scoobyis2cool
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| posted on 24/8/04 at 03:32 PM |
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Dash mounted switches and relays
I've just bought some dash mounted switches after killing the sierra column mounted ones. I'm using two dash mounted switches - headlight
and dip/main beam. Will these need relays or will they be OK?
These are the ones I've used (top of the page):
SVC website
It's not that I'm lazy, it's that I just don't care...
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silex
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| posted on 24/8/04 at 04:01 PM |
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You should be fine wiring direct. The page you pointed to had a spec of 15 Amps capacity at 24V. Because you are using halve the voltage you can
approx. double the current through the switch, equating to about 30 amps.
All things being equal - depending on your exact lighting setup, etc. you would not normally draw more than 20 amps (and thats allowing for some
pretty good bulbs).
To keep the switch safe, dont put a fuse in the feed wire to the switch any bigger than 25Amp.
You could test this on the bench if you wish before wiring the whole thing into your car.
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JoelP
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| posted on 24/8/04 at 06:40 PM |
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im not convinced that you can double the amps just cos you halved the volts. This would equate to power delivered, but you have to think about what
the switch is rated for. The 24v rating indicated how much insulation there is between the contacts. ie, 240v would arc between the contacts. The heat
developed in the contacts is related to amps of current, not watts of power, so i would limit the current to 15 amps anyway. But hey, if you are sure
im wrong, please explain cos i couldnt guarentee im right here.
However, headlight total draw is likely to be around 120w. or 10 amps, so its likely that the switch will manage. me, id use a relay anyway. If the
one switch is responsible for turning on both sides, dips and mains it could end up with (55x2, 60x2, 5x6) 260w, 20 amps. So its worth making sure!
Beware! Bourettes is binfectious.
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wilkingj
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| posted on 24/8/04 at 10:02 PM |
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JoelP... you are right.
The contacts on the switch are rated for 15Amps. Putting double the current through them will burn them out eventually or probably sooner. The
Voltage rating is also the insulation factor for safety, ie you can use it safely at 24Volts but 240 volts will
1: Arc or flash over the contacts,
or
2: flash up you arm when you operate the switch!
Would you want to see all your hard work / pride and ultimate joy go up in smoke cos you skimped on the proper switches / fuses. The fire hazard
from incorrect wiring is not worth the risk. Also your insurance investigator would soon spot the cause, and reduce your payout as you contributed
to the situation.
If you really want a "HOT" vehicle... just screw up the electrics! 
You know it makes sense 
Best Regards
Geoffw
1. The point of a journey is not to arrive.
2. Never take life seriously. Nobody gets out alive anyway.
Best Regards
Geoff
http://www.v8viento.co.uk
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silex
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| posted on 25/8/04 at 05:43 AM |
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But he's not going up in voltage, he is going down. What I am looking at is the wattage that the switch will handle
W=va So 24*15 = 12*30
Believe me when you first turn a bulb on, lets say a 70w, for that first split second when you turn on it could pull 300W or more. This is because
element is cold and bulbs are rated when hot.
This will cause a small arc anyway, even on a relay. Besides, a little bit of arcing like this is good for the switch - it keeps the contacs clean and
free from corrosion.
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mad4x4
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| posted on 25/8/04 at 08:29 AM |
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silex YOUR RIGHT ,
I would however, question you formula notation of W=va where it should have been P=VI
P=power in Watts
V= Volts
I= Current in Amps.
At 24v we know
Power = 15x24 = 360Watts
so for 12v
P=IV = I=P/V = 360/12 = 30Amps
That means that the switch theorectically can handle 30Amps at 12v - However, when an engine is running it produces 13v-15v (Charging the battery).
I would never run 30Amps @ 12v through this switch, as you would be over or on it design limit. Normally when you design something you build in a
safety factor of at least 25% so that would equate to 22.5Amps at 12v giving a 25% over current safety factor.
This safety factor would allow for the chages in voltage due to the Altenator charging the battery
Scot's do it better in Kilts.
MK INDY's Don't Self Centre Regardless of MK Setting !
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scoobyis2cool
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| posted on 25/8/04 at 09:54 AM |
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Wouldn't it work like this:
Power per bulb of say 70W
12V supply
P=IV, therefore current, I = 140/12 = 11.7A
This means that the switch needs to handle a current or 11.7A and a voltage of 12V, which apparently it would do with ease.
This may not be right but it's how I would have calculated it with my fairly limited (A level) knowledge of electrics.
Pete
It's not that I'm lazy, it's that I just don't care...
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mangogroove
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| posted on 25/8/04 at 10:16 AM |
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smoke pipes..........smoke leaks out thing stops working......must have run on smoke.!!!!!!!!!lol
You want it when!
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silex
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| posted on 25/8/04 at 11:28 AM |
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Well the notation does not reall matter, I put it into simple terms V=Volts, A=Amps, W=watts for the non technical, the formula is just the same.
I have been looking through test data here at work and although I cannot find that exact switch manufacturer, I have data from one of the worlds
leading supplier of that style of switch with the same claimed 15Amps at 24V rating in their test data.
There results are taken using lamp loads (Not the normal ohm loads but bulbs)
1. Test Voltage at 27.5V (includes alternator charge)
Amps - Life Expected
6 120,000 cycles
10 80,000 cycles
14 20,000 cycles
1. Test Voltage at 13.5V (includes alternator charge)
Amps - Life Expected
12 250,000 cycles
20 150,000 cycles
29 20,000 cycles
Now making a couple of assumptions,
a. The main beam is on a seperate switch to the driving lights
b. Dip Beam = 70W and Main beam = 100w (assuming a little upgade after the SVA)
Therefore,
Driving lights = 2 x 70Watt + 4 x 5Watt = 160w Total
so 160W/12V = 13.33Amps
Main Beam = 2 x 100Watt = 200w Total
so 200W/12V = 16.66Amps
So as the test data shows above, the switch should be good for about 30Amps, you should have no problem putting 13 and 16 Amps through the switches.
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Steve @ sVc
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| posted on 25/8/04 at 04:16 PM |
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scoobyis2cool
These switches are just fine for the lights. We have been selling them to builders and kit car companies for nearly 10 years now and have not (to my
knowledge) ever had a problem with them).
If you find you would like brighter lights, I use the H4 Xenon filled bulbs.
Osram claim an additional 50% more light on the road at the same current consumption as the normal halogen bulbs.
AND they are "E" marked so ok for the SVA & Mot.
Best wishes
Steve @ sVc
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scoobyis2cool
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| posted on 25/8/04 at 04:30 PM |
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Hi Steve,
I was planning on asking you this question when I came up and got the switches yesterday but you weren't there, isn't that always the
way?! 
Anyway, glad to hear you haven't had any trouble, i'll stick to the standard bulbs for now and maybe replace them at a later date. Just
rushing to get it on the road at the moment!
Cheers,
Pete
It's not that I'm lazy, it's that I just don't care...
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