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Author Topic: Switch Capacity  (Read 1000 times)

CJK

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Switch Capacity
« on: June 01, 2019, 07:42:38 AM »

I would like some advise please. Picked up a nice little switch rated at 3A 250VAC - 5A 125VAC from my local model shop and would like to use it as a main switch between battery and ESC. I will be using either NiMH 7.2V or 8.4V, 20A ESC, so my question is - will it be man enough?
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DaveM

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Re: Switch Capacity
« Reply #1 on: June 01, 2019, 08:21:09 AM »

The only way to be certain would be to measure the current drawn by the motor under load. That said, I wouldn't use a switch this size for anything other than a 385 or smaller. Soldering even light-gauge wire to those terminals is tricky - the plastic switch case melts very easily.
DaveM
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CJK

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Re: Switch Capacity
« Reply #2 on: June 01, 2019, 09:59:37 AM »

Would this be better? 25A@12V
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DaveM

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Re: Switch Capacity
« Reply #3 on: June 01, 2019, 10:10:30 AM »

Quite definitely better!
DM
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malcolmfrary

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Re: Switch Capacity
« Reply #4 on: June 01, 2019, 10:12:24 AM »

Counter intuitively, low voltage DC ratings for current switching tend to be lower than the figres quoted for AC.  The life shortening damage to switches happens when being switched off, if there is and inductive load on at the time, it creates an arc.  AC lets go about 100 times a second, allowing the contacts to separate further stopping the arc from re-forming.  DC doesn't let go until the contacts have moved far enough for the arc to collapse.
Switches can usually carry a lot more current than they can control.

Apart from that, what Dave said.  The tabs and contacts are often part of the same pressing, a very slight melt of the plastic will re-align the contacts, so speed is essential.  An iron that has been allowed time to get really hot helps to get the join done and the source of heat moved away before the temperature from the iron gets to the plastic.

2 posts while writing - yes the newly proposed switch looks way better - make sure that the panel that it gets mounted to is up to the job.  And whatever THAT is fastened to.
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Martin [Admin]

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Re: Switch Capacity
« Reply #5 on: June 01, 2019, 11:20:42 AM »

Quote
Picked up a nice little switch rated at 3A 250VAC - 5A 125VAC from my local model shop and would like to use it as a main switch between battery and ESC. I will be using either NiMH 7.2V or 8.4V, 20A ESC, so my question is - will it be man enough?

 
I theory yes, because under most situation, you won't be using the switch under load.... but as you know, it under those 'special situations' ... when you need it most, it may let you down!


Malcolm: Am I right in thinking, that switch 'Amp rating', in this case, 3A, refers to the max "switching" current, not the load current?


... depending on above, accept or disregard above, above!     
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imsinking

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Re: Switch Capacity
« Reply #6 on: June 01, 2019, 11:44:41 AM »

you may not be 'switching' under load , but at some time during ESC operations it will take maximum load internally and you could get arcing / heating up etc . . .that's got to be considered , i've seen mini switches in a puddle of plastic (LIPO supply) along with inadequate wiring melting . . .things you 'got away with' in the DC motor era will def' trip you up in the brushless / LIPO world , worth bearing in mind . . .
Bill 
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plastic

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Re: Switch Capacity
« Reply #7 on: June 01, 2019, 02:06:05 PM »

I don't understand the concept of "This is what I bought, how can I make it do some purpose it wasn't designed for?"  Why not just buy either a properly rated switch or use the little switch to drive a heavier power relay?

Odds are the switching current will be zero unless the motor is running when switched - but these tiny switches get hot and melt really easily with current running through them - and the stall current of a motor is significantly higher than its running current - so if things stall, you've got loads of things damaged and needing fixing.  A 385 is supposed to have a 4A stall current.
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CJK

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Re: Switch Capacity
« Reply #8 on: June 01, 2019, 04:13:00 PM »

I don't understand the concept of "This is what I bought, how can I make it do some purpose it wasn't designed for?"  Why not just buy either a properly rated switch or use the little switch to drive a heavier power relay?

Odds are the switching current will be zero unless the motor is running when switched - but these tiny switches get hot and melt really easily with current running through them - and the stall current of a motor is significantly higher than its running current - so if things stall, you've got loads of things damaged and needing fixing.  A 385 is supposed to have a 4A stall current.

No 'concept' involved here! It was an impulse purchase. Up to now I haven't given any thought to how I will wire up the conversion from static to RC. When I got home doubt about the suitability set in. Luckily I have this forum to guide me.

I think it would be a good idea to actually put some serious thought into what I want to achieve and knock up a wiring diagram and get the approval of those on here that know what they are doing.  :embarrassed:
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Martin [Admin]

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Re: Switch Capacity
« Reply #9 on: June 01, 2019, 05:07:23 PM »

 
.... and don't forget the  'shop keepers advice' factor..... still popular selling "feature" by some independents!   ok2 :-))
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malcolmfrary

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Re: Switch Capacity
« Reply #10 on: June 01, 2019, 07:21:05 PM »


I theory yes, because under most situation, you won't be using the switch under load.... but as you know, it under those 'special situations' ... when you need it most, it may let you down!

Malcolm: Am I right in thinking, that switch 'Amp rating', in this case, 3A, refers to the max "switching" current, not the load current?

... depending on above, accept or disregard above, above!     

 


Probably.  Usually. 
But there are places in the world, well known for making electric devices, where it seems that they look at the spec sheet, find the biggest number on it and use that, either in marketing or when labelling the product.  320A ESC anyone?

