Ok so as not to hijack another post here is my question. I motorized an athern snowplow with a Cannon dn22 or cn22 dc motor. I run the bachman dcc system right now but plan on upgrading in the future. It is a 16v ? system I think.. I have a decoder in it (bachman but upgrading to a sound controller) and would like the wheel to turn at something slower than a huricane. What size resistor should I use? The guy at the hobby shop said a 8 ohm 32 watt resistor, but all I can find are 8 ohm with 1/4,1/2, or 1 watt. What am I missing here?
Thanks
Mike
alco's forever!!!!! Majoring in HO scale Minorig in O scale:)
A 32 watt resistor will be of the wire wound type to begin with. As for the accuracy of the "Hobby Shop guy's" recommendation, I can't say.
Here is some info on resistors.
Voltage dropping resistors are sized by the current draw of the motor. Rheostats (variable resistors) make poor DC motor controllers because DC motor current draws vary widely. The size of the resistor you want is going to depend greatly on how much current the motor draws in operation, and what voltage you want to operate the motor at.
Let's say your motor in the snowplow draws 0.4 amps (probably less in reality) turning the plow, and you like the speed with the motor at 4 volts. This means the resistor needs to drop 8 volts (assumes 12 volts output from decoder). An 8 volt drop divided by 0.4 amps gives 20 ohms. Resistor power rating needs to be greater than 8 volts times 0.4 amps, or 3.2 watts. So a 5 watt, 20 ohm resistor is probably in the ballpark. If the current draw is less, you will need a higher resistance to keep the motor from going faster.
A 5 watt resistor will not be a simple find, but they can be found. A voltage regulator circuit may be a simpler way to achieve the goal.
But in reality, a decoder should be able to control the motor speed just fine. If the decoder can't control the motor RPM sufficiently, you will probably need a gear or belt reduction drive.
my thoughts, your choices
Fred W
If you are going with a DCC decoder in the Snowplow just use the motor output and set it to a low speed. I'm not sure what the resistor is for is my point. if you are going to have a decoder in it use the motor leads. That will give you an adjustable speed on your plow.
Can you please clarify why you would use a resistor. the only thing I can think of is that your are using a motor that is not rated at 12 volts in which case you would need a resistor. Otherwise i resistor is not needed if you use built in speed control of the decoder ( which can take speed curves to limit the max speed depending on the decoder)
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/
You technically wouldn;t need a resistor even if the motor isn't rated for 12 volts - just hope you never mess up the decoder programming. Just set mid and max to low values so that even at full throttle the motor only gets half ot 1/4 power and it will spin nice and slow. Don't risk melting the shell after you do all this work with a resistor that ends up getting hot.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I set up a little experiment for this. I used a 330 ohm resistor in series with one lead to the motor. I did not use a light output. DC motors have a cogging action and pure DC from the light output is not the way to go.
I used the decoder output that goes to a motor. The PWM the decoder sends to motors overcomes the cogging action fairly well. The current is about 22ma.
I did this test using a Micro-Tsunami and a NCE Power Cab.
A 1,000 ohm miniature potentiometer would be ideal as you might want to optimize the rpm.Maybe position the potentiometer where a small screwdriver can adjust it though a window opening.
You might try program the decoder to do this without a resistor. Just my thoughts.
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.
A small potentiometer will almost certainly fry from the motor current. This is somethign I learned very young, before I truly understood anythign about electronics, when I tried to use one to control a 6 volt motor powered by a battery. They are usually rated at 1/8 watt or possibly 1/4 watt, and assume you're dropping half the volts with a .25 amp motor witht he pot set to halfway - 500 ohms. That definitely exceeds 1/8 watt.
Also, the function outputs on most decoders aren't designed to handle more than about 200ma, and the decoders uusally also have a total limit for all functions which is NOT the same as all functions times the limit per function - so if there ar eadditional lights and so forth, it probably won;t be enough. The motor outputs on the other hand are designed for the load of a motor, anywhere from 1 amp to 2 amps depending on the decoder. With adjustment of start, mid, and max you can set a very limited range of motor speed to it looks liek a plow and not an airplane no matter how much you crank the throttle. And a neat effect, although having never seen a rotary in action I don't knwo if it's accurate or not, would be to keep the start set to default so the blade doesn;t start at speed step 1 and put a LOT of momentum on it, for both acceleration and decelleration, so when you cut the power to the blade motor is takes a long time to wind down and finally stop.
All good thoughts. Randy in the above post has the idea that I am looking for. I have a bachmann decoder and a bachman decoder. I know a flywheel will help the start up and shut down momentum. I also plan on a coreless motor with 0 cogging. As I have not upgraded my dcc system yet I was hoping a resistor would help to slow down the rotation at the first speed step. I guess I will just have to put up with the airplane rotation speed for now.
I tried a can motor with a 300 ohm resistor and without a resistor. The current was 22 ma. through the resistor. The resistor is really not needed.
An open frame motor driving only a flywheel draws 400ma at 12 volts dc. I hooked up the motor to the decoder motor wires of a SoundTraxx Micro-Tsunami which is rated for 750 ma. Using a NCE Power Cab on 128 step mode, I have very good control of the motor. I stepped from 0 through 3. I interrupted the DCC signal to simulate poor track connection and the motor picked up where it left off.
The motor and flywheel are from an older MDC Box Cab without the gearbox.
Dial up users beware, 14mb file.
richg1998 I interrupted the DCC signal to simulate poor track connection and the motor picked up where it left off.
On DCC, the speed is constantly broadcast over and over again. This prevents runaway locos due to dirty track. So it would make sense it would pick up where it left off.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
rs2mike I have a decoder in it (bachman but upgrading to a sound controller) and would like the wheel to turn at something slower than a huricane
Mike,
Are you using the decoder (QSI/Tsunami/Loksound) to drive wheels, or just the snow plow?
If you are just driving the snow plow wheel, then any high speed/silent decoder with back-emf will work. I have to go back and read the manuals, but I think it's either QSI or Tsunami has a user adjustment parameter for the motor type (pancake, open frame/open core, or can) to fine tune the back emf.
If you need both motor drive, and snow wheel plow control, you might still be able to do it with a QSI-Revoloution decoder, which supports variable output functions. It all depends on your max current output.
I also have a video showing a small can motor turning just a 16mm flywheel and it worked ok on speed step one and two using the NCE Power Cab in 128 step mode. The video is fuzzy like the first one I posted.
No doubt, you will have to get a multimeter and experiment. Try to let everyone know your results. Others may want to do something similar. The Micro-Tsunami has a lot of fine tuning capabilites.
I use only the Micro-Tsunami so I cannot comment on other brands of decoders. Realize you might be able to use sound as many rotary snow plows used two cylinder steam engines for the rotary fan. Whistles, also as the plow operator could communicate with the pushing engine engineer. Many rotary snow plows also had headlights.
Rotary snowplow links.
http://www.google.com/search?hl=en&sa=X&oi=spell&resnum=1&ct=result&cd=1&q=steam+powered+rotary+snow+plows&spell=1
Here is another link concerning rotary snowplows that were converted from steam to diesel. You could then use a diesel decoder.
http://espee.railfan.net/sp_rotaries.html
To the best of my knowledge, the rotaries that are self powered are the more modern single unit ones, that is, without a tender.