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 20:45:10 GMT
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Viewed:
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12158 times
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In lugnet.trains, Rob Hendrix wrote:
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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
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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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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 diode up to
a point. More LEDs in sereies will only improve this. For a lit light in bothe
directions, you’ll need a bridge rectifier. I build my own out of 4 Schottky
diodes, as these only drop ~.3v as opposed to the .7v for a standard silicon
diode. I can then wire wire up directional lights to all or part of the bridge
and input wires. This works great on my RDC which has running lights, red tail
lights at each end and directional head lights. I also add a capacitor in
parralel with each LED string to reduce flicker.
15ma seems to be fine, Most LEDs are rated to 20ma. But the gain in briteness
from 15ma to 20ma is almost unnoticable. You need about .8v more then the LED
voltage to reach full brightness with the FET, but it remains constant after
that.
Mathew
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Message has 1 Reply:
 | | Re: Need Electrical Help
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| I appreciate all your suggestions. Is there a website you can suggest that has more information on this stuff? Also, beside Jameco, what are some othe good places to buy parts? Thanks, Tom (16 years ago, 25-Oct-08, to lugnet.trains, FTX)
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Message is in Reply To:
| | Re: Need Electrical Help
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| (...) 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 (...) (16 years ago, 24-Oct-08, to lugnet.trains)
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