Battery Master Solenoid (Contactor) Diode

How to keep the Cessna 170 flying and airworthy.

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GAHorn
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Battery Master Solenoid (Contactor) Diode

Post by GAHorn »

Later Cessna's came out with solid-state avionics and began to suffer excessive radio failures. This was ultimately traced to voltage spikes that occur when the battery Master switch was disengaged with radios still online. Early airplanes had their radios wired to be switched on and off individually rather than through a radio-master. Many Cessna 170's are still wired that way. If you have a radio/avionics master-switch then doubtlessly you turn it off before shutting down (except when you suffer senior moments and forget.) 8O
At any rate, Cessna came up with a Service Letter that addressed the issue (the exact SL # escapes me just now......what was it I was talking about??) Oh, yeah,...anyway the fix was to put a diode at the Battery relay/solenoid/contactor. This diode is attached to the small terminal that leads to the actual master switch and also to the large terminal that comes to the relay from the positive battery terminal. The diode points toward the positive battery cable terminal.
What was happening was that the electro-coil within the relay (solenoid) gets it's power directly from the positive battery cable, and it's circuit is completed to ground through the actual aircraft Master switch in the cockpit. This grounding of the coil activates the electromagnet formed by the coil and pulls the large internal contactor closed to complete the circuit across the relay, thereby sending battery power onto the main aircraft electrical buss.
When at the end of a flight, the pilot turns the Master switch back to the off position, the ground circuit of that coil is broken and a large voltage "spike" occurs as that coil's magnetic field collapses. This happens almost as fast as most central-Texas "B"-models. The 600+ volts formed by that spike gets on across the relay and into your aircraft's electrical buss faster than the relay contacts can physically open, which gives any appliances that are still online a 600+ volt shock! So, your avionics that are still online, as well as your strobe power supply, and your digital clock, etc., etc., are taking a beating, ...unless that diode is in place. That diode allows that 600 volts to find a place to be absorbed in the ship's battery (which is slower than a Grand Prairie "A"-model to re spond to large voltage spikes, and the actual current is small, so it isn't harmed.)
In other words, if you don't have such a device wired across from your battery master relay's small terminal to it's large terminal that the battery positive cable is attached to,...then your radios, etc., are at risk.
These diodes can be bought expensively I'm sure, from Cessna, or you could get one cheaply from Radio Shack (Cat. No. RSU 11928397) for about 89-cents.
Or you could do what I did. I mentioned it to Wendell Wyborny who promptly produced a variation on the theme for me. He used a small 18v/100Amp MOV (Metal Oxide Varistor) to accomplish the same thing. It was only slightly larger than a pin-head. (MOV's will equalize large voltage differences across their leads, but will not bother with voltages that are essentially the same. In other words, while the battery relay is ON, the MOV is like an open circuit. But when the voltage spikes high when you switch the relay off, then the MOV shorts the excess voltage back to the battery positive terminal. There's no advantage to using a MOV other than they are not polarity sensitive, so if you made certain that the diode is not installed backwards, then you'll be fine.)
Remember, you want the current to flow from the small terminal toward the large postive battery cable terminal. (The diode will be marked to indicate direction of flow.... >| means the current will flow to the right.)
Last edited by GAHorn on Wed May 01, 2002 2:35 am, edited 1 time in total.
zero.one.victor
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electronic razzmatazz

Post by zero.one.victor »

Why not just turn the radio's off first,then the master?

Eric
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GAHorn
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Re: electronic razzmatazz

Post by GAHorn »

zero.one.victor wrote:Why not just turn the radio's off first,then the master?

Eric
Like I said, "....ultimately traced to voltage spikes that occur when the battery Master switch was disengaged with radios still online..."
If you can always remember to do that, then the radios will do fine. What about your strobe power supply? And your digital clock? (if you have any such items that remain on until the master is turned off.)
Otherwise, if you can remember to always turn them off,...then you should have no problem. If you forget and have to pay an avionics shop $70/hour for troubleshooting repairs..... For 89-cents tho'......
n3439d
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Post by n3439d »

The same problem occours from the starter relay in the key starter systems.

ken
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flyguy
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GREY MATTER OR MUDDY WATER

Post by flyguy »

I have no use for all the avionics piled up in the back of my hangar that I 'spiked' by turning "off" the master while having the radios still 'on'. That is because there are none. I think the spike comes from another activity. Sometimes an operator listens or talks to ground control before starting the engine then energizes the starter which creates a power fluctuation that is harmful to the avionics.

Here is the replay of events that resulted in the trashing of a fairly new stack of Narco avionics in my 170. We were returning from OSH and had a friend riding back to Kansas City with us. She and her husband had flown to OSH in their Swift but due to the limited seating and w/b, accepted the offer of a chance for her to fly back with us in the 170.

This lady, a relative newcomer to 'small plane travel', was enjoying, or so I thought, the view from the back window of the 170. Apparently she became bored with the un-eventful flight and decided to spend the time doing some beauty reclamation from a week at Oshkosh. Her choice of activity was to remove her nail polish. Of course the agent for removing said "polish" had a very strong odor, similar to overheated electronic components, and immediately filled the cabin with this odor!

I "percieved", because of what I smelled, that there was an electrical problem, possibly a fire, and it caused me to "PANIC"! I figured the best way to combat it was to shut down the electrical system! Before turning off my master switch, I called "MAYDAY" to my friend in the Swift! I and told him that I had an emergency and to follow me to the nearest airport! I didn't wait for him to respond but immediately turned the electrical system off and headed for the nearest airport!

