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-   -   Push button starter question (https://vansairforce.net/community/showthread.php?t=169230)

rv7charlie 03-10-2019 10:39 PM

20A should be more than adequate. Only reason for the caution is that some have eliminated the separate contactor, when using a starter that has a built-in solenoid (that can on the side of the starter motor) that both moves the pinion out, and provides the high current connection for the starter. Those can stress a typical 10A switch.

N804RV 03-10-2019 10:42 PM

Quote:

Originally Posted by RandyAB (Post 1330737)
Thanks for piping in Charliie. I know from previous posts that you are quite knowledgeable about electrical stuff. There are a few Honeywell swithches in the Grainger link provided several posts up that handle 20 amps. How much current are the switches expected to handle? The solenoid does the heavy lifting for the most part does it not?

What are those $400+ PMA'd Bendix keyed switches rated for? I bet you could look it up.

N804RV 03-11-2019 08:37 PM

Quote:

Originally Posted by Carl Froehlich (Post 1330648)
Interesting as this is opposite of my priorities. I worry most about some kid getting in an hitting the start button. Not so much that the engine will start, but the prop hitting someone else. So steps taken to not let a push of the start button engaging the starter.

If someone wants to steal avionics, canopy security makes little difference. For this reason I did not put a lock on the canopy (other than a canopy cover when away from home and parked on the ramp). Insurance will pay for avionics, but repairing a broken canopy would be heartbreaking.

Carl

Pulling out the fuse would serve to keep accidental starter engagement from a "kid getting in and hitting the start button" as well as slow down an airplane thief.

RandyAB 03-11-2019 08:48 PM

Quote:

Originally Posted by N804RV (Post 1330740)
What are those $400+ PMA'd Bendix keyed switches rated for? I bet you could look it up.

5 amps according to the spec sheet provided on ACS.

https://m.grainger.com/mobile/produc...fc=MWP2IDP2PCP

I think I might use one of these with the “momentary on” for the starter and the “on” position for the ECU master of the SDS EI. I will have to check if the “on” stays on while the starter is engaged though or that obviously won’t work.

N804RV 03-11-2019 10:06 PM

I wouldn't think the switch in the cockpit would see more than about 4 amps.

Looking at the wiring diagrams, it looks to me like the only current going through the start switch in the cockpit is from the 12vdc from the "Start" circuit breaker, through the switch, then coil of the starter contactor, and then to ground.

So, discounting any resistance of the wire, the switch, and the circuit breaker and their connections, the current in the circuit would only be 12vdc divided by the resistance of the coil.

I was looking at a popular starter contactor spec sheet today. It listed the coil resistance at ~3.5 ohms. That's less than 4 amps, nominal.

BCP Boys 03-12-2019 06:38 AM

Keyed OFF/ON L/R
 
I've had a keyed switch in the -7 and kept it that way after the panel upgrade. I thought about going to a push button but that would have required 2 more switches for the Mags and possibly another solenoid. I'm not sure the "failure rate" on the Keyed Cessna type switches are that bad. The key is also the same key that opens my canopy so there is that. About the only complaint I have is that if you have too much "stuff" on your key ring it will flop around when you do aerobatics, so I do agree with that. I only have the 1 key on mine so it doesn't bother me. I did change the contacts at around 700 hours. I think it comes down to a personal preference again. I think the next plane will have a push button with e-Mag switches.

Mel 03-12-2019 07:15 AM

Quote:

Originally Posted by BCP Boys (Post 1331009)
I'm not sure the "failure rate" on the Keyed Cessna type switches are that bad.

It's not the "fail rate" so much that the failure may not be caught. When you do a mag check, you switch between both, right, and left. Few people check the "off" position. That's where the failure often occurs. And since you normally shut down the engine with the mixture, a failure of the "off" position is not obvious.

lr172 03-12-2019 08:36 AM

Quote:

Originally Posted by RandyAB (Post 1330737)
Thanks for piping in Charliie. I know from previous posts that you are quite knowledgeable about electrical stuff. There are a few Honeywell swithches in the Grainger link provided several posts up that handle 20 amps. How much current are the switches expected to handle? The solenoid does the heavy lifting for the most part does it not?

most external auto solenoids (i.e. ford style typically used in our planes) draw around 3-4 amps in my experience. As Charlie mentioned, those mounted on the starter that also pull the drive gear will draw more. I would expect those mounted solenoids that are only contactors would also draw the same 3-4.

