Author Topic: LED resistor value?  (Read 1290 times)

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peteski

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Re: LED resistor value?
« Reply #15 on: February 09, 2023, 08:08:37 PM »
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I'll try to whip something out Lee.  If it helps, 1k 5% resistor will have brown, black, red, and gold bands.  1.5k 5% will have brown, green, red, and gold bands.

I remember resistor color codes since I took electronics courses in the trade high school, when I learned a silly mnemonic: Bad Boys Rape Our Young Girls, But Violet Gives Willingly
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wm3798

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Re: LED resistor value?
« Reply #16 on: February 10, 2023, 12:05:39 AM »
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Odds are that will get you on some sort of list...

Did some tests with the 9v battery.  Looks like 2.2k will be a good candidate for headlights and some structure applications.
1k is pretty bright, but could be useful for street lighting and interiors for industrial buildings
4.7k gives a nice soft glow, I think that's what David used for the mood lighting in Phil's Bayou Another.
That's all I tested tonight.

Curious what the application might be for a 1million Ohm resistor...  Would that be for running an LED off of the 220v line my dryer runs on?
Or could I simulate the glowing tip of Raymond Burr's cigar in "Rear Window"?

Lee
Rockin' It Old School

Lee Weldon www.wmrywesternlines.net

mmagliaro

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Re: LED resistor value?
« Reply #17 on: February 10, 2023, 04:52:09 AM »
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Exactly.  I totally spaced out in my post and failed to explain that. :facepalm:
Actually those k, M, G numeric prefixed are the same for any units.  Most people are familiar with computer memory or disk space sizes.  Kilo (k), mega, (M), or giga (G) are multipliers most of us are familiar with.  Kilogram is thousand grams, or thousand ohms, or thousand bytes, and so on.
Mega byte is million bytes, mega ohm is million ohms.   LOL, actually most aluminum electrolytic caps have rather broad tolerances (like -10% +50%). Tantalums have tighter tolerances.
Well, sometimes.  I've only been using ones rated at 20% or better lately.  But what I was really referring to is the way
no-name electrolytics from China are sometimes complete fakes that aren't even the right value or voltage rating.

oakcreekco

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Re: LED resistor value?
« Reply #18 on: February 10, 2023, 12:00:40 PM »
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Checked out two Katos last night.

One a Dash9, early run had a surface mount resistor with a "271" mark.

The other was a SD45 latest run, with a "561" mark.

Not sure if these marks are Kato part numbers or resistor values??

The Dash 9 has brighter lights, but the light comes on as soon as the throttle is cracked, and before the loco moves.

The SD45 lights aren't as bright, and the engine has to be moving at about 30smph to be constantly "on". Any speeds lower, the lights will flicker and going slow enough go out completely.

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Any input appreciated, but this "when does it light up" makes me scratch my head.
A "western modeler" that also runs NS.

peteski

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Re: LED resistor value?
« Reply #19 on: February 10, 2023, 01:07:50 PM »
+2
Checked out two Katos last night.

One a Dash9, early run had a surface mount resistor with a "271" mark.

The other was a SD45 latest run, with a "561" mark.

Not sure if these marks are Kato part numbers or resistor values??

The Dash 9 has brighter lights, but the light comes on as soon as the throttle is cracked, and before the loco moves.

The SD45 lights aren't as bright, and the engine has to be moving at about 30smph to be constantly "on". Any speeds lower, the lights will flicker and going slow enough go out completely.

Any input appreciated, but this "when does it light up" makes me scratch my head.

First of all, yes those numbers on the SMD resistor are the value of the resistance.  They are equivalent to the color codes on larger leaded resistors.
Easy way to figure those out is to use online calculator:
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-smd-resistor-code
Here is a more techincal explanation (most popular are the 3- and 4- digit codes).
https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-smd-code/

271 is 270 ohms and 561 is 560 ohms.
Next question is if both models have white LEDs, or does one model have white LEDs, while the other one has yellow LEDs? Also does each model have same type of LEDs (like 3mm leaded package or small surface mount LEDs).  That all makes a difference in brightness.

White LEDs need around 3V to light up while Yellow will only need around 2V. White LEDs are also usually brighter (more efficient) than yellow LEDs

Of course the lower value resistor will also allow higher current to pass through the LED, so it will glow brightly.

I don't like the 270 ohm resistor Kato uses for their LEDs. With a 270 ohm resistor, at full throttle (12V) it will pass 37mA through a yellow LED and 33mA through white LED.  The 570 ohm resistor is a bit more conservative as it will pass 18mA through a yellow LED and 16mA through white LED.
Of course at lower throttle settings (lower voltage) the current will be lower.

