Switch Wiring

[Andy J]

About to wire up a switch for a 8ft wing span model powered by a petrol 2 stroke motor using 2 off DPDT switches with 2 charging sockets as pictured. Intention is only to use 1 battery for the Rx and 1 for the engine.  At the moment the switch is wired such that both the live and neutral are switched on each switch but that provides no redundancy on each switch.

 

What is the current thinking on the forum about power switching. My initial thoughts are to only switch the live connection from each battery using both poles of each switch and common up the neutrals.  At least that way I can switch both the engine and Rx batteries rather than relaying on a separate kill switch controlled by the Rx.

 

Aware a number of people will recommend two batteries for the Rx, but personally I have never had a battery fail in flight plus my radio gear can be set to alert me of any abnormal drop in voltage so don't think that approach is required.  The more likely scenario is that the outward power connection from the switch to the Rx could fail due to say a wire breaking so do accept it would be better if two separate power inputs to the Rx are used if input connections to the Rx permit.

 

 

[Gary Manuel]

Switching the positive and negative is not a good idea. It effectively results in 2 switches in series and therefore 2 failure points.

 

Unswitched negative is the way to go. Use both poles of a DPDT switch in parallel as you have suggested and you have switch redundancy (it will tolerate 1 pole failing).

 

Input from the battery can either be 1 battery per switch (by keeping wiring to both switches separate) or both switches from 1 battery (by commoning the positive of the inputs to both switches). I like to have separate batteries in order to keep any ignition noise isolated from the receiver.

Edited September 27, 2021 by Gary Manuel

[Andy J]

Gary, do you keep the neutrals from each battery separate?

[Gary Manuel]

1 minute ago, Andy Joyce said:

Gary, do you keep the neutrals from each battery separate?

Yes I do. I even try to run the wiring of ignition and receiver as far away from each other as possible, though they do come quite close together using the type of switch in your photo.

[Andy J]

Thanks Gary, will modify the switch accordingly to keep the neutrals separate from each battery. 

 

Interested to know if anyone has made a simple power bus system such to allow a better distribution of power to each servo within a model.

[Gary Manuel]

 

[Andy J]

Totally agree Gary.  Given the model has 8 servos up for any method of improving the power distribution.

 

Not totally sold on using a powerbox as this only introduces multiple plugs into the equation but cant currently think of an alternative.

Edited September 27, 2021 by Andy Joyce

[Andy J]

Just looked at that thread for a power safe distribution board and sent Chris a PM on the topic as it looks perfect for my application.

[MattyB]

XPS in the states do a number of these type of solutions - we have a number of the X10+ units in service with Frsky RXs, but they work with pretty much any serial receiver…

 

https://www.xtremepowersystems.net/products.php?cat=8

 

Then there are the Frsky redundancy buses too, there were issues with the early iterations but they seem to work fine now…

 

https://www.t9hobbysport.com/search?s=Redundancy+bus

[Mike T]

See this thread.  I included a diagram, but you can see from the subsequent posts that switching the +ve contacts is the better way to go:

 

 

[Andy J]

Thanks Mike, do recall now seeing your posts and diagram. Always better to switch the live connection.

[Martin McIntosh]

I have a petrol model with two batteries via heavy duty switches into a JR Rx and a separate ignition battery. Not entirely foolproof but it works. However, I soon found that when recharging, one battery must be disconnected from its switch otherwise I can only get a few mA/h of charge in. Must be a common ground problem.

When they are available I now use two packs via a HK Failover Switch which will switch out a failed pack and run from the other. No mechanical contacts either, just push buttons.

 

[Martin Harris - Moderator]

Hard to see why the common earth might affect charger detection in this case although I have experienced similar when trying to charge 2 packs from chargers fed by a common power supply.  The cure for this was to provide a separate power source (12V battery) for the second (and on occasion, when charging an onboard glow, 3rd) charger.  I quite frequently charge dual NiXX and LiFe battery systems one at a time without having to disconnect anything (and I don't use diodes except with LiFe packs for voltage reduction purposes).

[Martin McIntosh]

Got me beat too. The packs are 5 cell Eneloops feeding into batt 1 and batt 2 on a JR 921. I soon discovered that on recharging, one would only take 20 mA/h and the other, say, 200. This is a very real situation.

[MattyB]

3 minutes ago, Martin McIntosh said:

Got me beat too. The packs are 5 cell Eneloops feeding into batt 1 and batt 2 on a JR 921. I soon discovered that on recharging, one would only take 20 mA/h and the other, say, 200. This is a very real situation.

Think you may have the wrong thread Martin!

[Andy J]

Could you draw a crude sketch Martin of your power lines schematic as difficult to see how this would happen as the battery switch should isolate any feedback possibly though the receiver. Just realised you are using an electronic switch so perhaps this is the cause.

[Martin McIntosh]

Eh. Matty B, I think not.

Andy, I was referring to the mechanical switches not the electronic ones I now use.

Edited October 3, 2021 by Martin McIntosh

[Andy J]

Well as I suspected re-soldering the switch was a failure as one of the poles was damaged due to excessive heat from the soldering iron.  Did think that would happen so better quality switches now on order.

