Bachmann (Shay) LED bulb?

DGE-Railroad

DGE-Railroad

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The rear light on my Shay had stopped working. I've traced it to a failed LED and pulled it out.

I'm thinking to replace it with a standard warm white LED but does anyone know what voltage it should be running at? I just want to make sure I get the right spec...20200206_160246.jpg
 
DGE-Railroad said:
The rear light on my Shay had stopped working. I've traced it to a failed LED and pulled it out.

I'm thinking to replace it with a standard warm white LED but does anyone know what voltage it should be running at? I just want to make sure I get the right spec...View attachment 261448
Click to expand...
Have you tested it outside the loco to be certain it has failed? I would try 2 x aa to it to see if that gets anything. Then go up one aa at a time if you can to tempt it into working or not. If it works you will not only know the value but also if something else is amiss. Led’s generally have a good long life, well unless you put too high voltage or wire them the wrong way. No need to ask how I know this.
 
dunnyrail said:
Have you tested it outside the loco to be certain it has failed? I would try 2 x aa to it to see if that gets anything. Then go up one aa at a time if you can to tempt it into working or not. If it works you will not only know the value but also if something else is amiss. Led’s generally have a good long life, well unless you put too high voltage or wire them the wrong way. No need to ask how I know this.
Click to expand...

NO!

A LED ALWAYS NEEDS A SERIES RESISTOR!

By all means test the LED with a battery, but use a resistor as well.. If you limit the current with a resistor, you should survive connecting a LED the wrong way round.

For 'belt and braces' use a 1k resistor (any voltage up-to around 12V).

Almost any warm-white LED should be a straight swap, but i have found the LED's Bachmann use give a very yellow light.
 
Thanks both. I'm pretty positive it's a goner (which I was dubious about...) I out a multimeter across it in both directions, set on the diode position but it reads nada!

I can't help wondering if the legs shorted at some point in the past as the installer certainly didn't protect either of the legs..

It's driven by the Bachmann PCB but I'll pop a resistered 3.3v led in and see how I go!
 
A volt meter on the terminal to the LED, and run the loco, this should give a good indication, however looking at the photo I would guess somebody has replaced this, and the resistor is down the line, so nominally 3 volts.
 
JimmyB said:
A volt meter on the terminal to the LED, and run the loco, this should give a good indication, however looking at the photo I would guess somebody has replaced this, and the resistor is down the line, so nominally 3 volts.
Click to expand...
Thanks Jimmy. I had wondered about doing that. I reckoned it was likely to be tricky (it at least fun to watch :D ) on a moving loco. I'm waiting on a rolling road to arrive, as I could do with one anyway. This could be a good candidate for that..
I may curb my usual desire to get everything done and just take a quick measurement first as you say, when the road turns up
 
If you do not have a rolling road..

Can you put a block of wood under either end of the loco, and arrange power somehow?

PhilP.
 
PhilP said:
If you do not have a rolling road..

Can you put a block of wood under either end of the loco, and arrange power somehow?
PhilP.
Click to expand...

Or turn it upside down - most of my testing is done like that :oops:
 
PhilP said:
NO!

A LED ALWAYS NEEDS A SERIES RESISTOR!

By all means test the LED with a battery, but use a resistor as well.. If you limit the current with a resistor, you should survive connecting a LED the wrong way round.

For 'belt and braces' use a 1k resistor (any voltage up-to around 12V).

Almost any warm-white LED should be a straight swap, but i have found the LED's Bachmann use give a very yellow light.
Click to expand...
A 1.2k resistor is 20 milliamps at 24 volts, so your 1k is good for more than 12v...

Please, everyone, help people that ask "what voltage" for a LED, reinforce the LEDs need the current limited, not the voltage.

There are many sites that talk about LEDs.. perhaps I could suggest mine:

LED Lighting Basics

Greg Elmassian web site on large scale trains and garden railroads, cigars, and computers
elmassian.com elmassian.com

Also links to LED resistor calculators.

And in this specific case, there is most likely a resistor in heat shrink near the LED, but checking the voltage BEFORE connecting the LED won't tell you anything, I can explain for the curious.

Greg
 
Greg, you are of course correct, and having read the page (which could be classed as interesting) some none technical types would just “glaze over”. My solution is based on the premise that when the locomotive is running, if the voltage is around 3v then the correct resistor is already fitted, however if the voltage goes up there is no resistor. Just trying to keep it simple.
 
DGE-Railroad said:
The rear light on my Shay had stopped working. I've traced it to a failed LED and pulled it out.

I'm thinking to replace it with a standard warm white LED but does anyone know what voltage it should be running at? I just want to make sure I get the right spec...View attachment 261448
Click to expand...

