Using Ohms Law, I calculated I need a 4 Ohm, 4 Watt resistor to drop 9 volts supplied by my power source to 5 volts to slow down my operating semaphores.
R=E/I, R=4/1, R=4 ohmsP=I squared x R, P=1*1*4, P=4 watts
Circuitron says their Tortoise is rated at 1 amp. However, as someone has pointed out, in reality, the machine probably draws less current than that. What would be a realistic value to use to determine how many watt resistor I should use for my application? Utilizing a 1 amp rating will result in a lower watt product and perhaps running the resistor too hot. The last time I took electronics was in junior high. Perhaps I am making too much of this.
Victor A. Baird
Fort Wayne, Indiana
Victor,
The 1 amp rating is for the contacts. They switch that much current without damage. The Tortoise itself draws around 15 milliamps, so way less than a whole amp.
Mike Lehman
Urbana, IL
Thanks Mike. Circuitron says the machine can handle 4-5 amps...
Hi Victor:
I have recently been trying to figure the power requirements for my layout and one of the topics that came up was the typical draw from a tortoise.
According to Randy Rinker the tortoises (tortii?) actually only draw 15 - 18 ma each. Randy is pretty knowledgable about these things so I trust his advice.
Dave
Edit:
I just checked the Circuitron web site and they confirm Randy's figures on the Tortoise draw. Those numbers are at stall. They draw 4 ma while operating.
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
I have put a bipolar 20 ma LED in series with one lead to a Tortoise. No problem.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Victor:
I think you may be confusing the amount of current the Tortoise can switch vs the amount of current it takes to run the Tortoise motor. For your purposes, i.e. slowing down the speed of the Tortoise, you only need to look at the current required for the Tortoise motor (15 ma). The 4 - 5 amps is what the contacts can handle if you are using the Tortoise to power a frog, etc. It has nothing to do with running the Tortoise motor itself.
Dave:
Thanks. I understand that as did not used that in my calculations. I was just offering that out for what the contacts can handle when Mike said it was 1 amp. But if .015 amp is what the motor will use, I'll revise my calculations with that. So rounding up, it sounds like a 1/4 watt 300 ohm resistor would work. If I use 4 mili-amps, it will be a 1000 ohm resister at 1/4 watt.
If you are going to wire LEDs in series with the Tortoise's power supplies for signal or panel indications, I believe you have to take the LED draw or voltage into account. I'm not sure which. Don't ask me to do the math!
I find that running the Tortoise motors on 8 or 9 volts gives the best slow-motion speed for the points.
It takes an iron man to play with a toy iron horse.
If you need to reduce the Tortoise voltage to lower then 9V but not use a different poser supply, I suggest putting a 2 wire bicolor LED in series witht hemotor, even if you hide the LED. Because of the relatively large difference in stall current vs running current, a resistor would be a very poor voltage dropper. The LED will drop 2.5-2.7V either way - stalled or running. If that's not enough, a second LED would probably make it too low, much below 5-6V and the things will barely move. You can drop in smaller increments by using back to back pairs of regular diodes - they need to be in pairs, connected antiparallel to each other (cathode to anode, anode to cathode) so they pass current in both directions to allow the motor to reverse - the 2 wire bicolor LED is exactly that, a pair of LEDs connected antiprallel inside the case. Adding pairs of regular silicon diodes will frop .6 to .7 volts per diode pair, so you can somewhat fine tune the drop by adding or removing diode pairs.
I have a far better idea - Tortoises are HUGE and bulky. A micro RC servo is barely the size of the edge connectro on a Tortoise and the control circuits are not expensive - some even have the capbility to emulate the 'bounce' that happens when an upper quadrant semaphore drops - look at a video of a real one and you can see this happen. The servo has fine enough movement that it can do this, for that extra little bit of realism. Plus a servo can accurately do a 3 position indication, with a Tortoise you play games with the make/break of the contacts to sort of have a center position but it's never the same coming from red to yellow as it is going green to yellow due to the required gaps in the switch contacts internally.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
wabash2800...I was just offering that out for what the contacts can handle when Mike said it was 1 amp...