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CJK

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Re: Switch Capacity
« Reply #11 on: June 02, 2019, 08:28:33 AM »

OK 1st attempt, would appreciate advice. Thank you
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malcolmfrary

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Re: Switch Capacity
« Reply #12 on: June 02, 2019, 09:23:28 AM »

Moving the 10A fuse to between the EC3 and switch will help protect both the switch and the boat from anything that the switch and its wiring might offer.
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CJK

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Re: Switch Capacity
« Reply #13 on: June 02, 2019, 10:02:47 AM »

Moving the 10A fuse to between the EC3 and switch will help protect both the switch and the boat from anything that the switch and its wiring might offer.
Now you've got me! If you put the fuse where you suggest and the switch is in the ON (RUN) position what is the fuse protecting there wouldn't be anything on that subcircuit (charging) - would there?
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Netleyned

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Re: Switch Capacity
« Reply #14 on: June 02, 2019, 10:14:09 AM »

Think he means EC5???
Ned
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CJK

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Re: Switch Capacity
« Reply #15 on: June 02, 2019, 10:53:24 AM »

This?
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DaveM

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Re: Switch Capacity
« Reply #16 on: June 02, 2019, 12:36:07 PM »

Looks to me like that one would work.
DM
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CJK

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Re: Switch Capacity
« Reply #17 on: June 02, 2019, 04:51:03 PM »

I thought I would buy the 25A 12V switch pictured earlier and luckily checked the dimensions before doing so - its huge, like 2" height. Did some goggle searching for minimum 10A 12V and didn't get anything that was much smaller. Is there anyone who could point me in the right direction please.
Since I started this project Ive studied many builds and seen what look like smallish switches being used on lots of them. They were all similar volages/amps to what I'm proposing so I must be looking in the wrong places.  :embarrassed:
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DaveM

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Re: Switch Capacity
« Reply #18 on: June 02, 2019, 05:14:15 PM »

Why not adopt some lateral thinking? This says that if you have a removeable fuse (like a blade type) then use that instead of a switch. You can always remove the battery pack for charging  8)
DaveM

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clockworks

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Re: Switch Capacity
« Reply #19 on: June 02, 2019, 07:29:21 PM »

I'm new to RC boats, but have raced cars and flown planes for years. I've never fitted a switch to isolate the battery.


On cars, the usual way is to have a small switch on the ESC, and keep the battery disconnected until it's time to race.
The small planes that I've built haven't used a switch at all - just unplug the battery. Saves weight, and one less thing to go wrong.


I've stuck with this for boats. Connect up the batteries at the lake, turn on Tx, switch on ESC.


I guess the difference is that, with cars and planes, it's normal to change the batteries several times during a session, whereas most boats run for long enough without swapping or recharging?


I wouldn't charge a battery inside a model either, incase something goes wrong. I've seen people doing it with SLA and a trickle charger, but anything else seems a bit risky. Batteries out before charging, and keep the battery/charger away from anything flammable. Maybe I'm just over-cautious?
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Netleyned

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Re: Switch Capacity
« Reply #20 on: June 02, 2019, 07:50:08 PM »

Resettable blade fuses can be used in lieu
of an ordinary blade fuse.
Press the button to trip and press again to reset.
You will still need to charge the battery by disconnecting.
Ned
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Colin Bishop

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Re: Switch Capacity
« Reply #21 on: June 02, 2019, 08:01:53 PM »

Gosh! 20 posts on a really simple issue!
Colin
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DaveM

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Re: Switch Capacity
« Reply #22 on: June 02, 2019, 08:03:36 PM »

I'm new to RC boats, but have raced cars and flown planes for years. I've never fitted a switch to isolate the battery.
On cars, the usual way is to have a small switch on the ESC, and keep the battery disconnected until it's time to race.
The small planes that I've built haven't used a switch at all - just unplug the battery. Saves weight, and one less thing to go wrong.
I've stuck with this for boats. Connect up the batteries at the lake, turn on Tx, switch on ESC.
I guess the difference is that, with cars and planes, it's normal to change the batteries several times during a session, whereas most boats run for long enough without swapping or recharging?
I wouldn't charge a battery inside a model either, incase something goes wrong. I've seen people doing it with SLA and a trickle charger, but anything else seems a bit risky. Batteries out before charging, and keep the battery/charger away from anything flammable. Maybe I'm just over-cautious?
You and I have the same hymn-sheet. 

I shall investigate resettable blade-fuses, Ned.

DaveM

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Bob K

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Re: Switch Capacity
« Reply #23 on: June 02, 2019, 08:38:47 PM »

Gosh! 20 posts on a really simple issue!
Colin

Maybe not for the more experienced model boater, but everyone started from scratch once and the learning curve is not always smooth.  I have seen too many boats break down on the water, usually with wiring issues.

Ended up an informative set of posts, and will be useful to many,  Nice wiring diagrams BTW  :-))
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Colin Bishop

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Re: Switch Capacity
« Reply #24 on: June 02, 2019, 08:51:11 PM »

An interesting philosophy above, but not mine I'm afraid.

I generally build scale models up to four feet long or so and the last thing I want to do is to delve inside them at the pondside. That's when you start breaking off all the delicate details and/or rigging.

So I prepare them on the bench at home beforehand, take them to the pond and use concealed switches on the decks to power them on and off. I then check them over again when I get home. Works for me. I even have external charging points for the NiMH or SLA cells and never take the batteries out of the boat as they are usually buried well down below and are not readily removable. Again, this has worked fine for over 40 years and I have never experienced a problem. Battery capacity is always sufficient for a day's sailing sesion and can be topped up overnight for a two day event without dismantling the boat.

I have made an exception for my recent SLEC Fairey Huntsman which has a LiPo pack but that is very easy to access.

Horses for courses?

Colin
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