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Discussion Starter #1
I was thinking that I want to replace an old 'grain of wheat' incandescent bulb in a headlight on a Locomotive.

Since my train is DC only, I thought about how to make the light bright at any train speed.

My solution is to use a Buck Converter.
If you do not know what a buck converter is they are simple voltage converters that generally take a wide input voltage range say 24V to 4V and output a stable Voltage say 3V in this example. There are even Buck converters increase voltage.

Of course beecause the train goes in Forward and reverse by reversing the voltage we would need a Bridge Rectifier first so that the buck converter always gets properly polarized power and does not burn up.

So now for the math...
My train starts to move at about 3V on the rails, although slowly. We subtract from that the forward voltage loss of the Bridge rectifier 0.7V and that leaves us with about 2.3V. Now most Buck converters that convert down also require a higher input voltage than output voltage. , so let us assume we would have a similar loss of 0.7V across the buck Converter. THat leaves us with 1.6V

I wonder how bright a 1.6 V LED could be ? I see 1.5V LEDs on ebay . I also see that 3V SMD LEDs with leads are targeted to the train hobby market.

Here is a 1.5V LED
https://www.ebay.com/itm/COOL-WHITE...491662?hash=item33d89cdfce:g:f0IAAOSwEyhZ4RSu

This little guy will run on 0.9V and up to 1.9V
https://www.ebay.com/itm/world-smal...a=0&pg=2047675&_trksid=p2047675.c100008.m2219

With this last one I think it would be possible to have the lamp iluminate to full brightness even when the train was barely moving. Getting 1.6V to the bulb even when the train is at a crawl.

Luckily for me this steam Locomotive has a red reflective interior where the bulb fits in, and the red really shows up (maybe too much with an incandescent bulb) so I am not concerned about putting a daylight LED in there as it will still pick up the red.

HAs anyone tried anything like this?
 

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you are out on your conversion factors ... the bridge will drop 1.5 volt rather than 0.7 volt, and white LEDs are 3 to 3.3 volt rated .. you will need about 6 volts to start with to get full LED brightness [without the joule thief circuitry]
 

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This is the hard part of the problem, "....My train starts to move at about 3V on the rails.."
In the good old days, we built a diode bridge INSIDE one lead to the motor, and used the voltage drop across the bridge to provide constant voltage to the GOW bulb. Also, the good old days, motors needed more than 3 VDC to move.

Just put an LED in with voltage dropping resistor and skip the extensive elecrical circuit theory.

Here's my science project
 

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If you do not know what a buck converter is they are simple voltage converters that generally take a wide input voltage range say 24V to 4V and output a stable Voltage say 3V in this example. There are even Buck converters increase voltage.
Those are called boost converters.
 

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Discussion Starter #6
This is the hard part of the problem, "....My train starts to move at about 3V on the rails.."
In the good old days, we built a diode bridge INSIDE one lead to the motor, and used the voltage drop across the bridge to provide constant voltage to the GOW bulb. Also, the good old days, motors needed more than 3 VDC to move.

Just put an LED in with voltage dropping resistor and skip the extensive elecrical circuit theory.

Here's my science project
https://www.youtube.com/watch?v=cW3Z8v1p46A
Nice video but you do not show how/if bulb dims when train slows. That is the whole point of my post
 

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you are out on your conversion factors ... the bridge will drop 1.5 volt rather than 0.7 volt, and white LEDs are 3 to 3.3 volt rated .. you will need about 6 volts to start with to get full LED brightness [without the joule thief circuitry]
You only have one diode conducting at a time in a bridge rectifier, so I believe you are incorrect , not I.
 

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I’m not sure about all that complicated math and joules amps volt conversion stuff but they offer multi volt leds that work on a range of voltages this example doesn’t go down to 3 volts but somebody probably makes them.

I know many will work in a wide voltage range. They wil also get brighter and dimmer with voltage that is the pont of this original post that seems to be IGNORED
 

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I’m not sure about all that complicated math and joules amps volt conversion stuff but they offer multi volt leds that work on a range of voltages this example doesn’t go down to 3 volts but somebody probably makes them.

I know many will work in a wide voltage range. They wil also get brighter and dimmer with voltage that is the pont of this original post that seems to be IGNORED
The original post says you want to replace an old school incandescent bulb with an led so it will be bright at any speed. It does not say the same brightness at any speed! Hard to ignore something that’s not there!
 

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You only have one diode conducting at a time in a bridge rectifier, so I believe you are incorrect , not I.
Um, no. 2 diodes are used at a time in a bridge rectifier.

No it is a "buck converter"
https://www.google.com/search?q=buc...46j0j7&client=ubuntu&sourceid=chrome&ie=UTF-8

It is in no way "boost"ing the voltage
I don't really understand what you are saying in the quote below, but if a device is increasing the voltage, then it's a boost.

There are even Buck converters increase voltage.
 

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it's desirable to have a steady voltage supply given a wide range of track voltage.

it's simple enough to use a low-drop voltage regulator to limit the voltage when track voltage exceeds some amount. I don't believe the complexity of a buck-converter is needed. It's conceivable that the no series resistor is needed if the output of the regulator matches the LED voltage.

The problem is what is the minimum track voltage that the circuit will work at. Certainly greater that the LED voltage

You only have one diode conducting at a time in a bridge rectifier, so I believe you are incorrect , not I.
there are 4 diodes in a bridge rectifier, 2 conduct at a time
 

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one thing you neglected to mention ... a 'joule thief' LED is about 100 times the cost of a regular led and current limiting resistor .. a factor for many users ..
 
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