posted
I have an old car and would like to have dome lights that stay on for a few seconds after the door is shut, like in most new cars.
I was thought about it...Can I just use a capacitor? It would charge while the door is open, and when the pin contact opens (door closes) the small cap, would hold the voltage above 12 for a few (10) seconds.
Would this work? What value cap should I start with?
posted
yes, it should work...the hard part will be getting the time constant to be high enough where it won't shut off right away. the time constant is engineering term for the decay rate of the voltage in the capacitor. for every time constant that passes the voltage in the cap is reduced by 63.2%.
anyway...the time constant, t, is equal to 1/RC
so to get the time constant large, the resistance value must be small. i would need to know the capacitance value of the capacitor to figure out what size resistor might work. but, to get the voltage to stay above 12V for that long would prob require a fairly large cap...and to keep it above 12V would mean it would have to be charged to over 12V before the door was closed...to do this you'd need a voltage source before it to step up the voltage high enough where it wouldn't drop below 12 very fast.
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posted
I don't think a cap would work unless it was pretty darned big.
Maybe easier to do would be a simple timed relay? You should be able to get relays that are "quick on, timed off", but the capacitor would give you a nice fade effect
ShadowStar
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Cap, resistor and transistor for the timing... fairly obvious how I hope.. then use the transistor to run a relay with the relevant diode on it and a largish cap on the relays outputs as well as your dome light.
Sorted.
Do you need a diagram?
Colin
-------------------- *hmm need to think of a new sig*
posted
dude, Just do it the easy way. Use one cap and one diode. The kind of cap you want is called a super cap. it has a higher internal resistance so it discharges slowly, but for your app that doesnt matter. Your not trying to filter anything. you can get half a farad or upto like 1.5 farads and they are really small! maybe the diameter of a nickle and half and inch high for the 0.5farad. that will keep your light on more than long enough. http://www.evanscap.com/Capattery.htmlhttp://www.maxwell-technologies.com/ultracapacitors.htm
use a 16V! 11V is not enough. there is a 50% rule.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Dank, I'm merely posting the exact setup that manufacturers use. It works well so I assume it works out cheapest as well. Very easy to do as well. I'd say it gives slightly better results than a super cap would.
Colin
-------------------- *hmm need to think of a new sig*
posted
LOL, sorry about that. You can get them for cheap. I cant remember the mfg though. Ill see what we use at work.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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