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Master switch, revisited

wireweinie

wireweinie

FOUNDER
Palmer, AK
Just ran into, what I consider, an improper master switch set up a few days ago. This was on a Supercub with a B&C charging system installed. One of the complaints was that on shutting off the master switch, with the engine running, the electric gauges were staying on. Upon inspection, I found the master switch was a single pole type that only activated the battery solenoid. I also found the 'field wire' running directly from the field breaker to terminal #6 on the voltage regulator. What was happening was, when the battery solenoid was opened (master off) the alternator was still supplying power to the bus. This power was going back to the regulator, through the field breaker, keeping the alternator on line.

The fix is simple although the installation can be a b*&^%. Install a double pole master switch. Look at the attached diagram for this regulator. The correct way to install a master switch is shown on the wire that goes from the 5 amp field breaker to the #6 terminal on the reg. Route the wire from the breaker to one half of the double pole switch and from that same half of the switch out to the #6 terminal. That way, when the engine is running, EITHER turning off the master switch OR pulling the field breaker will physically open the path for power to the regulator. Remember that a double pole switch is actually two switches in one housing. Turning the switch 'off' will open the field circuit, on one half, and open the master solenoid control circuit on the other side.

On a side note, If you have a voltage sensing terminal on your system, like terminal #3 on the B&C diagram, make sure that terminal is connected to the bus bar. We found that terminal jumpered to terminal #6 on this aircraft. This works 'okay' while every thing is new and shiny. But as the field breaker ages and the terminals start to tarnish, there will be a voltage drop between the bus bar and terminal #6. That means the reg will sense low voltage and start to increase the voltage coming out of the alternator. High voltage (15.1 volts) was another complaint on this aircraft. The customer installed a fused wire directly from the bus bar to terminal #3, as per the diagram, and the problem was fixed.

My observation is that some people overthink wiring. If you don't have experience with DC wiring and controls, PLEASE!, ask questions. You may have discovered a new, neat way of wiring. But on the other hand you may simply not know why something is done a certain way. So ask away. No tech worth his salt will give you grief for trying to learn.

Web
 

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As long as you said we could ask away. In my posting of a non-electric experimental plane I would like to know the proper way to wire the starter and switch that I need for that without the alternator. A diagram of the wiring would be nice.
 
Keep it simple. If you have the battery close, like under the seat or at the kick panel, use a mechanical disconnect like the automotive guys use, to connect or disconnect the battery (in flight if necessary). Then wire the starter solenoid just like Cessna or Piper do. The disconnect functions as circuit protection for the big cables. Use a fuse or breaker to protect the wires to your starter switch. Look at a diagram as mentioned and let me know if you have any questions.

Web
 
I converted my Dec to a Plane Power alternator and regulator, after going through two standard regulators and an alternator rebuild in ten years. They recommended a separate switch for the alternator field and a small breaker in that line. I installed both. Now all I have to do is remember to turn the alternator on after start. It has been trouble free for two years.

Web - me too on the starter. I am planning what you suggest, but in the J-3 it is really difficult to get to the battery after start. Even though it adds complexity and weight, we are considering two solenoids - one to connect the battery, and the second to engage the starter.
 
Remember that the battery solenoid is drawing power from the battery as long as it is 'on'. That extra power draw can shorten the useful charge on a small battery.

Web
 
Yes thanks. I was thinking of turning it off after start. I guess all airplanes have full battery potential at the starter solenoid continuously during flight. So need only the starter solenoid, and a disconnect for storage and charging?
 
bob turner said:
Yes thanks. I was thinking of turning it off after start. I guess all airplanes have full battery potential at the starter solenoid continuously during flight. So need only the starter solenoid, and a disconnect for storage and charging?
Click to expand...

You need some way of quickly disconnecting the battery in flight. On an aircraft with full electrical, you turn off the master switch to disconnect the battery in a catastrophic electrical failure. If you don't use a master switch/solenoid, you'll need to be able to throw a mechanical disconnect to do the same function. If you only power the starter with the battery, I guess you could mount the start solenoid right at the battery. But if the solenoid sticks, you'll have to sit there and watch as the battery runs down, just like a Piper with the old style, original electrical system.

If you want to run any radios or lights, etc., you're going to need that disconnect so that you can leave battery power on. You can calculate the amount of power a master solenoid will draw continuously and see how big a battery you'd need to power it along with your equipment. I feel that a small battery with a mechanical disconnect is a better set up than a larger battery with the solenoid.

Web
 
Of course. Just had a momentaty lapse. We have already wired one up with a master solenoid at my insistence. The tap for the radio is a separate, fused line with another switch. The fuse is 5 amps in line at the battery.

For mine, I plan to use a separate battery for the radio, since it is already wired.

The day of the starter motor is coming. I am getting too old for the pretzel maneuver after engine start, and refuse to walk around the strut after it is running.
 
Hang in there, Bob - last time I looked you were pretty darn good at pretzeling; better than me anyway!
 
With a little imagination you could mount a mechanical master switch at the remote battery with a cable or push rod lever action control.
 
Put the battery at the kick panel or under the seat and use the mechanical master without the push rod.

Web
 
For a J-3, the extra distance might mean the weight of the wires and cost might rival that of the solenoid? I agree - no pushrod. I have room for wires under the floorboard, but not big ones.
 
wireweinie said:
Just ran into, what I consider, an improper master switch set up a few days ago. This was on a Supercub with a B&C charging system installed. One of the complaints was that on shutting off the master switch, with the engine running, the electric gauges were staying on. Upon inspection, I found the master switch was a single pole type that only activated the battery solenoid. I also found the 'field wire' running directly from the field breaker to terminal #6 on the voltage regulator. What was happening was, when the battery solenoid was opened (master off) the alternator was still supplying power to the bus. This power was going back to the regulator, through the field breaker, keeping the alternator on line.

