ACK ELT Turning on at startup

[Wayne]

I have my ACK ELT installed aft of the baggage compartment and the remote control on the panel.

Occasionally on starting the plane (master switch on, no avionics switch on) the ELT will activate. Must be picking up a electrical spike from starter or....? and causing it to turn on. There is no wires running from the ELT through the cabin that are live at start up.

Any suggestions?

Thanks

[mgomez]

Could it be reacting to the shake and shudder of the airframe as the engine cranks?

To isolate that, try removing it from its mount and resting it on some foam (or even a seat cushion), and trying it again a few times.

[lucky333]

I helped a guy with the exact same problem. He did not have diodes across the master and starter contactor coils. The transients generated tripped his ELT almost every time. Adding the diodes eliminated the problem.

John

[az_gila]

Originally Posted by lucky333

I helped a guy with the exact same problem. He did not have diodes across the master and starter contactor coils. The transients generated tripped his ELT almost every time. Adding the diodes eliminated the problem.

John

If that was the case... then fixing it probably prevented early failure of the expen$ive avionics in his panel.....

Gil in Tucson

[Wayne]

Originally Posted by lucky333

I helped a guy with the exact same problem. He did not have diodes across the master and starter contactor coils. The transients generated tripped his ELT almost every time. Adding the diodes eliminated the problem.

John

Please tell me more on these diodes...

Thanks

[lucky333]

Relay and solenoid coils generate a negative spike across themselves when they are switched off. A diode across the coil with the band-end towards the (+) supply will become forward biased (turns ON) to 'short out' the spike generated as the magnetic field around the coil collapses.

Aero electric explains it pretty well here:
http://www.aeroelectric.com/articles/spikecatcher.pdf

Bob discusses several transient suppression ideas in this one and arrives at the diode across the coil. This may be a bit misleading as ALL coils should have a diode across them first. If you need to suppress arcs across contacts etc, that's a separate issue. His other schematics also show which terminals are which on the contactors and how the diodes hook up. The 'simple electrical schematic' in the RV7 preview plans also shows where to put them.

Note how with a 12V coil, he shows a -300 volt spike. That's about right and may actually be a bit low. I scoped one for grins at lunch and my battery contactor generated -500V across the coil when it let go (without a diode). It ALSO injected much of that into the BUS terminal. Even with internal protection, imagine how delighted your EFIS will be to have this presented on its power lines

Vans and lots of others sell ready made diode assys with install directions. Or you can roll your own. I used some 3Amp 1KV ones that I had around. Something like a 1N5408 will set you back a whoppin' 41 cents at Mouser and will handle about anything you throw at it. Put em right at the contactors keeping leads as short as possible.

1N5408 at Mouser

However you decide to do it, diodes across these (and any other DC coils) should always be included.

BTW: be careful when measuring voltage across large coils. A client of mine once called to say that every time he tried to measure the coil voltage on a (larger) 24V contactor, his DVM blew up, actually 3 of them did. Scoping it out revealed a 2500volt spike when the coil was shut off. Smoked the protection circuits on his el cheapo DVMs. He later added diodes across the coils and bought better meters.

And of course, I am sure that you NEVER would tell anyone to hold onto that no-diode 12V coil terminal while you cycled the master switch.. would you..

Have fun,
John

[swisseagle]

Hello all!

There is a product that help for this situations, does anyone has experience with

WhackJack or SnapJack from http://www.periheliondesign.com/suppressors.htm

I do not have any relations to this company, just found theyr products are may be helpfull.

Dominik

[DGlaeser]

Originally Posted by lucky333

I helped a guy with the exact same problem. He did not have diodes across the master and starter contactor coils. The transients generated tripped his ELT almost every time. Adding the diodes eliminated the problem.

John

My ELT is not connected to the electrical system in any way - how would those transients cause the ELT to trip?

[Bryan Wood]

Originally Posted by DGlaeser

My ELT is not connected to the electrical system in any way - how would those transients cause the ELT to trip?

Dennis,

Did you use the antenna supplied by ACK? The owner/designer of the ELT that you are talking about is a family friend and he gave me a correction a couple of years ago for using a rubber ducky and mounting it in the baggage area. I did it because it is so common among builders to do this and have one less antenna outside the airplane creating drag. Here's a basic expanation as he gave it to me.

The provided antenna has a very narrow bandpass and drops off sharply for frequencies other than 121.5mhz. The reason that this is important is not for when it is being used as a transmitter in an emergency, but to protect the electronics from electric noise from tripping the thing on. He said that your own radios can trip the thing on, or others can when just rolling by and keying their mikes. This is not unique to this ELT either. There is a chance that noise from the alternator, starter, electronic ignition, etc. could be setting the thing off.

If this isn't your problem you should call Mike at ACK monday and give him a shot at your problem. I'll bet that he will know exactly what your problem is right off of the top of his head.

Let us know what you find if you could. This has me curious and I'd like to learn also.

Best,

[Wayne]

I talked to Mike at ACK and he gave a few test to do to help isolate the problem. It was also discussed how the Lightspeed electronic ignition that I have might be triggering it as well. The LSE II control box is only 1 ft away from the ACK remote head.

So off to the airport I went Saturday afternoon with tools in hand ready to tackle this problem only to find that I can get it to go off! Geez, I hate thinks that are temperamental and inconsistent!

Here is the plan if (when) the ACK ELT goes off again at engine start up. Shut down and:

1. retry starting again with the cable unplugged from the remote head. If this is the problem I will follow Johns advice with the diodes.

2. retry starting again with the fuse breaker pulled on the LSE. The LSE is identified as the problem I was told to purchase mu metal from Aircraft Spruce and wrap the LSE control box.

I will advise further when I become wiser.

Appreciate the input from all