Need for master relay

[Britguy59]

I read and search the internet and groups such as ours and occasionally find out about things I hadn't considered. I ran my electrical bus feeds from the battery through a master auto reset CB then to a master switch to the regular CB's. Now I find out about "master relays" and wonder....should I have put one of these in, and why?" I can still do it, but wonder if its worth it. My loads are fairly light, master switch expected to handle the current. Don't see one in the plans. What is the purpose and benefit?

[Drew]

Not an electrical expert but will RELAY my observations.  :lol:

The relay appears to be able to handle a larger load than a normal switch---but more importantly, it takes just a little bit of current to operate the switch.  You might get away with less large wire if you locate the "switch" by the battery or firewall.  If you insist on carrying the "large load" all the way to the panel, you can do this, but you are going to have to run a rather large wire just to put a switch on the inst panel.  There are a couple of different relays that look sort of the same.  The two favorites are the one used for a master relay----which is designed to be used continuously-----and the starter solenoid which is designed to be used for a very short duration----don't know how they are different.

Other type smaller relays would be for either a speedbrake or trim.  You probably don't have enough room in the stick to put all those fat wires to go to the flap/trim switches.  However, you can put a lot of smaller wires in the stick that carry a very small current to actuate the proper relays---which carry the larger current.

So...electronic experts...please shred the above if not correct.  Still learning----part of learning is trying to explain it.

[GlennBob]

Drew,

The continuous relay is wired kinda cool.  For a long time, I couldn't figure out how it worked because there was no power from the battery to my master switch to operate this relay.  I finally found out (tracing) that the relay is hot all the time and the master just completes the "ground" for the relay, which is then energized all the time.  This relay only has one low amperage wire running to the Master switch. When grounded, then energized, the relay then carries all the load of the ship.

That relay wasn't that hard ! ! !   :lol:

Glennbob

[Britguy59]

The above is all true and known. For my larger loads, gear, landing brake, trim motors, I built individual relay controllers. The power source still comes from the breakers, fed through the master switch though. My question is more asking if the relay has been found to solve any found issues relating to master switch falure? I have heard of none.
I can put one in, just like to ask as each added componant adds weight.

[Drew]

But shouldn't the master switch be completing the circuit for the master relay?

Also---I thought I heard of someone installing a switch to bypass the master relay in case of master relay failure. And I thought I heard an argument about that being a very low prob of fail.

[easyrider]

I have just changed out both the starter and master relays,they became intermittant. What is interesting they failed within 20 hours of each other.
The hours on the airframe are 700 and the life is 19 years. They are both mounted up in the nose next to the battery, visually they looked OK no corrosion.
Easyrider

[LongEZDaveA]

If I understand your question correctly, there is no need for a relay in your case IMHO.  The main thing to be concerned with is a good margin between the rating of your switch (at your battery voltage) and your total loads that will go through the switch.  If the rating given for your switch is AC only, you have to REALLY have a large margin.  If you care to, give us the rating on the switch and the maximum total load that it will see, plus your buss voltage.

I did not install a master relay and so far have no regrets.

[Britguy59]

I believe I will measure actual current through the switch and compare to rating. Data to follow for info to others.
Painting at last, such a rush to see pretty white wings.....

TTFN

Martin