Larry

Saville 03-12-2019 09:19 AM

Is there a circuit diagram that shows how to modify a key-switch system to push button?

Thanks

N804RV 03-12-2019 10:05 AM

Quote:

Originally Posted by Saville (Post 1331044)
Is there a circuit diagram that shows how to modify a key-switch system to push button?

Thanks

The easiest way is to buy the "Aeroelectric Connection" book or the "Aircraft Wiring Guide" that Vans sells.

How you do it in your aircraft is going to depend on a couple of different things.

If really want some help with it, PM me and I can give you some ideas.

tracy 03-12-2019 10:56 AM

I've also got the spring loaded toggle switch with red cover protector. I'd go this route again.

Scott Hersha 03-12-2019 11:54 AM

RandyAB,
I used the momentary start push button + keyed ?enable switch? for the reasons someone else explained. It can prevent unauthorized or unintentional starter activation - much the same as a standard aircraft (Cessna) start switch, but at a much less expensive price, and either switch could be bypassed if there was a switch failure and you were stranded somewhere.

RandyAB 03-12-2019 07:28 PM

Quote:

Originally Posted by Scott Hersha (Post 1331078)
RandyAB,
I used the momentary start push button + keyed ?enable switch? for the reasons someone else explained. It can prevent unauthorized or unintentional starter activation - much the same as a standard aircraft (Cessna) start switch, but at a much less expensive price, and either switch could be bypassed if there was a switch failure and you were stranded somewhere.

Where did you put it Scott? I?m trying to be as economical with the panel real estate as possible given what I have planned.

MJarreau 03-12-2019 09:13 PM

Switch Rating Question
 
Just a question for technical types here.... Most switches I've seen are rated for AC (think industrial switches from Allen-Bradley, Cutler Hammer, SquareD, and etc.). When switching DC don't they have to be de-rated somehow? :confused:

Mike

Bicyclops 03-12-2019 09:31 PM

Quote:

Originally Posted by MJarreau (Post 1331175)
Just a question for technical types here.... Most switches I've seen are rated for AC (think industrial switches from Allen-Bradley, Cutler Hammer, SquareD, and etc.). When switching DC don't they have to be de-rated somehow? :confused:

Mike

DC is harder on switches than AC is. A diode on the start contactor helps the switch by absorbing the spike as the magnetic field collapses. A start contactor could pull as much as 5A which can be tough on a lot of switches.

I use an "enable" switch with a flip up cover on it and a start button on my stick which pulls a relay closed to power the contactor. The relay coil current is very low so I think the switch should last a long time. If it or the relay were to fail, the enable switch also has a momentary position that bypasses the relay straight to the contactor.

Ed Holyoke

rv7charlie 03-12-2019 10:25 PM

Quote:

Originally Posted by MJarreau (Post 1331175)
Just a question for technical types here.... Most switches I've seen are rated for AC (think industrial switches from Allen-Bradley, Cutler Hammer, SquareD, and etc.). When switching DC don't they have to be de-rated somehow? :confused:

Mike

According to Nuckolls (Aeroelectric) 12-24v dc is about the same stress as 125vac (much lower ability to drive an arc when the switch opens). That's supported by ratings on switches that do have both ac & dc ratings; they're typically the same current at 12-24vdc & 125vac. Plus, our lifetime cycle count in a/c is about 1% or less of a typical switch's life rating.

Mich48041 03-13-2019 07:35 AM

I agree with Charlie. The fact that the voltage is being reduced by a factor of
10, 120 down to 12, automatically reduces the rating. The switch carries 1/10
of the power (volts times amps). The important thing is for the switch to be
snap action so that the arc is broken quickly. A diode should be connected in
parallel with a relay coil with the banded end of the diode connected to the
positive side. Even a small diode like a 1N4148 will work.

Scott Hersha 03-13-2019 07:34 PM

Randy,
I can send you a pic of my switch installation if you would like. I?ve tried posting pics here on this site, but haven?t been able to (I?ve been trying new ideas for a couple hours now). When I eventually figure it out, the next time I try to post a picture, it doesn?t work anymore, and I have to load some other picture posting ap. This usually takes more time than I?m willing to devote. I?ve probably used maybe 5 picture posting applications for posting pictures here, but they seem to be a temporary solution. Sending you an email with a picture attached is very easy, so send me your email address and I will be happy to send you what I have done, and where my switches are located on my RV8. If you happen to be at Sun N Fun, come and take a close look at JB Aircraft Engines exhibit booth, near building C I think (N60-N61).


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