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Soft Switch - Hard Lesson?

A very graphic lesson learned.

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Martin Harris22/09/2018 13:13:01
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A clubmate was working in his modelling shed, surrounded by all the paraphernalia of a traditional modeller, mostly highly flammable, plus many models with substantial value and countless hours of work invested in them and wondered why the back of his legs were feeling warmer than expected. Glancing behind him, he was horrified to see substantial flames leaping from a largish petrol powered Yak.

Fortunately, the hastily lobbed fuselage didn't leave any sparks or flames behind it and the combustion was beaten into submission in the safety of his garden before spreading to the part filled petrol tank.

img-20180920-wa0000.jpg

An AIB (Amateur Investigators Bullsdoodoo) inquiry has concluded that the most likely cause was a fault in the receiver, which has (had!) a soft switch facility which allows the receiver batteries to remain connected with minimal discharge (a few microamps) for weeks or months.

In this case, the "safe" option of LiFe cells (2 pairs) was being used but the sizeable conductors would allow high currents to be extracted under short circuit conditions.

For those who may not have recognised it, the next picture details what was left of the receiver - I believe the level of destruction allows us to draw the conclusion that this was most likely to be the seat of the fire:

img-20180921-wa0001.jpg

With a traditional switch for the receiver battery there is (a) a physical disconnection and (b) thinner gauge wiring which will probably burn out before a short circuit would allow conflagration to get to these levels but although soft switches have a remote switch, in most cases it fails "safe" i.e. the open condition activates the receiver.

How lucky was he that this happened while he was actually in the shed?  Or more importantly, that he didn't incapacitate himself due to the result of panic!

So (with my safety officer hat on) I would strongly recommend that if you use this type of system (although this was a Spektrum receiver, similar products are available from other radio and power distribution box makers, you make a point of physically disconnecting any batteries at the end of a flying session.

Edited By Martin Harris on 22/09/2018 13:38:02

jrman22/09/2018 18:49:21
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Martin,

"but although soft switches have a remote switch"

Not sure what this means.

Martin Harris22/09/2018 18:58:37
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A switch located (usually) on the exterior of the model (this may be a physical, magnetic or radio controlled switch) which initiates the power-up of the receiver or power distribution box. The failsafe characteristic is normally that should the switch fail or become disconnected, the power remains connected to the receiver and servo.

Edited By Martin Harris on 22/09/2018 19:07:17

jrman22/09/2018 19:03:57
402 forum posts
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I understand what a soft switch is, I just thought you were adding another switch as well.

Martin Harris22/09/2018 19:07:58
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9404 forum posts
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The point I was trying to make is that although you may have a switch which appears to turn off the power, there is always a direct connection from the battery to the receiver or distribution box (I would say power box but this is too similar to a trade name).

Denis Watkins22/09/2018 19:29:30
4528 forum posts
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As always, when our gear works it works

But the odd time it goes pear shaped

A switch that does not switch off, does not seem a good idea

Good quality off switches are available

Hindsight is a great thing

But we can use as little as 500mah during the day, so why have 1000s of mahs in reserve?

" oh chaps, I am OK, I have a 3000mah pack back up" this is a lot of amps waiting to dissolve the innards

Judging or risking enough power needs consideration

Callsign Tarnish22/09/2018 19:43:10
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What brand was the receiver?
Martin Harris22/09/2018 20:35:01
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I'd disagree with a couple of your points, Denis. The principle of an electronic switch which fails safe (for the model in flight) is very sound, in my opinion. Mechanical switches can be a weak spot, which is why I like to have 2 of them and 2 batteries on my larger models. The point of my OP (which does mention the make CT but I don't want to make this brand specific) was to highlight the dangers of leaving the batteries connected to any similar equipment even though they can be from the perspective of battery drain.

Secondly, although we do tend to overspecify packs these days based on capacity, this is largely because we have the option of better power density, meaning that the higher demands of more powerful and digital servos in generally larger models than were common can be accommodated without danger of voltage drops under load. Packs are less stressed by lower C rates and cycle lives.

Edited By Martin Harris on 22/09/2018 20:37:47

Steve J22/09/2018 21:40:37
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Posted by Martin Harris on 22/09/2018 13:13:01:

So (with my safety officer hat on) I would strongly recommend that if you use this type of system (although this was a Spektrum receiver, similar products are available from other radio and power distribution box makers, you make a point of physically disconnecting any batteries at the end of a flying session.

I already do so with my Spektrum Powersafes for two reasons, a) I have to disconnect the batteries to charge them, and, b) if I don't disconnect they discharge at c. 1000 mAh/month. If the batteries (LiFes) are disconnected, I can connect months later and go flying.

Steve

Steve J22/09/2018 21:50:07
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Posted by Martin Harris on 22/09/2018 20:35:01:

Secondly, although we do tend to overspecify packs these days based on capacity, this is largely because we have the option of better power density, meaning that the higher demands of more powerful and digital servos in generally larger models than were common can be accommodated without danger of voltage drops under load.

Indeed. My main criteria when deciding what batteries to use in a model is how the receiver/servo power rail holds up under in flight.

Steve

MaL23/09/2018 16:03:20
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140 forum posts
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Why not fit a DPDT slide switch is series with the electronic on/off switch?wink 2 that way you could charge the batts in situ too....

where did I leave me coat......

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