[Chris Walby]

I don't get the rational for using a double pole switch with them linked together.

 

Either select a switch that has the correct rating or use something else. Why? if one pole stops working then you can't tell it has, which means the other pole becomes overloaded and is more likely to fail.

Better still locate the RX battery in a position (say under a canopy) where as part of the start up sequence you just plug it into the RX wiring loom, alternatively use  a suitably rated switch for the intended load.

 

IMHO - keep it simple

[Martin Harris - Moderator]

Good point, especially if the rating for each contact is less than or only marginally sufficient for the load. 

 

I'd be a little cautious over the advice over using the plug in method though as there may be a small amount of arcing as the receiver and servos power up which coupled with the design life of something like 50 - 100 operations for a typical gold plated connector - as little as 10 for tin plated - means that the reliability of the connection may be compromised rather quickly.  I know we often use these connectors for aileron connections but they're unpowered on assembly and only unplugged at the end of the day.

[Andy J]

20 hours ago, Chris Walby said:

I don't get the rational for using a double pole switch with them linked together.

 

Either select a switch that has the correct rating or use something else. Why? if one pole stops working then you can't tell it has, which means the other pole becomes overloaded and is more likely to fail.

Better still locate the RX battery in a position (say under a canopy) where as part of the start up sequence you just plug it into the RX wiring loom, alternatively use  a suitably rated switch for the intended load.

 

IMHO - keep it simple

Don't agree Chris, using a SP switch could cause the loss of the model unless a dual battery installation is used. What could get round this issue of not knowing if both poles of a DPDT switch are working is to simply use two LED's driven of either pole of the live side of the switch.

[Mike T]

My logic for using linked poles was straightforward.  It left one side of the wiring unswitched, eliminating a point of failure, which enabled the second set of contacts to be doubled up, thereby increasing the load capacity.  I wasn't looking for redundancy because (as pointed out) you can't tell if one pole has failed, but reducing the load on each pole by sharing it does reduce the likelihood of failure.  As our friend Don Fry is fond of pointing out: 'it's a game of blood' ?.

 

@Chris WalbyI agree that switches with sufficient individual contact ratings should be used. I wasn't able to find any suitable for our purposes.  If you know of any then please give us some links. 

 

I mentioned in the other thread that 'heavy duty' usually refers to the casing, not the contacts and if you've ever opened up a heavy duty Futaba/JR switch you'll see that apart from the switch body and the cables, the actual switch innards are the same as the standard off-the-shelf versions.  There is one popular switch,complete with LED and a charge socket with a sliding cover that has innards which are frighteningly small.  If you look at the switch in the OP, note that while the hefty alloy double casing looks sturdy, the actual switches fitted in the back appear to be miniature types listed by Maplins or RS which had a contact rating around the 2A mark...

[Martin McIntosh]

I recently bought a double toggle switch unit, mistakenly reading it as an electronic one, in a nice alloy casing which has a fuel filler in the centre! Why would anyone want to risk squirting fuel all over the switches? They are however of standard miniature construction with a reasonable current rating so if I ever wish to go down the simple dual battery route again I would be happy to use it. Shame that the switches are mounted upside down but easily corrected.

A well known jet jockey and dealer stripped and examined many switches and genuine JR ones were the only ones he would use. The chances of a parallel contact slide switch failing are minimal and I confess to using the same ones for many years because they are at least the genuine article.

If anyone is paranoid about this then even on a single battery they could wire in two switches in parallel.

[Mike T]

I tend to use the same switches over many years as well!  If it ain't broke...  Wouldn't you feel sick if you swapped out a known good 'un which was technically past its sell-by date, only to fit a new one which failed after a few flights?

 

The switch that came with my old McGregor gear could be taken apart to clean the contacts.  It was the most reliable part of the system...

 

[Chris Walby]

Andy, my point exactly, why have a weak link in the chain when you don't need to, just don't have a switch. If you can arrange the RX battery to an easy access point (under the canopy) then just plug the the battery straight into the RX loom, sorted.

 

As for the LED idea, sorry I don't get that as it won't tell you a pole has failed until both have, unless you are talking two switches configured and you can switch each pole separately (what a faff to double check everything before each flight). 

 

Mike T How about 

G.T. Power Electronic Switch 8A/10A £9.49 from 4-Max

or 

G.T. Power Electrical Switch 8A/10A £13.99 from 4-Max

or HK used to do a dual input version but its not double diodes so both batteries discharge at the same rate

or how about this

Mini Servo Power Distribution Box - Yellow fully featured version for Petrol Engines £69.99 from 4-Max

 

There is a wide range to choose from

 

If you are unconvinced then I suggest a chat with George at 4-Max will help. With my 180 powered Yak I don't use a switch, just plug the 3S2200 straight into the UBEC (20A). The rational is that firstly keep it simple, secondly if a servo fails the the battery and UBEC can produce enough current to clear the fault without browning out the RX, thirdly is I use 3S2200 as flight batteries so they get a good work out on a regular basis thus I know they are more than good enough as a RX battery

 

Lastly although I have not used any I understand there are some very good quality/expensive switches on the market from top end suppliers, but the name escapes me at the moment!