What Size Resistor?

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#1
OK, I am not an electrical guy. Ohms, watts, whatever.:confused:

I want to power my control panel bi directional (red and green) LEDs from my Tortoise power wires so that when the polarity of the machines changes the lights will turn from red to green and vice versa. The voltage going to the Tortoise machines is 9 volts. The LEDs are 3mm units that require 2 to 2.1 volts depending on which way the current flows. I don't want to wire them in series, yes I know you don't need a resistor if you do it that way. The problem with this setup is if, or when, an LED fails the Tortoise will be rendered inoperable as well. The LEDs will be wired in parallel with one lead of the LED attaching to one power wire of the Tortoise and the other lead of the LED attaching to the other power wire of the Tortoise.

Question: What size resistors do I need?

Thanks for the help.
Greg
 

Iron Horseman

Well-Known Member
#2
I want to power my control panel bi directional (red and green) LEDs from my Tortoise power wires so that when the polarity of the machines changes the lights will turn from red to green and vice versa. The voltage going to the Tortoise machines is 9 volts. The LEDs are 3mm units that require 2 to 2.1 volts depending on which way the current flows. I don't want to wire them in series, yes I know you don't need a resistor if you do it that way. The problem with this setup is if, or when, an LED fails the Tortoise will be rendered inoperable as well. The LEDs will be wired in parallel with one lead of the LED attaching to one power wire of the Tortoise and the other lead of the LED attaching to the other power wire of the Tortoise.

Question: What size resistors do I need?
One could go through all the math but a 1K resistor is the "go to guy" for most LED applications that are model railroad related. If the LEDs end up being too bright go to a 1.2K if they are to dim try an 800 ohm. I usually "wire them up" using alligator clips first to get my brightness where I want it. Nothing worse than a control panel glaring one in the face all the time.
 
#3
One could go through all the math but a 1K resistor is the "go to guy" for most LED applications that are model railroad related. If the LEDs end up being too bright go to a 1.2K if they are to dim try an 800 ohm. I usually "wire them up" using alligator clips first to get my brightness where I want it. Nothing worse than a control panel glaring one in the face all the time.
Iron Horseman,
That is exactly what I was looking for.

Thanks much,
Greg
 
#5
Those are interesting. I have no idea how they work.
Before you posted I went ahead and ordered some 1k ohm 1/2 watt resistors from Digikey. I got 35 of them for $1.85. Hard to beat that.
 

tootnkumin

Well-Known Member
Staff member
#6
Seeing we're having a natter in here about LEDs and resistors, I'm looking at these on my local ebay
https://www.ebay.com.au/itm/L0805GW...8V-3-4V/182873939477?var=&hash=item2a9422de15
Scroll down to where it says features in the description and tell me if these would be suitable for the new Econami PNP decoders. On Soundtraxx's website, Technical note #18, has a diagram showing 680 ohm resistors being used with LEDS for the FX functions, but doesn't mention the forward voltage of the LEDS being used. I noted from elsewhere in their info that the decoders FX voltage output is at track voltage i.e. about 14V, so how can I tell if these LEDS are suitable in that regard as well.

Additional to this, I have a 1st edition Tsunami decoder from a Genesis loco, and I believe the bulbs used are 1.5V . Now they had no resistors fitted, whereas Soundtraxx advise the fitting of resistors with their OEM decoders, where 1.5V bulbs are to be used with them. Does this mean that the Genesis boards only output 1.5V to their lighting FX connections? And if so, are there any known LEDS that will match, or would these LEDs I'm looking at be OK as direct bulb replacements?
 

tootnkumin

Well-Known Member
Staff member
#7
Actually I've just found a very interesting calculator. You need to know or be able to find out the voltage of the output of the FX tab on the decoder (Soundtraxx e.g. is the track voltage, probably 14-15V) which is Vs in the calculation (top line), the "forward voltage" of the LED (middle line) and the forward milliamps (current) (bottom line). This will give you the resistor value needed for the LED.

The calculator is down the page, about the middle.
https://www.electricaltechnology.org/2014/02/LED-resistor-calculator.html

I have some LEDs with a forward voltage of 12V~18V (1000ohm resistor included prewired) and 20mA.
Calculating on 14V as the Vs, the forward V as also 14V(average) and the mA as 20, shows I don't need an extra resistor at all for use with the OEM Tsunamis'. That's good news, cause I bought quite a few. Also explains why a similar FV12-18V without any resistor, wouldn't light up when attached to the Genesis Tsunami's probable 1.5V output.

The calculation for the ones in my previous post, indicated a resistor of 550ohms being required.

Think we should sticky that calculator?
 

Iron Horseman

Well-Known Member
#8
Seeing we're having a natter in here about LEDs and resistors, I'm looking at these on my local ebay
https://www.ebay.com.au/itm/L0805GW...8V-3-4V/182873939477?var=&hash=item2a9422de15
Scroll down to where it says features in the description and tell me if these would be suitable for the new Econami PNP decoders.
See figure 16 on page 14 of the Econami Installation Guide. They will need 1k resistors.

