Dusty
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| posted on 27/3/08 at 02:15 AM |
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Senior moment
I need a shift light. My Omex ECU will drive one but a max of 1 amp. So 12 watts, right? Watts over volts = amps. Checking in my bag of bulbs for one
marked 12w I can't find one but 21w has a resistance of about 1.4ohms and 5w about 3.7ohms. So 12w is between those two at say 2ohms. But volts
over amps = ohms so I was looking for a 12 ohm resistance bulb. Where are the missing ohms or do they magically appear as the filament heats up?
It's more than 40 years since I did O level Physics and I wasn't much good at it then.
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matt_claydon
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| posted on 27/3/08 at 02:30 AM |
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The resistance of a bulb increases dramatically as soon as the filament is hot.
As an aside, this is why they generally always blow when you turn them on - the overall resistance is much lower and therefore the current much
higher. This means that any part of the filament that is slightly thinner than the rest (this happens naturally over time) gets heated beyond melting
point and pop, with a bright flash the bulb is dead.
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Agriv8
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| posted on 27/3/08 at 07:52 AM |
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You can get some VERY bright LED's these days, worth a look at maplin.
Most modern one are happy running at 12V and dont take much Juice ( ie run 2 or 3 in a line )
Regards
Agriv8
Taller than your average Guy !
Management is like a tree of monkeys. - Those at the top look down and see a tree full of smiling faces. BUT Those at the bottom look up and see a
tree full of a*seholes .............
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02GF74
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| posted on 27/3/08 at 08:09 AM |
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^^^^ wot he says. car electrics run at 14 V.
three 5 mm LEDs in series would be blinding 
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MikeRJ
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| posted on 27/3/08 at 08:49 AM |
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Use an LED (with a suitable current limiting resistor), that's almost certainly what the OMEX was designed for. Ultrabright LED's take
little current (<30mA usually) and are usualy fairly directional so you don't need a reflector etc. They also turn on (and off) much more
quickly than an incandescent lamp, which tends to draw your attention more.
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Mr Whippy
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| posted on 27/3/08 at 08:52 AM |
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quote:
Originally posted by 02GF74
^^^^ wot he says. car electrics run at 14 V.
three 5 mm LEDs in series would be blinding
Do take into account how bright the light is at night as it is very easy to install something that really is blinding
Fame is when your old car is plastered all over the internet
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jlparsons
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| posted on 27/3/08 at 09:09 AM |
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quote:
Originally posted by Mr Whippy
quote:
Originally posted by 02GF74
^^^^ wot he says. car electrics run at 14 V.
three 5 mm LEDs in series would be blinding
Do take into account how bright the light is at night as it is very easy to install something that really is blinding
Or be clever and wire in two resistors with one switched in when the lights are on, so it's ultra-bright in the day and you can see it even if
the sun's in your face, then at night it's dimmed so it doesn't blind you. Proper buck-rogers stuff that. 
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02GF74
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| posted on 27/3/08 at 09:32 AM |
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quote:
Originally posted by jlparsons
Or be clever and wire in two resistors with one switched in when the lights are on, so it's ultra-bright in the day and you can see it even if
the sun's in your face, then at night it's dimmed so it doesn't blind you.
yeo - but wiring becomes non trivial.
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jlparsons
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| posted on 27/3/08 at 11:00 AM |
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True, but another 5 minutes work, well worth it i'd say or you've got to choose between dull in the day or blinding at night. Just run a
relay off the light circuit from the fusebox or the light switch that bypasses the second resistor when activated. Job done.
Assuming all my dash lights will be LED I'm going to run the earth for the whole lot through the same arrangement so every button and dial light
dims at night.
Any resemblance to real persons, living or dead is purely coincidental. Some assembly required. Batteries not included. Contents may settle during
shipment. Use only as directed. No other warranty expressed or implied. Do not use while operating a motor vehicle or heavy equipment. Subject to
approval, terms and conditions apply. Apply only to affected area. For recreational use only. All models over 18 years of age. No user-serviceable
parts inside. Subject to change. As seen on TV. One size fits all. May contain nuts. Slippery when wet. For office use only. Edited for television.
Keep cool; process promptly.
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Dusty
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| posted on 27/3/08 at 11:08 AM |
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There is a 5mm led in circuit already and I have never seen it light. This may be due to wiring it wrong or some error in settings so it just occurred
to me last night to get the manual out and see if I could use proper electrics and proper light bulbs to get it to work.
I'm getting to the age where gizmos and gadgets are becoming difficult to operate. Bit like my Dad who is regularly defeated by his TV and
can't find any channels because he has inadvertently pressed one of 40 odd buttons on the remote which has switched it to svideo or something
and it's not listening to the digibox or the standard aerial so his telly ceases to work. Why don't big companies make mobile
phones/TV's/DVD/CD/gizmos for the elderly or easily confused?
ps thanks for the answers. Forgot to say that first time round.
[Edited on 27/3/08 by Dusty]
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