The currents with the 270 ohm circuit at full throttle are a bit over the typical maximum current rating for small LEDs Kato uses.  I guess they assume that most modelers will not run those models at full throttle (since we all know Kato models are geared way too fast). 


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peteski

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Re: LED resistor value?
« Reply #20 on: February 10, 2023, 01:10:43 PM »
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Well, sometimes.  I've only been using ones rated at 20% or better lately.  But what I was really referring to is the way
no-name electrolytics from China are sometimes complete fakes that aren't even the right value or voltage rating.

While I'm not talking about cheap parts, I seem to recall that aluminum electrolitics in general (even the "good"ones from Digikey) had a fairly wide tolerance, especially in the "plus"direction.  But I have not bought any of those for some time.
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peteski

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Re: LED resistor value?
« Reply #21 on: February 10, 2023, 01:16:28 PM »
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Curious what the application might be for a 1million Ohm resistor...  Would that be for running an LED off of the 220v line my dryer runs on?
Or could I simulate the glowing tip of Raymond Burr's cigar in "Rear Window"?

Lee

LOL!  Sure Lee, that could be one application. But seriously the higher value resistors have lots of uses in timing circuits, oscillators, low power amplifiers, voltage dividers, etc.

On the other end of the spectrum, there are fractional ohm resistors, usually used for high current measurements (a voltage drop across them is used to measure the current passing through). So you could find  0.01 or 0.001 ohm big-a$$ resistors.  Some resistors are nothing more than a piece of wire (since wire has resistance).
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oakcreekco

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Re: LED resistor value?
« Reply #22 on: February 10, 2023, 01:45:55 PM »
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First of all, yes those numbers on the SMD resistor are the value of the resistance.  They are equivalent to the color codes on larger leaded resistors.
Easy way to figure those out is to use online calculator:
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-smd-resistor-code
Here is a more techincal explanation (most popular are the 3- and 4- digit codes).
https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-smd-code/

271 is 270 ohms and 561 is 560 ohms.
Next question is if both models have white LEDs, or does one model have white LEDs, while the other one has yellow LEDs? Also does each model have same type of LEDs (like 3mm leaded package or small surface mount LEDs).  That all makes a difference in brightness.

White LEDs need around 3V to light up while Yellow will only need around 2V. White LEDs are also usually brighter (more efficient) than yellow LEDs

Of course the lower value resistor will also allow higher current to pass through the LED, so it will glow brightly.

I don't like the 270 ohm resistor Kato uses for their LEDs. With a 270 ohm resistor, at full throttle (12V) it will pass 37mA through a yellow LED and 33mA through white LED.  The 570 ohm resistor is a bit more conservative as it will pass 18mA through a yellow LED and 16mA through white LED.
Of course at lower throttle settings (lower voltage) the current will be lower.

The currents with the 270 ohm circuit at full throttle are a bit over the typical maximum current rating for small LEDs Kato uses.  I guess they assume that most modelers will not run those models at full throttle (since we all know Kato models are geared way too fast).



Thanks Peteski. The Dash9 looks like a white one, and it lights up as soon as the throttle is cracked open and loco isn't moving.(MRC9900 and my old Control Master 20 have the same results)

The SD45 has the yellow/orange LEDs. They come on after train is up to speed.

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peteski

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Re: LED resistor value?
« Reply #23 on: February 10, 2023, 02:15:46 PM »
+1
White LEDs are magnitude more efficient than yellow LEDs.  While yellow LEDs require less voltage to start glowing, they will need much more current (thus, higher voltage) than white LEDs to give out any appreciable light.  If you were to replace the yellow LEDs with white (the 560 ohm resistor can stay), you could have brighter headlights in that unit.

Flicker indicates intermittent contact with track.  It is just when loco runs faster the flicker is so short (and the LED is bright enough) that your eye does nto pick it up. If you cleaned the wheels and track really well, there should be much less (if any) flicker.  Well, to really take care of all conductivit problems, the bearing cups in the trucks and the pickup strips in the chassis would also have to be cleaned out.
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oakcreekco

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Re: LED resistor value?
« Reply #24 on: February 10, 2023, 02:28:30 PM »
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Thanks again. The locos were recently serviced and cleaned.

The "flicker" that I described is not like a dirty section of track flicker. It's like a quick pulsation with a few brighter moments in between.

May just need to find some white LEDs and my little world problem will be solved.

