Date: Mon, 31 Jul 2017 15:24:52 -0700
Reply-To: Alistair Bell <albell@SHAW.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Alistair Bell <albell@SHAW.CA>
Subject: Re: Time Delay LED Lighting Circuit
In-Reply-To: <8E9736C0-E715-4EAB-AA6F-951B54F2C298@NBNet.nb.ca>
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Yeah I know Roy. I use pwm dimmers on the van. But what I guess I was trying to say is the voltage reduction dimming from the headlight switch rheostat didn't play that well with my home made dashled .
Back to Neil's delayed off circuit. As it is what do you think will happen? Will the cap output voltage drop ?
What I'd like to see is a circuit that delays off, full brightness held for a set time then a slow dim. It's so very elegant :-)
Cheers
Alistair
> On Jul 31, 2017, at 3:05 PM, Roy Nicholl <RNicholl@NBNet.nb.ca> wrote:
>
> Apples and Pears ;-)
>
> Most incandescent dimming systems use phase control (AC world) or constant current reduction (CCR) (DC world).
>
> LEDs can also be dimmed via CCR (constant current reduction) which is the simpler method, but tends to have a minimum threshold around 10% below which output is unreliable.
>
> In AC applications (buildings) where drivers are being used Pulse-width modulation (PWM) - effectively switching the LED on/off thousands of times per second - produces a much better dimming effect (down to 1% brightness).
>
>
>> On 31-Jul-2017, at 18:40, Alistair Bell <albell@SHAW.CA> wrote:
>>
>> Haven't found your basic led to dim very well david. Back in the day I made up led and resistor combo and fitted into the bulb holders for the dash lights. It dimmed to a certain point ( via headlight switch dimmer) then off. Not the same dimming as the incandescent.
>>
>> Alistair
>>
>>> On Jul 31, 2017, at 1:27 PM, David Beierl <dbeierl@ATTGLOBAL.NET> wrote:
>>>
>>> The resistor is to limit current through the LED which is a "constant
>>> voltage" device like any diode.
>>>
>>> The LED will start dimming the instant power is removed. I would expect it
>>> to dim rapidly at first and then more slowly. It might retain a minimal
>>> glow for some time -- LEDs will visibly emit with microamps of current
>>> running through them.
>>>
>>> LEDs dim perfectly well. If you put 2-watt LEDs in your instrument panel
>>> they would dim similarly to the 2-watt incandescent lamps they replace --
>>> at high brightness. At low brightness they would still dim much more
>>> slowly than the incandescents because the latter have to be red hot before
>>> they begin to emit, which takes considerable current. But if you replace
>>> your two watt panel lamps with 0.2 watt LEDs, the dimming rheostat meant to
>>> work with a total of ~15 watts will have little effect on the 1.5 watt load.
>>>
>>> The formula for energy in watt-seconds (aka joules) stored in a capacitor
>>> is Estored = Cfarads x Evolts^2 / 2. So your 4,000 uF cap charged to
>>> twelve volts would in theory contain enough energy to run a two watt
>>> festoon bulb for about three tenths of a second. In practice it would run
>>> longer than that because it would only be using the full two watts at the
>>> instant power was disconnected.
>>>
>>> The way the commercial circuits work is by using a timing circuit (no doubt
>>> containing a timing capacitor) to operate a transistor that switches
>>> battery current on and off.
>>>
>>> Yrs,
>>> d
>>>
>>>
>>>
>>>> On Mon, Jul 31, 2017 at 3:45 PM, Neil N <musomuso@gmail.com> wrote:
>>>>
>>>> Yes. I too think the resistor is for voltage drop. Parts list
>>>> specifies "an LED" so.....
>>>>
>>>> My sense is that voltage stored in the cap would naturally taper off
>>>> but kind of moot if I used an LED(s).
>>>>
>>>>
>
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