Switch Voltage

by Paul Simmonds-Short in 912iS Technical Questions 7 years ago

Hi everyone, newbie here who hasn't even ordered the aircraft and engine yet (that'll be June) but too keen and planning FAR too early :huh:

In the installation manual the voltage recommendation/requirement for the switches is 28V and various currents. I've had some fancy rocker switches manufactured (I know, I know, far too soon) and they can handle 28V but the LEDs in the switches are rated for 14V. Now, it's not a huge deal to put in a resistor so the LEDs don't burn out but I just wanted to confirm whether 28V is actually flowing through those switches or just 14V. The switches in question are the LANE, BACKUP BATTERY & FUEL PUMP switches. If it's 28V then a resistor it is; if it's 14V then no resistor.

Also, does anyone have any idea of the current that is drawn when the backup battery switch is in use? The circuit diagram has a 30A CB and the switch requirement is 20A. The rocker switches I have can handle 20A but I was just wondering if the CB 'let through' 29A and then that went through the switch (no relay, obviously) then that would not necessarily be a good thing. I've read the forum regarding relays for this switch and I think it would be a good idea if I could avoid using one but that really depends on what the actual current draw is. If it's actually comfortably below 20A then no snags.

Thank you for your time in considering my beginner's questions and I'll no doubt have a million more when I finally get the toy and its power plant.

Cheers,

Paul

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Re: Switch Voltage

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

The voltage present on the Lane, Pump, and Backup Power switches 12v Generator and the 12v Aircraft Battery.
The Actual Current will be 10 amps or less.

Fuses/Circuit Breakers are rated to protect the Wiring.
You will only see 30 amps if wiring shorts out the battery.
It will never blow in normal operation.

Switched are rated for the amount of current they can switch and the voltage they can withstand.

The contacts in higher voltage switches will need to produce a larger air gap when open.
The contacts in higher current switches will need to be larger.
A switch may be rated for 120v/20A (AC), and 28v/15A (DC) in anticipation that the AC Current passes through Zero, 120 times a second and so is easier to stop than DC currents.

28 Volts is the lowest voltage rating you will find in panel mount switches because any voltage lower is not any more likely to arc across the open switch.

Lighted switches are going to be difficult to implement in these circuits.
You do not want to sink the lamps to the airframe ground.
This destroys the ECU to Airframe ground isolation.
The installation Manual specifically prohibits installing lamps in these circuits.

In The Backup Power Circuit, under normal operation with the switch OFF, the battery will be present on one side of the switch and the working "A" Generator will be on the other side. All terminals will be showing 12 volts in both the OFF and ON position. The light will never go OFF.
You may need 3 pole switches with one pole designated exclusively to the lamp just to indicate the position of the switch handle or the switches will have to operate relays that do the actual switching. That of course, will also introduce a bunch of additional failure points.

Keep it Simple.
There is a good reason that switches in aircraft are very utilitarian looking.
They work and they are reliable!

This is NOT like Blinging out a car or a Boat panel where you are only dealing with One Battery, One Alternator, and a Common Ground.
The 912iS engine has 3 power sources and two ground systems.
Be sure not to cross wire them.
There are different "Ground Rules" here. (Pun Intended!)
Getting it wrong can be disastrous once in the Air.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: Switch Voltage

by Paul Simmonds-Short » 7 years ago

Paul Simmonds-Short

4 25 7

Thank you, Bill, for such a comprehensive and detailed reply; it is exactly what I needed to ensure the switches are correct and I will pass these details on to the 'switch man' to make sure they comply fully.

I'll leave out the LEDs in the Backup Battery and Lane circuits. If you are able to advise if I should do so with the fuel pump switches that would be great but I do appreciate that I've already hogged a lot of your time. I have studied the wiring diagrams but the outputs of the HIC A & B modules doesn't really get detailed in the Installation Manual. I've also looked in the parts catalogue and Maintenance Manual but I'm none the wiser. Again, if it turns out that I cannot use the LEDs in the switches then I won't upset about it :-)

Thanks again for your help.

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Re: Switch Voltage

by geoffrey klestadt » 7 years ago

geoffrey klestadt

27 94 94

What Bijl said. Use aircraft grade switches from aircraft spruce.

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Re: Switch Voltage

by Paul Simmonds-Short » 7 years ago

Paul Simmonds-Short

4 25 7

Thanks again Geoffrey.

Yes, these are aircraft grade and not just from the auto store :-) They can carry different voltages on each side of the switch (not that they're going to) and each side is rated for 20A, so it looks like I'll be good to go (minus the LEDs). I've sent Bill's reply to the supplier (who knows how electrical 'stuff') and will check all is okay before they go anywhere near a panel.

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Re: Switch Voltage

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

Directly from the Installation Manual.
[quote]NOTICE: Do not connect additional loads (Additional Fuel Pumps, Lamps, LEDs, etc.) to the power supply of the Fuel Pumps.[/quote]

No Leds On the Pump Switches!

All Lighted switches need a ground connection for the lamps.
The Engine/ECM Ground does not, and should NOT exist in the panel.
You can use lighted switches on the Airframe side just not on the engine side.
Lighted switches for NAV, Strobes, Avionics, etc., even the Master are OK.
Lighted switches are not recommended for ANY of the engine switches.

The Battery Backup switch should also be a covered switch so that two actions are required for activation.
You should be required to open the cover before activating the switch so that it is never activated accidentally.

The Lane Switches also typically have side shields so that accidentally bumping them is difficult.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Switch Voltage

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