If it is a bog standard Bachmann 55ton 3 truck the limiting resistor is on the board in the coal bunker fed to the tender then to the LED from the connection board by a yellow and a grey wire.
Link to parts/wiring diagram will show connection. https://www.bachmanntrains.com/home-usa/dwg/dwgs/82494.pdf

In my case that board is not fitted and the LED is connected directly across the motor terminals to give a form of directional lighting with a 560 ohm in series, battery voltage is 14.8V.

Edit
Remembered that the Yellow and Grey wires transition to Blue and Black on mine halfway along so the connections at the LEDE are Blue and Black the polarity is on the plug in board in the tender.
 
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JimmyB said:
Greg, you are of course correct, and having read the page (which could be classed as interesting) some none technical types would just “glaze over”. My solution is based on the premise that when the locomotive is running, if the voltage is around 3v then the correct resistor is already fitted, however if the voltage goes up there is no resistor. Just trying to keep it simple.
Click to expand...
Jimmy, the problem is that the LED has failed, thus an open circuit.

So as I said, trying the measure the voltage tells you nothing.

The reason is that the only current flowing in the circuit would be through the voltmeter, which is very small... thus no matter whether there is a resistor or not, there will essentially be NO voltage drop across the resistor V=IR where I is almost zero, then V (voltage drop across the resistor) is zero, thus the voltage reading is the SAME with or without a resistor.

There is no keeping it simple with a blown LED and an open circuit.

Greg
 
Greg Elmassian said:
A 1.2k resistor is 20 milliamps at 24 volts, so your 1k is good for more than 12v...
Click to expand...

Yes, but the OP did not state whether they were analogue, DCC, whatever..
If they do not have a 1k resistor, they would probably use 'whatever is nearest' that they do have. They may also use their ?controller? to supply the test voltage.
I maintain they will most-likely get something out of the LED, and it would survive reverse polarity, using a 1k resistor.

Trying to keep it simple for the OP.. And I did say limit the current. :)

Greg Elmassian said:
Also links to LED resistor calculators.
Click to expand...

Current limiting Resistor calculator for leds

ledcalc.com ledcalc.com

Measuring the voltage (no load) will not help the uninitiated deduce whether there is already a series resistor in the circuit, but would confirm the polarity of the supply.

In my experience on the Bachmann lighting, it does seem a little 'fragile'. - I wonder if they are over-driving the LED's? :wondering:
 
Greg Elmassian said:
Jimmy, the problem is that the LED has failed, thus an open circuit.

So as I said, trying the measure the voltage tells you nothing.

The reason is that the only current flowing in the circuit would be through the voltmeter, which is very small... thus no matter whether there is a resistor or not, there will essentially be NO voltage drop across the resistor V=IR where I is almost zero, then V (voltage drop across the resistor) is zero, thus the voltage reading is the SAME with or without a resistor.

There is no keeping it simple with a blown LED and an open circuit.

Greg
Click to expand...

Greg, I doubted what you said, having only a light knowledge of electronics, so, 9 volt battery, 1k resistor and multimeter in hand, voltage across the terminals with or without the resistor was just about identical, so again you are correct!
 
Ok as per post #4 the LED is driven by the board so the limiting resistor is already installed.
To answer the original question the voltage should be in the vicinity of 3V (white LED) if it is a direct replacement of the existing LED then the current limiting resistor on the board should do its thing and let the LED light.
Just be careful of wiring it in to avoid shorts.
 
Jimmy, LEDs are indeed "tricky" since they really only care about current (once you have hit the minimum conduction threshold voltage).

The sneaky thing is they are still a diode, just one that produces light, so measuring them is as tricky as trying to measure the resistance of a battery, and as anyone who has accidentally tried this, good for at least popping a fuse in your meter if not destroying it.

So the fallback is good old OHMS law, and realizing that the voltage "dropped" by a resistor in the circuit depends wholly on the current going through it.

This is along the lines of people trying to find a poorly conducting joiner, and measuring voltage with no load on the rails... even though there is resistance in a joiner, with no real load (several amps) on the rails, the voltage "lost" at the joiner is minimal because the current is minimal.

I have a big box of resistors that gives me an 8 amp load at 24 volts, and I place that at the "far end" of a powered section, and you can easily find the bad joiner by putting a voltmeter on either side of a joiner.

Interesting stuff, makes sense when you dig into it.

Greg
 
I have a little 18v can motor for testing poor track joints.

I set the controller fairly high, and place the two motor leads on the track - if the motor kicks out of my hand, then the track power is OK :emo::emo::emo:

I told you I was simple :)
 
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