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Author Topic: will this switch be safe?  (Read 5902 times)

jenga

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will this switch be safe?
« on: November 27, 2014, 05:59:11 PM »

Greetings,
Ive been looking for a toggle switch for my future wiring project. What I am looking for in an ideal world is a switch that will control power (+&-)supply from battery to esc/motor on one throw and power (+&-) from charger to battery from the other throw  eg on-off-on.
now, I can find switches, but all rated for 250v eg 2A250v, 6A125v (rating from same switch) and the highest I can find is rated at 16A.
it seems to me, that as voltage decreases amp capacity rises,based on example above, so will 16amp be ok on power supply carrying 20amp 12V?
the 20amp supply rating is governed by inline fuse (quick blow) as advised by esc manufacturer.
could some sparky guru advise me please or suggest better option. best be safe than sorry
regards Jenga
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inertia

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Re: will this switch be safe?
« Reply #1 on: November 27, 2014, 06:50:48 PM »

This is one of those areas where theory and practice differ. In theory 16A is the maximum current that switch will carry at any voltage, whereas 16A toggle switches have been used to operate many higher-current model boat power circuits for yonks. Naturally if your installation includes two 60A brushless motors and speed controllers then it would be a good idea not to use a 16A switch, but for "normal" purposes (e.g. a couple of 500 brushed motors and a 12v SLA battery) then you're on pretty solid ground.
An alternative is the ACTion P106 or P107. These use a very low-resistance MOSFET to switch the high current while you can connect a tiny low-current switch to turn the MOSFET on and off, because that's the way semiconductors work. Click on this link then scroll to the bottom of the page. http://www.componentshop.co.uk/action-electronics/power   
So simple that I wish I'd thought of it................... <:(
BTW there's no need to use a SPDT switch to put the charging jack in circuit. You can wire it up permanently to the battery terminals as long as you remember to switch off the supply to the model's circuits before you start charging.
DM
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Bob K

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Re: will this switch be safe?
« Reply #2 on: November 27, 2014, 06:54:23 PM »

Hi Jenga.  I assume that you want it for a boat, to be accessible, and may be subject to ocassional water.

For c/o switching of up to 5A at 28 VDC I would recommend this compact Maplin 2 pole c/o toggle switch rated at IP67 (up to 1m depth).
http://www.maplin.co.uk/p/ip67-sealed-toggle-switch-n25ka?gclid=CjwKEAiAtNujBRDMmoCN46aB8noSJAC7SYv7lagP-zaNf1KYlP4uoRgHqNgkM21tL06uqmgjzvvuOxoCefvw_wcB

Wire the common to your battery, the n/o side to your motors ESC etc, and the n/c to your charging socket.  That way you charge without the motors ESC etc being in circuit. 

Switch ratings are normally based on what power they can switch on and off.  As long as you don't touch the TX controls when switching "on" the contacts will not be switching under load so you don't need "full power" or surge rating. 

Anything much over 5A and you are going to need a heavy duty relay as well, the switch turning the relay on.

(PS:  DM's reply was almost at same time - excellent advice  :-)) )
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plastic

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Re: will this switch be safe?
« Reply #3 on: November 27, 2014, 07:00:56 PM »

I would use a teeny tiny switch powering the coil of a big-assed 12V 40A Automotive relay - then you know it's designed for the job!  :-))
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jenga

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Re: will this switch be safe?
« Reply #4 on: November 27, 2014, 07:16:35 PM »

thank you for your swift response guys, the reason I wanted the on-off-on switch was to make it idiot proof (needed).
im not being lazy wanting to charge in situ, just that my left hand thumb was neatly trimmed off below knuckle by chopsaw a few years ago so grip is poor. rather spend on insitu set up than drop battery into hull. easiest option would I guess be have Tamiya type connector and swap leads that way.
keep suggestions coming, all gratefully recieved
regards Jenga
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Stavros

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Re: will this switch be safe?
« Reply #5 on: November 27, 2014, 09:37:58 PM »

40 amo switches available form Demon Tweeks
 
http://www.demon-tweeks.co.uk/performance/switches-start-buttons/longacre-heavy-duty-40-amp-ignition-switch
 
Alos Please DO NOT Use those stupid TAMYA connectors they are usless.....use deans connectors or similar available form Component shop
 
http://www.componentshop.co.uk/leads-and-connectors/connectors/deans-connector
 
 
Far more reliable and will take high currents
 
Dave
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flashtwo

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Re: will this switch be safe?
« Reply #6 on: November 28, 2014, 12:07:04 AM »

Hi,

How about this one rated 20 amps at 12 volts.

http://www.rapidonline.com/electronic-components/high-current-dpdt-centre-off-toggle-75-0252

or perhaps a DPDT knife switch ( the old Frankenstein movie type!) simple design and high current. Perhaps a practical friend could help you make one out of copper and brass.

Ian

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NFMike

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Re: will this switch be safe?
« Reply #7 on: November 28, 2014, 12:08:26 AM »

As BobK says, most current ratings for switches are what they can switch (usually OFF as that is generally the worst case). Commonly there are two ratings given, one each for AC and DC and these are usually different voltages and currents (DC V and A being lower as a rule because DC is harder to break than AC).
Switches can usually safely carry a lot more current than they can break. If you go to RS or Farnell you'll often find the full spec including carrying capacity in a data-sheet.
If you already have a switch it may have an 'RS' number on it: nnn-nnn even if you got it elsewhere, so you might be able to find it on RS to get the full spec.


