but the idea is the same:Image

If the diode is not installed across the coil, the spike generated by the coil shows up as a high negative voltage across the contacts of the master switch in the cockpit, and can cause arcing in the switch, possibly reducing its life. For what it’s worth, as far as I know my airplane has its original master contactor and cockpit switch, and has worked reliably for over 50 years, 2800+ hours, and Lord knows how many switch cycles, all without benefit of said diode. That is likely a testament to the quality of Cessna’s cockpit switches. That said, it is still a good idea to install the diode.

Also, only a tiny fraction, if any, of the voltage spike gets into the electrical system in the airplane, even if the diode isn't installed. (Explanation: voltage in a circuit distributes itself proportionally according to the impedance of the components in that circuit. The impedance of an open switch is MUCH higher than the impedance of other components in the circuit, like light bulbs, avionics, and even the battery. ) If you feel you need to “protect” avionics from this small spike, and the spike generated by the collapsing magnetic field in the aircraft wiring, place an identical diode from the power bus (or the bus side of the master contactor) to ground. Orient the diode with the band on the bus end.

If this isn’t technical enough for you, take a look at this..

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Re: electric relay

Postby gahorn » Thu Sep 24, 2009 12:27 am

The pic posted above is not applicable to our Battery Master Relays, and hopefully no one will be confused by it.

Here's an schematic which Cessna provided for aircraft post-1962, and which should apply to this discussion. (The diode should be connected between the solenoid/relay SMALL terminal and it's large, Battery Positive terminal.) Sorry for any confusion.

Batt Mstr Diode.JPG
click to ENLARGE


Having read thru the link* which Miles posted, I notice that none of their drawings depicts the schematics of our airplanes. Specifically, they do not place their relay-activation switches in the circuit at the same place that our airplanes are wired.

I suggest we should all follow Cessna's suggestion and install the diode as they depict in their later drawings and their Service Bulletin.

Here's the link Miles posted:
spikecatcher.pdf
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Re: electric relay

Postby cessna170bdriver » Thu Sep 24, 2009 3:10 pm

gahorn wrote:Having read thru the link* which Miles posted, I notice that none of their drawings depicts the schematics of our airplanes. Specifically, they do not place their relay-activation switches in the circuit at the same place that our airplanes are wired.

I suggest we should all follow Cessna's suggestion and install the diode as they depict in their later drawings and their Service Bulletin.

Here's the link Miles posted:
spikecatcher.pdf


Fully agreed.

The salient point is that to suppress the spike, the diode needs to be in parallel with the contactor coil. On that, Aeroelectric Connection and Cessna seem to agree. Whether the coil is switched in its positive or ground leg doesn't change how the diode works. I happen to like switching the ground leg as Cessna does it, especially when the control wire has to penetrate the firewall, because if that wire happens to short to ground the worst that happens is that you can't shut down the electrical system. If that wire is in the positive leg and shorts to ground, you lose the whole system.

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Re: electric relay

Postby lowNslow » Thu Sep 24, 2009 6:30 pm

Miles, question on diodes (specifically the ones were talking about here) : when and if they fail, do they fail to an open or closed circuit condition?
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