Watts to amps eqn.

[Spike_Sierra]

just wondering whats the equation for seeing how many amps a certain light draws, saw 100w light running on 12v(do you say 12 or 13.4 or whatever its charging on)..

any help?

need to see what size cabling i need, and fuses. ive got some wire thats got 630 on the side of it, pretty thick stuff, and all lights together ive got 600w so im guessing this should be ok then?

[festy]

P=VI (watts = volts x amps)
So, amps = watts / volts (I=P/V), and volts = watts / amps (V=P/I).

to work out the current a 100w light would draw at 12v,
I=P/V, so I=100/12, or 8.33 amps.


600w at 13.8v is almost 45 amps, hope you're got a big alternator

[muzza_fattire]

Power (Watts) = Voltage (Volts) x Current (Amps) or;
Power = Current^2 x Resistance or;
Power = Voltage^2 / Resitance


So, 100W at 12V will draw 8.3 Amps
At 13.4V it will draw 7.46 Amps.

[Spike_Sierra]

ahh i thought thats what it was, i have been using that equation for ages, just thought i better make sure..

so it draws less amps when running at a higher voltage eg draws more when cars off..

standard zook alternators are 35 amp
luckily i have a 70amper
as long as im still over 12v when everything is on its all good right?

[chimpboy]

so it draws less amps when running at a higher voltage eg draws more when cars off..

Not really, you'll find the lights get dimmer when the alternator's not pushing the voltage up. In other words, a 100W lamp is only a 100W lamp at full voltage.

The lamps have fixed resistance, which means in effect that the current drops when the voltage drops (because V=IR). It also means that power output is voltage squared divided by resistance, ie P=(V^2)/R

Good formulas here:



It is because power = voltage squared divided by (fixed) resistance that inadequate cable makes a big difference to light output; every voltage drop is multiplied by itself when it comes to the effect on light power. For example if your lights are 100W each at 13.8 volts, they will only be about 75W at 12 volts.

Jason

[murcod]

The lamps have fixed resistance,

Actually that's not quite right- the resistance of a lamp filament varies from cold (ie. no voltage applied) compared to hot (turned on).

[murcod]

See here for an in depth explanation: http://www.allaboutcircuits.com/vol_1/chpt_2/5.html

[DamTriton]

The lamps have fixed resistance,

Actually that's not quite right- the resistance of a lamp filament varies from cold (ie. no voltage applied) compared to hot (turned on).

[electronic trades hat on]

Whilst this is correct, for the purposes of the example, and near enough in real life, the resistance is stable once comforably above "yellow light".

[electronic trades hat off]

[chimpboy]

The lamps have fixed resistance,

Actually that's not quite right- the resistance of a lamp filament varies from cold (ie. no voltage applied) compared to hot (turned on).

Hrmph!

[Spike_Sierra]

anyone know if i could get say a 50amp fuse somewhere, highest i could find is 30..
where should i look..

[murcod]

The lamps have fixed resistance,

Actually that's not quite right- the resistance of a lamp filament varies from cold (ie. no voltage applied) compared to hot (turned on).

[electronic trades hat on]

Whilst this is correct, for the purposes of the example, and near enough in real life, the resistance is stable once comforably above "yellow light".

[electronic trades hat off]

Actually if you read that blurb in the link the resistance does vary depending on the voltage applied . It's not just a matter of it being a linear relationship once the lamp is lit.

Guess I went to a different Electronics school!

[r0ck_m0nkey]

anyone know if i could get say a 50amp fuse somewhere, highest i could find is 30..
where should i look..

You can get higher amps in both glass and blade, or you could look at using a circuit breaker instead. Any Auto Shop should have them, at 50A the fuses start costing a bit more and are a lot bigger in size.

Although i would wire the driving lights in pairs off their own circuit and fuses, that way you can have more control over how many you have running and if you blow a fuse it will only take one pair out and not all. Would cut down on cable size and fuse sizes aswell.

[murcod]

www.jaycar.com.au
www.whitworths.com.au
www.narva.com.au
www.hella.com.au

Browse the above links and you'll find lots of nice fuses, fuse holders etc. Car stereo places are good sources of high current fuses too.

I'd recommend the same as Rock Monkey- run separate fuses/ wires/ relays for each pair. Circuit breakers can be good, but they won't provide the same protection a fuse will (they do not trip straight away once the rated current is reached and can supply way over their rating for a period of time.)

[Spike_Sierra]

what i have is 4 rooflights a 2 spotties on bar, paired up, on 3 relays and 3 switches, but only one power wire going to all of them. will see how we go
thanks again guys

[Pinball]

www.jaycar.com.au
www.whitworths.com.au
www.narva.com.au
www.hella.com.au

Browse the above links and you'll find lots of nice fuses, fuse holders etc. Car stereo places are good sources of high current fuses too.

I'd recommend the same as Rock Monkey- run separate fuses/ wires/ relays for each pair. Circuit breakers can be good, but they won't provide the same protection a fuse will (they do not trip straight away once the rated current is reached and can supply way over their rating for a period of time.)

C/B less reliable and responsive than fuses?... must be pretty sad sorta breakers.. sounds more like thermal cutouts than real breakers... why do you think the electrical industry has taken to installing c/b in preference to fuses, excepting really high fault current containment points?

Spock