Subject:
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Re: Need Electrical Help
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Newsgroups:
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lugnet.trains
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Date:
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Fri, 24 Oct 2008 02:34:23 GMT
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Viewed:
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12005 times
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> If the voltage is going to vary, don't use a resistor as current source,
> consider a current regulator instead. That way the LED stays brightly lit
> over a
> wide voltage range without getting burned out at max voltage. This works
> for
> one or more LEDs in seires. The simplest and cheapest is shown here
> <http://www.elecfree.com/electronic/fet-current-source-by-bf256/> The LEDs
> light
> up at full brightness when the voltage exceeds the LED voltage by a small
> amount
> and stays constant up to 18v or more. I use this for the headlights in my
> trains.
>
> You need a bit of overhead voltage, higher then the LED rated voltage, to
> pass
> through the FET. If the track voltage is going to vary a lot, build your
> sting
> of LEDs with this in mind. If you run 2 red LEDs(2.1v) in series, they
> won't
> light up much at less then 4.2v. A single white LED(3v) lights up to full
> brightness on the second 9v controller notch.
>
> I bought my BF256c FETs at Jameco online. An eazy source for colored and
> white
> LEDs is the LED Christmas strings. I pull the LEDs and toss the wire
> harness.
> Most are the 5mm type(~ sixe of a technic hole). But you can also find the
> 3mm
> (size of a hollow stud).
>
> Mathew Clayson
Great idea. I hadn't thought about whether Tom was going to use a variable
speed control. I just assumed batteries for some reason. Have you had any
brightness issues with the seemingly low 15mA output for multiple LEDs or
are you hooking 1 to 2 LEDs max to this FET? To the human eye it would be
hard to distinguish 7.5mA / 15mA unless they were beside each other.
Also, with a few well placed diodes in circuit, you could have headlights
while going forward and backup lights when going backwards too. :) BTW,
how's your BF256c handle reverse voltage? I assume fine.
-Rob
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Message has 2 Replies:
 | | Re: Need Electrical Help
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| (...) Hey Rob, It would be safer to place a regular doide in series with this if the voltage is going to be reversed some of the time. But the circuit shown in the link handles reverse voltage up to 9v fine. The LED acts as a reverse protection (...) (16 years ago, 24-Oct-08, to lugnet.trains, FTX)
| | | Re: Need Electrical Help
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| (...) As a crazy idea, use a MAX882/3/4 configured to output the required voltage. It is a step-up regulator that will work with as little as 1.5 volts. There are also PWM solutions. (16 years ago, 26-Oct-08, to lugnet.trains)
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Message is in Reply To:
| | Re: Need Electrical Help
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| (...) If the voltage is going to vary, don't use a resistor as current source, consider a current regulator instead. That way the LED stays brightly lit over a wide voltage range without getting burned out at max voltage. This works for one or more (...) (16 years ago, 24-Oct-08, to lugnet.trains)
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