Use a 240v switch in a 12v circuit?

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Hiya, hope this is a simple question :) I'm making up a wiring harness to run a 12v sprayer off my 12v ride-on mower battery.

This is the sprayer pump:
http://www.ebay.co.uk/itm/120978611777?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
Max current draw on it is 7A.

I need to put an inline switch in the wiring harness, so I was wondering if something like this would do:
http://www.ebay.co.uk/itm/UK-IN-LIN...3001?pt=UK_Light_Fittings&hash=item3f25acac89

Does it matter that it is designed for 240v applications rather than 12v? Will I still need one that is rated at 7+A, or because the power is so much lower on a 12v system, can I get away with a 2A switch?

Many thanks for any replies :)
 
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You just need to be careful if it is DC you are switching as it requires a greater rating than AC as there is no zero crossover on DC (as the voltage/current are zero in an AC system several times a second which assists the switch in breaking the arc current )
 
Thanks both, so a 10A switch should do it, whether it's designed for 240v or 12v then, if I've understood you correctly.

I can't seem to find any in the UK tho! There's these from Hong Kong:
http://www.ebay.co.uk/itm/2-x-Inlin...ome_Garden_Lighting_Parts&hash=item5d3bdafb2a

Or if I search for 12v 10A switch, I get these:
http://www.ebay.co.uk/itm/12V-250V-...al_Components_Supplies_ET&hash=item1c311fd7fd
but they're not inline.

How come it seems so difficult to get a 10A inline switch?? Am I looking in the wrong place, or is there a different solution that people use?? Thanks.
 
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The rated current for this pump is 7 amps. Fused at 15 amps

http://legacy.shurflo.com/pdf/industry/agri/911/911-8000-543-236.pdf

Therefor a normal 230 volt light switch will have a very short life switching the supply to the pump.

Use a relay with contacts design for 12 volt and at least 10 amp to switch power to the pump. Use the mains switch to switch 12 volts to operate the coil of the relay.

Most car spare shops will have something suitable, a headlamp relay or similar would be ideal.
 
Thanks both, so a 10A switch should do it, whether it's designed for 240v or 12v then, if I've understood you correctly.
As long as it is rated for 10A DC.

I can't seem to find any in the UK tho!
Have you tried RS Components/Farnell? Rapid Electronics?


There's these from Hong Kong:
http://www.ebay.co.uk/itm/2-x-Inlin...en_Lighting_Parts&hash=item5d3bdafb2a[/QUOTE]
That will not be rated at 10A DC.


Or if I search for 12v 10A switch, I get these:
http://www.ebay.co.uk/itm/12V-250V-...nents_Supplies_ET&hash=item1c311fd7fd[/QUOTE]
Did you not read down as far as CONTACT CURRENT - 10A AC?


How come it seems so difficult to get a 10A inline switch??
Maybe nobody wants to be grappling with a loose cable and switch swinging about whilst operating a moving vehicle, particularly a mower?


Am I looking in the wrong place,
Planet Earth is probably all you've got.


or is there a different solution that people use?? Thanks.
I would advise wiring it in properly, with a switch solidly fixed to the vehicle, ditto the wiring.
 
Links in this post may contain affiliate links for which DIYnot may be compensated.
Did you not read down as far as CONTACT CURRENT - 10A AC?

How come it seems so difficult to get a 10A inline switch??
Maybe nobody wants to be grappling with a loose cable and switch swinging about whilst operating a moving vehicle, particularly a mower?

Am I looking in the wrong place,
Planet Earth is probably all you've got.

Gee, thanks for the lecture.... No, you're quite right, I hadn't read as far down as the 10A AC on that listing - I was giving an example of the stuff that was being returned when I searched for 12v 10A switch and saying that that type wasn't what I was after as they weren't in-line.

I was just trying to replicate what was originally fitted to the sprayer, which is an inline switch (item 5):
http://shop.brinly.com/assets/images/ST-25BH.jpg
so give me a break!!

Thank you very much for all the constructive comments :)
 
There are voltage ratings on fuses because the fuse clearing/melting time is not supposed to be so fast that the inductance that all circuits have would generate very high voltage spikes.

I guess a 240v switch will interrupt 12v very quickly and may shorten the life of insulation, but how would you know?
 
There are voltage ratings on fuses because the fuse clearing/melting time is not supposed to be so fast that the inductance that all circuits have would generate very high voltage spikes.

.. No, there are voltage ratings on fuses because the fuse has to be able to open enough to quench the arc.
 
I guess a 240v switch will interrupt 12v very quickly and may shorten the life of insulation, but how would you know?
You guess wrong, Most 240 v switches are designed for use on AC and as has been said the arc created when the switch opens is extinguished when the alternating voltage goes through the zero crossing point. This means the switch can be opened slowly and still not have a prolonged arc.

Zero crossing point is when the voltage is zero as it changes direction.

Switches in a DC circuit have to extinguish the arc as there is no zero crossing point.. Most have a snap action to separate the contacts rapidly and far enough apart to break the arc before it can damage the contact surfaces.

Some large current DC switches use magnets to bend the arc (arc current in a magnetic field ) thus making it longer and quicker to break.
 
I think you need to think this one through and wire it in properly with fixed switches and secured wiring with appropriate protection,
The voltage's are low so there is no danger of shock the currents can be very high which introduces the risk of fire especially on mowing machines where there is dried grass etc floating around, I would go to a proper auto electrics shop. also can you post a pic of your mower as allot of the little dynamos on them wont produce anything near 7 amps so you might just flatten the battery or overload the charging system
 
I guess a 240v switch will interrupt 12v very quickly and may shorten the life of insulation, but how would you know?
You guess wrong, Most 240 v switches are designed for use on AC and as has been said the arc created when the switch opens is extinguished when the alternating voltage goes through the zero crossing point. This means the switch can be opened slowly and still not have a prolonged arc.

Zero crossing point is when the voltage is zero as it changes direction.

Switches in a DC circuit have to extinguish the arc as there is no zero crossing point.. Most have a snap action to separate the contacts rapidly and far enough apart to break the arc before it can damage the contact surfaces.

Some large current DC switches use magnets to bend the arc (arc current in a magnetic field ) thus making it longer and quicker to break.
How does that relate to the voltage generated by interrupting a circuit containing inductance L as in e = -L(dI/dT)? The L is small but so is the dT.
It's easy enough to plug in a range of typical values for L, dT and dI and see what kind of overvoltages are generated, but figuring the effect on appliance service lifetime is not so easy.
 
How does that relate to the voltage generated by interrupting a circuit containing inductance L as in e = -L(dI/dT)? The L is small but so is the dT. It's easy enough to plug in a range of typical values for L, dT and dI and see what kind of overvoltages are generated, but figuring the effect on appliance service lifetime is not so easy.
I'm not sure in what sense you think that would affect appliance service lifetime. In terms of insulation, brief 'overvoltages' will generally either have an immediately decisive effect, or no effect. As for arcing at the contacts, I would imagine that, unless one is talking of a very high inductance, such an induced voltage would only be able to sustain an arc for a very short period of time, probably usually no more than the maximum of half a cycle (10/8.3ms at 50/60 Hz) one would see with AC, and also probably less than the arcing due directly to interupting a high DC current (which is the main problem for DC switches).

Kind Regards, John
 
The back EMF from an inductive load can be absorbed by a contact suppressor fitted across the switch contacts. Most often a capacitor to absorb the energy with a resistor in series to limit the current when the contacts next close and short out the charged capacitor.

While the peak energy from the back EMF will discharge through the load the capacitor will retain a charge at the supply voltage.
 

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