It didn't take long to determine the cause of the odor and with a very red-faced passenger, as well as the pilot, we turned back on course. Without even thinking I flipped the Master Switch back on and the result was to immediately spike the radios. I heard the sharp snap in my headset and for a second the com and nav radio indicator lights came on then suddenly went out. Now another dilema. My friend in his Swift is in sheer panic! His wife, and friends, are in an airplane where the pilot has called MAYDAY and he can't communicate with me! At first we thought to continue on towards KC but then realized that it was prudent to land and explain to him what had happened! I also thought seriously about killing his wife!

To "de-George" :twisted: this story I will say that I always felt that the avionics failure was caused by the sudden input of voltage rather than the cutting off of the supply of voltage. I never could get the repeat of that sharp crack in the headsets again.

PS: I hate to diagnose eletronic failures so that is why I never turn my avionics "master Switch" to the on position. That way I don't have to deal with faulty radios. :wink:
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flyguy
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SNAP CRACKLE AND POP!

Post by flyguy »

WHERE THE HECK WOULD A 14V SYSTEM COME UP WITH 600V?

I am no genius at mathematics but what kind of transformer would it take to produce such a massive energy output? I certainly may be wrong but 600v would be un-controlable across any size of switch used in modern avionics!:roll:

My friend in the above story was an avionics tech at TWA when this event happened. We performed several tests and like I said were unable to recreate that sound in the speakers or headsets. Our assesment was the 15/16 volt charge to the avionics buss was the culprit.

There was a suggested fix from an avionics shop in KC to put a diode between the master buss and the avionics buss that would limit any voltage over 13v from going through that system. Never got around to doing that though.
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GAHorn
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Re: SNAP CRACKLE AND POP!

Post by GAHorn »

flyguy wrote:WHERE THE HECK WOULD A 14V SYSTEM COME UP WITH 600V?

I am no genius at mathematics but what kind of transformer would it take to produce such a massive energy output? I certainly may be wrong but 600v would be un-controlable across any size of switch used in modern avionics!:roll:

My friend in the above story was an avionics tech at TWA when this event happened. We performed several tests and like I said were unable to recreate that sound in the speakers or headsets. Our assesment was the 15/16 volt charge to the avionics buss was the culprit.

There was a suggested fix from an avionics shop in KC to put a diode between the master buss and the avionics buss that would limit any voltage over 13v from going through that system. Never got around to doing that though.
I realize that folks in East Texas/Western La are not living in the deep end of the gene-pool so I'll repeat this just for FlyGuy, :twisted: ..."a large voltage "spike" occurs as that coil's magnetic field collapses." This coil is an electromagnet that is inside the battery relay/solenoid. This is made up of multiple windings and becomes a transformer that creates the 600+ volts as it's magetic field collapses. (The 600 volt figure comes from the Cessna Service letter. I didn't make that up.) It's small ampereage so not much current, but it can kill solid state components.
In my opinion, it's much more unlikely that turning the battery master back on with radios already on would cause any problem, since the battery would act as a huge capacitor to alleviate surges. (Although actually starting the engine with avionics on certainly would be a likely cause, as Ken already pointed out. But in FlyGuy's example, his engine was already running.) I'd guess that FlyGuy's damage was indeed caused by the improper shutdown. Anyway, a diode is a cheap piece of insurance.
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flyguy
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POOLS FOR FOOLS

Post by flyguy »

NEW A FELLER OUT IN TEXUS HAD A BREACHY STEER. TRIED EVERTHANG TO GIT HIM TA STAY IN THE FIELD. GOT ALL THE FENCES DUN PURTY GUD THEN THE SUCKER LEARNED TO GO UNDR THE FENCE CROSS THE MIDDLE OF THU WATER HOLE. THIS FELLER FIXED UP THAT WATER GAP A LITTLE BETTER BUT THE DURN MULEY JES KEP GOIN UNER IT. HE'SA GITTIN PURTY MAD SO HE COME TO THE WATER HOLE THE NEX DAY WITH HIS CATTLE PROD. HE SHOOED THAT STEER BACK IN UNDER THE GAP AND JUST AS HIS ASS WAS NEARLY PAST HE HIT HIM WITH ALL THAT LITTLE BAM STIK HAD! THAT STEER JUMPED ABOUT 20 FEET FORE HE HIT THU GROUN. I CUD LIE TA YOU AND TELL THAT HE DIED OF THIRST FORE BEIN FEARED OF GOIN ANYWHER NEAR THAT WATER HOLE BUT I WONT. ILL TELL YOU WHAT HE DID DIE OF WAS LOSIN ALL HIS TEETH AND HAIR AND HIS TALE FELL OFF. I GUESS THE MORELL TU THIS SORRY TAIL IS THAT IT DONT TAKE MUCH VOLTS TO MESS YOU UP IFN YER STANDIN IN SHALLOW WATER! :mrgreen:
Rudy Mantel

Diode

Post by Rudy Mantel »

Is there a diagram available for the diode installation ? Perhaps a Cessna Service Letter ?
Rudy
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GAHorn
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Re: Diode

Post by GAHorn »

Rudy Mantel wrote:Is there a diagram available for the diode installation ? Perhaps a Cessna Service Letter ?
Rudy
I gave a text description of how it connects, but if you look at any of the post-67 electrical diagrams for the battery solenoid (master relay), it'll be dipicted.
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