# Electronics Question -- Resistors in Series



## MtRR75 (Nov 27, 2013)

My electronics knowledge is pretty basic.

I'm trying to install LED indicator lights on my control panel -- to indicate turnout direction. The resistors that I ordered for the LEDs are a little too small (low resistance). So I tried putting two resistors in series. Everything that I have read says that when you have resistors in series, you simply add the resistances together. So I expected that my two resistors in series would probably give me too low of a voltage to run the LEDs. However, two resistors gave me exactly the same voltage drop as one did. And adding the second resistor did not appear to have any effect on the LED brightness.

I was using the variable DC power supply on my Tech II transformer to protect the LEDS. Buy I also tried this with an old DC power supply from an electric razor -- same result.

I measured the resistance across both the single and double resistors. I got exactly twice the ohms across the two resistors, as compared to just one.

FYI, My LEDs are red/green three wire. I tested this on both the red and green sides -- same result.

What am I either doing wrong, or mis-understanding?


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## Lehigh74 (Sep 25, 2015)

V=IR

You doubled the resistance and halved the current.


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## gunrunnerjohn (Nov 10, 2010)

What you're misunderstanding is how LED's work. They have an operating voltage range that is very small. A typical green LED will light at somewhere around 2.5 volts. It'll do that with 1ma or 20ma of current flowing through it, the voltage difference will only be a few tenths of a volt. What you're seeing is normal.

Check out this voltage/current curve. As you can see, the current goes from zero at around 2.4v when it first draws any current to 2.7V when it's at 20ma.


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## DonR (Oct 18, 2012)

Did you have a 'load' on the resistors when you
metered them. It's my understanding that you
would get an incorrect reading when there is no
load.


Don


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## gunrunnerjohn (Nov 10, 2010)

Don, if you read my previous post, he results are to be expected. With the LED as a load, you can vary the resistors all over the map and get pretty much the same voltage drop. You'll have different currents flowing, but that won't affect the voltage readings across the resistors.


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## DonR (Oct 18, 2012)

GRJ

Yeah, I recall what you say happening some time
back when i was testing to get a dimmer glow from
LEDs. Approximately the same brightness regardless of resistance.

Don


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## MtRR75 (Nov 27, 2013)

gunrunnerjohn said:


> What you're misunderstanding is how LED's work. They have an operating voltage range that is very small. A typical green LED will light at somewhere around 2.5 volts. It'll do that with 1ma or 20ma of current flowing through it, the voltage difference will only be a few tenths of a volt. What you're seeing is normal.


Thanks for the response.

My LEDS are dual red/green with a Vf value of 2.1V on the green side and 2.0V on the red side. The current rating is 20 mA.

Question 1: I assume that if I exceed 2 V, the LEDS may burn out. Correct?

I plan to run these LEDS off of the fixed DC terminals of one of my TECH II Transformers. That transformer puts out 14.76 V (measured). I plugged these numbers (using 2.0 for Vf) into the LED resistor calculator formula any got 638 Ohms. The closest size I could get from Digikey was 634 Ohms -- which is what I bought.

Once the resistors arrived, I hooked one up to an LED and tested it using the variable DC terminals on the TECH II, so that I could control the voltage, and not exceed 2V at the LED. I tested the red and green sides separately. I found the following:

(a) Both the red and green lights came on somewhere around 0.8-0.9V at the LED.
(b) As I increased the voltage, the color brightened up. However, the brighter the green light got, the more yellow it looked. I decided that, for me, the best compromise between brightness and color is around 1.3 to 1.6V at the LED.
(c) That desired range of LED voltage corresponds to between 2.4 and 5.6 source V (from the transformer) -- by measurement, not calculation.

Question 2: Given that my actual source volts will be 14.67, from the Fixed DC Tech II, what do I need to do to achieve my desired voltage at the LEDS.

FYI. I will be running 15 of these simultaneously, and I know that each LED has to have its own resistor. As for current load, the red and green sides will never be on at the same time, but one color will be on in all LEDS at all times.


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## gunrunnerjohn (Nov 10, 2010)

Again, LED's are current mode devices. If you limit the current to their ratings or less, you're fine. You can put 100 volts on the circuit as long as you have high enough resistance to limit the current to the LED.

Very basic rule of thumb for determining the minimum resistance for current limiting for an LED.

Take the supply voltage and subtract the LED spec operating voltage from it. Take that result and multiply times 50 and you have the minimum resistor value for a 20ma common LED.

For example, 12V supply, and an LED with a 2.2 volt nominal operating voltage, the difference is 9.8 volts. That gives you a minimum resistor value of 490 ohms. More is not a problem, less is running the LED's over their limits.

Your transformer measurements make no sense in the real world. Obviously, that transformer has significant AC ripple, so your measurements won't really come out to what you think.

If you want to get the intensity for red and green right, use two resistors for each LED, one for red and one for green. Since virtually every brand of LED is a bit different, I just determine the values experimentally on the bench. For a common cathode LED, wire the common cathode lead to the negative lead of the power, and a resistor in series with each of the red and green inputs. Vary that resistor until the intensity of each is to your liking, then use that value for all the LED circuits.


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## time warp (Apr 28, 2016)

I would recommend that you have a separate power supply, just use the Tech II for running trains.


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## MtRR75 (Nov 27, 2013)

time warp said:


> I would recommend that you have a separate power supply, just use the Tech II for running trains.


Actually, I have two TECH IIs. One just runs trains. The second one runs the turnout controls and (hopefully) the LED indicators.


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## MtRR75 (Nov 27, 2013)

gunrunnerjohn said:


> Again, LED's are current mode devices. If you limit the current to their ratings or less, you're fine. You can put 100 volts on the circuit as long as you have high enough resistance to limit the current to the LED.
> 
> Very basic rule of thumb for determining the minimum resistance for current limiting for an LED.
> 
> ...


That is the formula that I was using. My mistake was moving down from the calculated 638 Ohms to 634 Ohms (the closest size). I should have gone UP to the next size.



gunrunnerjohn said:


> If you want to get the intensity for red and green right, use two resistors for each LED, one for red and one for green. Since virtually every brand of LED is a bit different, I just determine the values experimentally on the bench. For a common cathode LED, wire the common cathode lead to the negative lead of the power, and a resistor in series with each of the red and green inputs. Vary that resistor until the intensity of each is to your liking, then use that value for all the LED circuits.


That won't be necessary. The red side looked good over it's whole range. I just need to adjust the resistance for the green side.

Thanks for your help.


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## time warp (Apr 28, 2016)

MtRR75 said:


> Actually, I have two TECH IIs. One just runs trains. The second one runs the turnout controls and (hopefully) the LED indicators.


,
There you go! On our layout we have 2 Throttlepacks, plus a separate accessory power supply. Getting ready to add a second.


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## gunrunnerjohn (Nov 10, 2010)

You can use most any DC supply, or even AC with one diode for all the commons. You just need to "tune" the LED current limiting to what you have.


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## MtRR75 (Nov 27, 2013)

Another basic question regarding LEDs.

I have LEDS with a Vf rating of 2V and a current rating of 20 mA. What would cause the LEDS to be destroyed? Exceeding 2.0 Vf or exceeding 20 mA current?

I'd rather not find out by experimentation.


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## gunrunnerjohn (Nov 10, 2010)

Like I said, the current is all important in LED's. Some LED's are driven by pulse-width-modulation with much higher peak curents, but the average current stays at 20ma or less.

You should never put voltage on an LED without provisions for current limiting! If you put 5VDC with no current limiting on your LED, it's life will be very short!


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