When you use a switch to disconnect a circuit offload like this (where the switch is not rated to break the circuit under load) it is more commonly called an isolator.

jenga

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Re: will this switch be safe?
« Reply #8 on: November 28, 2014, 09:29:42 AM »

Again many thanks guys sound advise.
So as long as + positive side of circuit is broken, from battery to esc then it is safe to charge insitu? no need to break - side?
charging leads can be wired to battery as stavros said via carging jack.
As I said earlier the max amp on esc line , in theory would be 20amps, being protected by a quick blow fuse 20a ,
my concern was that too low a switch rating would either pit or fuse the contacts together, some good suggestions recieved and will be useful to loads of others too, cheers guys. I see no point in joining such a brilliant knowledge base as this and not using it, I will be back with more.
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jenga

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Re: will this switch be safe?
« Reply #9 on: November 28, 2014, 09:34:26 AM »

oops iit was DM not Stavros, sorry both. old age dont come alone....now what was I doing.........................................
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Bob K

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Re: will this switch be safe?
« Reply #10 on: November 28, 2014, 09:42:37 AM »


So as long as + positive side of circuit is broken, from battery to esc then it is safe to charge insitu? no need to break - side?


That's right, you only need to break one side (usually the +VE) to break the circuit.
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sparkey

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Re: will this switch be safe?
« Reply #11 on: November 28, 2014, 09:55:45 AM »

 :-)) Get a double pole 20amp switch and bridge terminals which will give you 40amps ,done this a lot of times and never had any problems,I have measured current up to 56amps on load,Ray. :-)) :-)) :-)) :-)) :-))   
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malcolmfrary

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Re: will this switch be safe?
« Reply #12 on: November 28, 2014, 10:43:29 AM »

:-)) Get a double pole 20amp switch and bridge terminals which will give you 40amps ,done this a lot of times and never had any problems,I have measured current up to 56amps on load,Ray. :-)) :-)) :-)) :-)) :-))
As long as the load is not active at switch-off time, an excellent idea.  As previously stated the pain for the contacts happens when they break the load when on DC.  There is a brief time when the contacts have just separated when an arc can form.  With AC, the supply drops to zero many times a second.  With DC, full voltage stays there and the arc can be maintained much longer.  This is what does the damage to the contacts.
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sparkey

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Re: will this switch be safe?
« Reply #13 on: November 28, 2014, 11:05:54 AM »

 :-)) Must admit Malcolm I always switch off load,some years ago I bought a lot of good quality D.P. 20amps toggle  switches from a old radio shop in Tooting which alas is no longer there,they are of the knife type with a heavy action,only setback is they are rather large the body being 35mmx25mm still I have always managed to fit them in,they have U.S.A on them so I take it they are American made,Ray. :-)) :-)) :-))   
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petermun

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Re: will this switch be safe?
« Reply #14 on: November 28, 2014, 11:39:54 AM »

In the past, I have used double pole 16 Amp switches connected in parallel, making sure the connecting cable is of the appropriate rating.   Pete
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NFMike

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Re: will this switch be safe?
« Reply #15 on: November 28, 2014, 12:51:58 PM »

I'd not use a knife type. They usually have a lot of exposed metal which is a short-circuit risk.

Bob K

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Re: will this switch be safe?
« Reply #16 on: November 28, 2014, 01:12:32 PM »

A small beware on double-ganging pole contacts, it is not an automatic doubling of current capacity.
Yes, under static conditions you have much greater contact area, but when switching load one pole may open/close ever so slightly before the other.  Arcing can be in the microseconds of break or make.
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Re: will this switch be safe?
« Reply #17 on: November 28, 2014, 01:56:25 PM »

I would use switches only under non load conditions let the ESC switch the load ,the switches I use are completely insulated and have no exposed parts,I have used switches this way for many years without any problems,Ray. 
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Netleyned

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Re: will this switch be safe?
« Reply #18 on: November 28, 2014, 02:33:43 PM »

Since most switching on and off will take place with no motors running
current will be minimal therefore there should not be any
appreciable arcing on the switch contacts.
The switch will probably only be used once or twice a week
there  should not be much pitting in a year's use. IMHO.

Ned
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inertia

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Re: will this switch be safe?
« Reply #19 on: November 28, 2014, 03:05:25 PM »

Exactly! What a sensible chap you are, Ned; a true proponent of one of my favourite adages "If it ain't broke then don't fix it".  :-))
It's usually cheaper that way, too!
DM
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jenga

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Re: will this switch be safe?
« Reply #20 on: November 29, 2014, 11:10:22 AM »

greetings
thanks for all advise. Update: went with Stavros idea (bows in respect) and used 40amp motor cycle ignition switch, this is designed to be weather proof and is for 12v dc loading, and will get 100's of more switching cycles on a bike per month than will in the boat, so should be bullet proof.
NOTE i did not say idiot proof
10 inc p&p
thanks for taking time to help out
regards Jenga
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Bob K

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Re: will this switch be safe?
« Reply #21 on: November 29, 2014, 11:52:13 AM »


NOTE i did not say idiot proof


"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.

Collorary: The difference between a thing that can't break and one that can, is that the thing that can't break can't be repaired once it breaks."   Douglas Adams
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plastic

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Re: will this switch be safe?
« Reply #22 on: November 29, 2014, 12:38:24 PM »

"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.

Collorary: The difference between a thing that can't break and one that can, is that the thing that can't break can't be repaired once it breaks."   Douglas Adams

Agreed - you can't make things idiot proof - idiot resistant is the best you can hope for.  :-)
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