The fix is simple although the installation can be a b*&^%. Install a double pole master switch. Look at the attached diagram for this regulator. The correct way to install a master switch is shown on the wire that goes from the 5 amp field breaker to the #6 terminal on the reg. Route the wire from the breaker to one half of the double pole switch and from that same half of the switch out to the #6 terminal. That way, when the engine is running, EITHER turning off the master switch OR pulling the field breaker will physically open the path for power to the regulator. Remember that a double pole switch is actually two switches in one housing. Turning the switch 'off' will open the field circuit, on one half, and open the master solenoid control circuit on the other side.

On a side note, If you have a voltage sensing terminal on your system, like terminal #3 on the B&C diagram, make sure that terminal is connected to the bus bar. We found that terminal jumpered to terminal #6 on this aircraft. This works 'okay' while every thing is new and shiny. But as the field breaker ages and the terminals start to tarnish, there will be a voltage drop between the bus bar and terminal #6. That means the reg will sense low voltage and start to increase the voltage coming out of the alternator. High voltage (15.1 volts) was another complaint on this aircraft. The customer installed a fused wire directly from the bus bar to terminal #3, as per the diagram, and the problem was fixed.

My observation is that some people overthink wiring. If you don't have experience with DC wiring and controls, PLEASE!, ask questions. You may have discovered a new, neat way of wiring. But on the other hand you may simply not know why something is done a certain way. So ask away. No tech worth his salt will give you grief for trying to learn.

Web
Click to expand...

Sounds similar to my experience helping a friend converting from a generator to a Denso alternator awhile back. I wired the field wire from the alternator to a 5 amp breaker on the main bus and with the engine running I turned the master off and the panel still had power on it. So the battery was out of the circuit and we have the alternator charging nothing but powering the plane, I didn't think an alternator would work without a battery in the circuit?

The plane had a DPDT master switch and original wiring the generator field was on one pole and battery power to the bus on the other pole. It was a simple fix, the two generator wires, it's own 12 volt input to the master and the output to the voltage regulator were still connected to the generator field side of the DPDT master switch. I disconnected the alternator field wire from the main bus 5 amp breaker that I had it connected to and re-connected it to the generator field wire off the master switch that was connected to the old generators voltage regulator and wired a 5 amp fuse in it between the master switch and alternator.

Because the generator supplied its own 12 volts to its pole of the master switch to start itself, it was missing 12 volts on that pole to power the alternator field. I connected the master switches 12 volt battery ON signal to the original generators 12 volt input wire to its pole of the master switch so both battery and generator poles would have 12 volts when selected on as it did with the generator.

I removed the jumper wire that connected the two on positions of the generator field side of the master switch so that that I could have one on position for battery only and the other for battery and alternator.
 
Last edited:
Alternator will still work after the is battery disconnected. But not well. If you try it, the voltage gets extremely unstable and eventually it will quit all together. It actually works better if there is some kind of load on the alternator. But the battery works to 'smooth out' the voltage and it provides a stable power supply to the voltage reg.

Web
 
wireweinie said:
Alternator will still work after the is battery disconnected. But not well. If you try it, the voltage gets extremely unstable and eventually it will quit all together. It actually works better if there is some kind of load on the alternator. But the battery works to 'smooth out' the voltage and it provides a stable power supply to the voltage reg.

Web
Click to expand...

That explains why it wasn't powering everything, some instrument lights were on and a few other things. I am still not quite sure how it was gettng power but sounds exactly like what you had happen. The Denso wired to the same 30 amp breaker on the main bus that the generator was wired to and the field was wired to the 5 amp breaker on the main bus also, at first I thought the power was coming to the bus backwards from the field wire.. I gave up and just did what I wrote above and no problems since, 10 years ago.
 
Drew all that out how I wired the alternator, and than read what you said about rewiring your problem, and looked at your diagram and see now how easy I could have fixed my problem, it was exactly the same as your problem. All I had to do was wire that bus 5 amp breaker directly to the original generator 12 volt input wire that powered the gen pole of the master switch, and than wire the alternator field the same way I did to the master switch output that went to the old generator voltage regulator wire and done. I wouldn't have had to mess with the 5 amp fuse in that field line, or jump 12 volts from the battery pole to the generator pole of the master switch.
 
wireweinie said:
Put the battery at the kick panel or under the seat and use the mechanical master without the push rod.

Web
Click to expand...

Is there a no solenoid, non electric, mechanical starter switch option? The non electric stomp switch on our families 1946 Farmall tractor has never failed in the 70 years we have had it. I filed the contacts 10 years ago and it is still going.

How about a stomp switch for the starter that was lightweight and lasted 50 starts. That would make it last till annual for most people. I see some at Napa for $12 bucks. The only weigh a few ounces.

Power would go straight from the battery, through the stomp switch, and to the starter.
 
Larry G said:
As long as you said we could ask away. In my posting of a non-electric experimental plane I would like to know the proper way to wire the starter and switch that I need for that without the alternator. A diagram of the wiring would be nice.
Click to expand...

Use the old wiring diagram from the early super cubs. it uses 2 fuses instead of a solenoid. that way it doesn't matter if you accidently leave the master on or not. I took the whole assembly out of a tri-pacer and mounted it under the seat.
 
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