Thanks again.
A "western modeler" that also runs NS.

peteski

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Re: LED resistor value?
« Reply #25 on: February 10, 2023, 03:03:03 PM »
+1
Thanks again. The locos were recently serviced and cleaned.

The "flicker" that I described is not like a dirty section of track flicker. It's like a quick pulsation with a few brighter moments in between.

May just need to find some white LEDs and my little world problem will be solved.

Thanks again.

Well, LEDs don't flicker (or momentarily change brightness) on their own.  Something between the source of power (throttle) and the LED is causing the fluctuations in voltage (and by definition in the LED current and its brightness).  I do think that you will be happy with white LEDs.
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peteski

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Re: LED resistor value?
« Reply #26 on: February 10, 2023, 07:07:50 PM »
+2
@wm3798  Here are some examples of LED headlight circuits using white LEDs.



Example 1 is often used by Kato and Atlas for models which feature headlight on each end (like most Diesels). It is the simplest, and has the lowest component count.

The capacitor shown in all examples is used to prevent the the LED in the opposite direction of travel from flickering during very brief disconnects from the track (which can happen during normal operation) When that happens (even for a microsecond) the windings in the motor produce a brief spike of reverse polarity voltage (BEMF) which will make the LED to flicker.  That capacitor shunts those spikes.  But the cap can be omitted for an even simpler circuit.

As I mentioned earlier, white LEDs are very sensitive to being reverse biased In Example 1 the LED which is illuminated in whatever direction the model is traveling clamps the voltage to around 3V, thus the reverse biased LED (the one which is dark) will never see more than 3V across it. That is safe for that LED.  Each LED protects the other.

Example 2 is for when only one headlight is needed.  Here, the LED is protected by a "plain" diode instead of LED, when the loco is running in direction in which the LED is not lit up.  A "plain" silicon diode has a forward voltage of around 0.7V - safely protecting the white LED.

This circuit is also handy in steam locos where it makes sense to install separate circuits in the loco and the tender (to reduce  the number of wires between the loco and the tender).  Actually if a plain diode is not handy, another LED can be used as a "protection diode", just as in Example 1. Just place that LED somewhere hidden from view, and paint it black to block the light.

Example 3 is a variant which uses a "plain" diode series with the LED. When reverse biased, the "plain" diode does not conduct. No current flows, there is no reverse voltage across the LED.

The resistor in each example can be place on either leg of the power feed.

« Last Edit: February 10, 2023, 07:15:55 PM by peteski »
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mmagliaro

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Re: LED resistor value?
« Reply #27 on: February 11, 2023, 05:03:20 PM »
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While I'm not talking about cheap parts, I seem to recall that aluminum electrolitics in general (even the "good"ones from Digikey) had a fairly wide tolerance, especially in the "plus"direction.  But I have not bought any of those for some time.
You just have to look for them.  If you select "+- 20%" in the Digikey filters, you'll see lots of them.  I tried that for a 1000 uf  35v cap just now.   If you go to + or - 10%, there is only one choice, a Nichicon.  It ain't cheap, at about $2 each.  At +-20%, you can get a lot of good ones for more like 50 cents.  So I usually go with 20%, since that's the sweet spot for tolerance vs cost.   The brands that commonly pop up are Panasonic, United Chemicon, Rubycon, Nichicon, and some others.  I tend to stick with those, since the Japanese manufacturers really seem to have "the touch" when it comes to electrolytics these days.

peteski

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Re: LED resistor value?
« Reply #28 on: February 12, 2023, 12:16:00 AM »
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You just have to look for them.  If you select "+- 20%" in the Digikey filters, you'll see lots of them.  I tried that for a 1000 uf  35v cap just now.   If you go to + or - 10%, there is only one choice, a Nichicon.  It ain't cheap, at about $2 each.  At +-20%, you can get a lot of good ones for more like 50 cents.  So I usually go with 20%, since that's the sweet spot for tolerance vs cost.   The brands that commonly pop up are Panasonic, United Chemicon, Rubycon, Nichicon, and some others.  I tend to stick with those, since the Japanese manufacturers really seem to have "the touch" when it comes to electrolytics these days.

Thanks Max.  I suppose that with this type of a capacitor (large capacitances which are mainly used in power supplies to smooth out voltage) the wide tolerance in the positive direction will not negatively affect the circuit's functionality, but lower than spec. capacitance would be detrimental to the circuit.  But when capacitors were used in timing circuits, the tolerance would be more critical.

But we are really getting off topic here. I'm glad to know that aluminum electrolytics are available with tighter tolerances.
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