CSX Robert:
Thanks for clarifying that.
from the circuitron manual for the toirtoise: (http://www.circuitron.com/index_files/ins/800-5615ins.pdf)
Connect a source of 12-18 volts DC to the [INPUT] terminals. Observe proper polarity.
Just hook up the 9 volts and you are good to go! No resistor necessary.Like Randy said, if you need to drop it more, use diodes. One attached to pin 1 with the silver band toward pin 1. Another attached to pin 1 with the silver band away from pin 1. (This is what Randy meant by anti-parallel). Each diode pair like this will give you a consistent .7V drop regardless of load. So 2 pairs in a series (4 diodes) will give you a 2*.7V = 1.4V drop or 9V-1.4V = 7.6V to tortoise.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
DigitalGriffinJust hook up the 9 volts and you are good to go! No resistor necessary.
I think part of the confusion is that no resistors are necessary when hooking up LEDs in the control line to a Tortoise. The stalled motor provides enough resistance that they're OK without resistors.
Whether that works the same for Victor's semaphores, I don't know. If the only concern is that they're illuminated with LEDs by power through the Tortoise, then I'd say they're OK without resistors. But if the semaphore drive is also somehow powered that way...?
It would eb the same no matter what. Semaphore or turnout, in the end the Tortoise motor is powered the same way. Semaphore it may go through the internal contacts to get a center position, and maybe a more complex switch to give teh 3 options, or be controlled via DCC, or other sort of detection, but in the very end you will have a wire going to pin 1 and a wire to pin 8, and it will need to provide the Tortoise motor with 12V or less, and that will have no effect on the current draw - it will draw what it always does.
Using 1N4001 diodes there will never be a problem - they can handle 1 amp. But since the peak stall current of the Tortoise is 18ma, it's plenty safe for any normal LED without any extra resistors.
Forgot to mention before, our club put a LM317 voltage regulator on the 12 vdc switch motor supply for the Tortoise for more realistic operating. Two resistors along with two caps for 9 vdc. Cicuitron I believe sells a commercial version that operates off of AC or DC. A pot is adjusted for required voltage.
LION does NOT put LEDs in series with the trutles. But LEDs, many of them are enervated by the same circuit that moves the Tortoise.
1 pole of tortoise is GROUND
1 side of each LED is GROUND
1 side of each RELAY (If relays are used for track and signal logic etc) is ued is GROUND
The other side of all devices is the single control wire for that tortoise coming from the tower.
LION *tried* when building the track relay panel of him, to put LEDs in series with each relay so him could see which relay was pulled and which was not. PLAN not work, too many issues with LED failure. Don't know why, but there you are, LEDs are gone from the relay board, but they are still present in the model board which also tracks the location of each train.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
You may have better luck using the run current in your calculation. Because that's when it will be moving, at stall we relly don;t care, because in this case there's nothign that needs power to hold position like a turnout with solid points (no hinge) where the springiness of the track can push the Tortoise back if theres insufficient voltage.
Say you use a 100 ohm resistor. E at 15ma stall would be .15 volts, but when running it would be 100 x .0004 or .04 volts. Less drop when running than when stalled. If you use a larger resistor to get sufficient drop at the run current, it will drop a lot more when stalled, but it won;t matter because the Tortoise doesn;t have to move.
This vast difference is why I say diodes are better. Diode response curves are only slightly current dependent, a resistor's is entirely current dependent. I've used diodes to drop 12V DC to feed light bulbs - just string them in series to get the drop you need, and it doesn;t matter if you connect 1 bulb or as many as takes you to the current limit of the diodes, they get the same voltage. If the semaphore uses an LED for the light - you can use that, just need to hook another one up anti-parallel under the table. White LEDs drop 3.1-3.5 volts, so that gets you even closer to the goal voltage.