Additional to this, I have a 1st edition Tsunami decoder from a Genesis loco, and I believe the bulbs used are 1.5V . Now they had no resistors fitted, whereas Soundtraxx advise the fitting of resistors with their OEM decoders, where 1.5V bulbs are to be used with them. Does this mean that the Genesis boards only output 1.5V to their lighting FX connections? And if so, are there any known LEDS that will match, or would these LEDs I'm looking at be OK as direct bulb replacements?
Yes, Genesis locomotives (and many Proto-2000) use 1.5V outputs (both DC and DCC) to their lamps.
No, there is no such thing as an LED direct replacement for a bulb. Bulbs are resistors and LEDs are diodes. Totally different things. A diode is a creature of current NOT voltage.
 

Iron Horseman

Well-Known Member
#9
I have some LEDs with a forward voltage of 12V~18V (1000ohm resistor included prewired) and 20mA.
And this is why people get confused. What you describe is not "an LED". It is "an LED with a resistor pre-wired with it." The LED could care less what voltage is applied as long as it is enough to make the current flow. After that the LED is essentially a short circuit and the resistor prevents it from being so. It is the resistor that makes it behave electrically like a light bulb.

So of course it will work on an decoder output of 12 to 18 volts. Which is also why it is so easy when someone asks, "What size a resistor to use with an LED in a model railroad application." Answer 95% of the time - 1K ohm. The only real variable is how bright is the LED and how bright do you want it to be.
 
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tootnkumin

Well-Known Member
Staff member
#10
And this is why people get confused. What you describe is not "an LED". It is "an LED with a resistor pre-wired with it." The LED could care less what voltage is applied as long as it is enough to make the current flow. After that the LED is essentially a short circuit and the resistor prevents it from being so. It is the resistor that makes it behave electrically like a light bulb.

So of course it will work on an decoder output of 12 to 18 volts. Which is also why it is so easy when someone asks, "What size a resistor to use with an LED in a model railroad application." Answer 95% of the time - 1K ohm. The only real variable is how bright is the LED and how bright do you want it to be.
So...what you are saying then is that the supply voltage has virtually nothing to do with the LED's resistor value, and yet a resistor is normally used to affect the voltage flowing through it? Or is it current (milliamps/amps) that is controlled by a resistor. Or does a resistor do both or either, according to which it is used to control? My understanding of electricity is basic, but I've been lead to believe that Voltage equates roughly to pressure e.g. PSI, whereas Current in amps, equates to volume (flow) e.g. pints/Gals per min. Does your statement therefore mean that the Voltage (pressure) could be as high as "whatever", so long as the Amps/milliamps (flow) alone is controlled to an LED.

I did do an experiment with one of these 12-18V that has the resistor in the negative wire (they are at least colored red and black) with my DC 12V throttle, while holding the resistor between the fingers. At the max, it got a bit too hot to hold for more than a few seconds. I would imagine it could burn at 18V. Half to 3/4 throttle, it was OK and plenty bright enough.
 

Iron Horseman

Well-Known Member
#11
So...what you are saying then is that the supply voltage has virtually nothing to do with the LED's resistor value, and yet a resistor is normally used to affect the voltage flowing through it? Or is it current (milliamps/amps) that is controlled by a resistor. Or does a resistor do both or either, according to which it is used to control?
I would say the resistor LIMITS the current rather than controlling it. But the answer is no, because more voltage will push more current needing more resistance to limit it. BUT for the voltages we use in model railroading 9-18V, or so, the variance isn't enough to make any significant difference. We don't need to be precise enough as if we are doing heart surgery or landing a vehicle on mars. Plus most general purpose resistors have a 10% variance anyway (that last color band), meaning one rated at 1K can be from 900 ohms to 1100 ohms. One can pay more and get better quality components clear down to 1% variance but they cost a bunch.

My understanding of electricity is basic, but I've been lead to believe that Voltage equates roughly to pressure e.g. PSI, whereas Current in amps, equates to volume (flow) e.g. pints/Gals per min.
That is exactly correct.

Does your statement therefore mean that the Voltage (pressure) could be as high as "whatever", so long as the Amps/milliamps (flow) alone is controlled to an LED.
Yes with in reason. Obviously very high voltages will break down electronic components for reasons beyond watts law.
 
#14
If you take a look at the devices in the links I posted, you will see that they will work on a voltage range of 5 to 90 volts, but will only allow 20mA through regardless of the voltage. A resistor is doing something similar, but the current output, will change based on the input voltage.
 

Iron Horseman

Well-Known Member
#19
I've been lead to believe that Voltage equates roughly to pressure e.g. PSI, whereas Current in amps, equates to volume (flow) e.g. pints/Gals per min..
I just thought of this. In this water and pipes analogy, the LED would be like a one way valve. There has to be enough pressure to open the valve but then the water just rushes through like an open pipe. Pressure in the opposite direction slams the valve shut and nothing flows.
 



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