# Resistor Size



## thompsondl (Dec 9, 2016)

I am wiring up a Tortoise switch machine and a couple bi-color LEDs. Seems like there are several different ways to wire these things so don't really know which is best. Anyways, I came up with a diagram that should work I think. I am not an electronics expert.










I originally tried this without the resistors but the switch machine wouldn't work but the LEDs lit up as expected. I started with a low voltage of 3V. I increased the voltage and ended up blowing the LEDs. (no problem, i got extras) so I figured I basically had a short and I need resistors before the LEDs to force some current to the switch machine and also reduce the current to the LEDs. My question is, first is the circuit look ok and second what size resistor should I use? I put 1K resistors on my diagram. That seems to be the most common that I have found in my searches but not sure how they came up with that value. I am sure there is some formula on how to calculate this.


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## thompsondl (Dec 9, 2016)

Here is my mock-up of a turnout and the switch machine so I can test my wiring. The switch machine work perfectly without the LEDs. Just need to figure out how to wire the LEDs in.


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## gunrunnerjohn (Nov 10, 2010)

It depends on how bright you want the LED's. Running with 10 VDC, you could have a value as low as 220 ohms for 20ma to the LED's. Anything above that just makes them dimmer, my pick would probably be around 470 ohms, plenty bright and not pushing the LED's too hard.


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## DonR (Oct 18, 2012)

The Tortoise turnout motor has special
SPDT switches built in that can control your
signal lights. Check your owners manual.

Don


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## gunrunnerjohn (Nov 10, 2010)

You still need resistors, you just get contacts to control the lights.









Although they discuss putting the LED's in series, I recommend against that, I don't see that as a particularly good idea, but it will work.


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## mtfrizzell (Jan 1, 2016)

thompsondl said:


> I am wiring up a Tortoise switch machine and a couple bi-color LEDs. Seems like there are several different ways to wire these things so don't really know which is best. Anyways, I came up with a diagram that should work I think. I am not an electronics expert.
> 
> 
> 
> ...




The resistor's job is to reduce the overall current is that leg of the circuit. There is voltage drop across the resistor. The diode typically has a voltage drop of 0.7 volts. The power supply voltage minus the 0.7 volt drop across the diode leaves the balance across the resistor. The current,I,through the resistor is the voltage drop, V, across the resistor divided by the resistance (R ohms) by Ohm's Law: I=(E/R).


Sent from my iPhone using Tapatalk


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## gunrunnerjohn (Nov 10, 2010)

Why not use the contacts on the Tortoise, they're there to make this job easy.


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## thompsondl (Dec 9, 2016)

The reason I was not planning on using the leads directly off the tortoise was the leds are to be on my control panel and will be close to the dpdt switch and the power source. If I use the leads off the tortoise I would have to run additional wires back from the tortoise to the control panel. Not a huge problem but more wires under my layout.

I am open for other wiring methods and suggests. I just thought about it and came up with that diagram.

Also the power source I am using now is that lab DC adjustable power supply a borrowed from work. I'll be getting some sort of power brick to use on my layout. I'm shooting for around 10v. Not sure what amperage I should be looking for either.


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## thompsondl (Dec 9, 2016)

gunrunnerjohn said:


> Although they discuss putting the LED's in series, I recommend against that, I don't see that as a particularly good idea, but it will work.


In my diagram the LEDs are in parallel. Does that make any difference? In some of the diagrams I have seen they are in series. Why would it be bad in series?


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## mtfrizzell (Jan 1, 2016)

thompsondl said:


> In my diagram the LEDs are in parallel. Does that make any difference? In some of the diagrams I have seen they are in series. Why would it be bad in series?




In series the two diode each have a 0.7 voltage drop across them and everything is the series has the same current thru it. 

In parallel the twi parallel legs have the same voltage and of course the diodes have the 0'.7 voltage drop, but by having different resistors in each leg the current is correspondingly different and the illumination follows the change it current. More resistance means less current thru the diode so the illumination is dimmer. If you put a potentiometer in place of the resistor you just built a dimmer switch.


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## DonR (Oct 18, 2012)

Your original wiring diagram should work.

The major considerations are the voltage 
available from your power source and the
resistor value to each LED. You could put
the resistor in the 'common' and need only one.

Start with NO LED connected. If your Tortoise
works, then add the resistors to the LEDs. Try
various resistance until you get desired brightness.

The amperage of your source power supply would
depend on the number of Tortoises you will have
on your layout drawing current. Keep in mind that
there is always a current to the Tortoises. Your
Tortoise manual should tell you the stall amperage.
Multiply that times number of Tortoises and you'll
have the required current needs. Each will draw
more current when operating but it's unlikely more
than one or two would be drawing motor power at
the same time.

Don


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## Mark R. (Jan 26, 2013)

You are trying to run a red and a green LED from a single 1000 ohm resistor. At 10 volts, I would think you have too much resistance for the two LEDs. Personally, I would use a resistor on each leg of each color (4 total) with a value in the 220 to 470 range. The red by nature will be brighter than the green, so you may want a higher value on the red to balance the brightness.

Mark.


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## thompsondl (Dec 9, 2016)

I built up a test circuit using 1K resistors. I see the difference in the brightness of the LEDs as Mark pointed out. I could maybe separate out the reds from the green and and use different size resistors for each. When I was at the electronis store buying resistors I bought a breadboard and a few different value resistors to play around with. I can experiment with different values until I get it right.










Here is a video of the switch machine and LEDs in action. Not the best video. Still learning how to shoot videos with my phone.


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## Lemonhawk (Sep 24, 2013)

You could also use bi color 2 lead LED's and just put it in series with one of the motor leads to the tort. That's the way I have my stall motor switch machines wired.


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## sachsr1 (Mar 3, 2016)

Are you using DC or DCC? I just have a single 2 lead bi-color led soldered to one of the leads running to my tortoise.


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## cid (Jul 3, 2014)

I'm not real familiar with the tortoise, however I'm pretty sure that the internal switches indicate the actual position of the switch motor as opposed to the command signal. (Personally) I would like to see that short delay just to know that everything is normal.😁


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## gunrunnerjohn (Nov 10, 2010)

The contacts are indeed driven by the mechanical motion and should indicate the actual position of the switch (or at least the switch machine).


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## feldon30 (Dec 30, 2012)

Just as another data point, I use LEDs directly off the tortoise and I do not use resistors. They might benefit from a capacitor though, as the LED dims as the switch moves.


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## Lemonhawk (Sep 24, 2013)

The dimming is due to the stall motor not being in stall condition where it draws more current. Consider it a "feature", it tells you when the points are moving!


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## feldon30 (Dec 30, 2012)

Yeah it's actually kinda cool.


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