Clarifications and answers..
Lots of good stuff in the thread. Thoughts and replies here:
I think there must be more about this issue that we are understanding. For example, Droopy reports that his starter circuit blew a 5 amp fuse. If your measurement of 3.5 amps draw is correct, you would not expect to blow a 5amp fuse.
Could that be because of the spikes associated with a contactor? Perhaps Droopy was using a different brand/type contactor that you are?
Different contactors and/or wiring setup. Spikes would not normally account for blown fuses.
Remember that this is a COIL that you are completing the circuit with your switch. If I remember correctly, a coil will pull a surge of 10 times its steady state current draw.
Basic coils do not exhibit an inrush current. Quite the contrary, a coil (inductor) will resist current changes through them. Inrush currents are more likely to be found in loads like incandescent lamps (where the cold-hot resistance ratio is indeed about 10X) or capacitive loads (initial power up of electronics etc).
Also do not forget that the collapsing magnetic field will create a reverse current flow. In other words, there can be arcing on the switch contacts that will cause pits and eventual failure of the switch contacts.
Absolutely true. Not only will arcing be a problem, transient voltages in the several hundred volts (about -450 volts in the case of the contactors
) can be generated. In these enlightened, microprocessed times, diodes across not only the contactors but ANY relay coil are a must.
And just my opinion but if you're using radio shack components for all this neat stuff in your plane you're really asking for trouble. They basically sell second rate components, or to be less politically correct, it's junk not fit for anything of any importance in an aircraft! Just my opinion take it or leave it.
Absolutely agree. Why get your shorts in a wad over non-spec AN fittings
AN Issues and then use hobby-junk in the electrix? Lots of better alternatives. Mouser. DigiKey, Allied, BC, Stein, Van's even. Lots.
A thought on your 3 amp limit, if you have any spare pins on your connector you could double up on the pins and raise the capacity.
Be careful on this one. As the contact pairs age, they will inevitably exhibit different contact resistances causing one set to carry more than 1/2 the current. I know its common and you probably can get away with it with these low currents but its a bit iffy. You just don't know how normal contact wear will affect resistance and current draws between the two. Fixed connectors would be much less affected than switches but its a matter of degree that I'd rather not explore. (Actually, I HAVE explored.. with negative consequences).
"Perfection in design does not come when there is nothing left to add....but when there is nothing left to take away!"
Don't overengineer a simple system just because you can.
Possibly the best advice on the forum. What's not there weighs nothing and can't break. Before adding a relay, why not try to find a switch that will fit where you want it and carry the load?
Decreasing operating weight of 9A by
Removing lead from Pilot's derriere.
Currently @ 239 - a 42# improvement.
My vote for best advice in the thread, Mike. I need to do it too. Rather than beating out the 8 x 2 1/4" holes in the steel battery box to save a louzy 4oz. of weight off the plane, spend the hour in the gym. GOOD JOB! Time for
me to lay off the chips...
PS what's your target? May just have to get a little competition going here..