3volt Throttle

[VonRyan]

While I hash out details for my HCD layout, and wait for the old pouring table to be removed from what will become the modeling center, I'm shifting some of my focus back to my box of bits that is my N-4mm modeling efforts.
The motor I have was sold as a 12 volt motor, but due to its size, and the fact that the documents that came with it say 1.5v with 3v max, I'm going to go on the 3v end to be safe.

My conundrum is controlling what will be a small loco. I don't want to just wire in a resistor to my 9v throttle, and I'd like to stay away from just using an on/off constant-power throttle.

I've built a simple 9v throttle (1k ohm pot, 100 ohm resistor, on/off switch, dpdt switch for direction control, and one of those fancy double transistors with the name I can't remember) but that was from plans, thus I have no idea what i'd need to make a 3 volt throttle.

If anyone can sketch up a plan/list of what I'd need, I'd much appreciate that. I'd also like to make this as small as possible, so i'd like to use the smallest parts possible.

Thanks All!


-Cody F.

[DKS]

I was able to control a model having a pager motor with a conventional DC throttle by incorporating a diode circuit into the model. It works like this: the motor is essentially treated like the headlight in a constant-brightness circuit. Opposing diodes are connected in series with a 12 volt G-O-W bulb, and the motor is connected in parallel to the diodes. Like this:



The throttle range is a bit limited, but I'm still able to safely run the pager-motor-powered critter on the same track as conventional DC locomotives.

If your motor is 3 volts, then simply double the number of diodes in series (each diode has a 0.75 volt drop).

BTW, the model in question is a hi-rail: http://whiteriverandnorthern.net/clinic_35.htm

[VonRyan]

I was able to control a model having a pager motor with a conventional DC throttle by incorporating a diode circuit into the model. It works like this: the motor is essentially treated like the headlight in a constant-brightness circuit. Opposing diodes are connected in series with a 12 volt G-O-W bulb, and the motor is connected in parallel to the diodes. Like this:



The throttle range is a bit limited, but I'm still able to safely run the pager-motor-powered critter on the same track as conventional DC locomotives.

If your motor is 3 volts, then simply double the number of diodes in series (each diode has a 0.75 volt drop).

BTW, the model in question is a hi-rail: http://whiteriverandnorthern.net/clinic_35.htm

Might be a bit of a stretch for a small steam loco. I was wondering if a small audio pot or one of the tiny ones that requires a screwdriver to adjust would be able to work. Since I'm planning on using an exterior power source of only 3v (or the closest as I can get), I'm trying to obtain the best range possible out of such a low voltage motor.

-Cody F.

[peteski]

Google is your friend. Google for  0-3V adjustable power supply schematic and here is the first hit:

http://www.circuit-projects.com/index2.php?option=com_content&task=view&id=147&pop=1&page=0&Itemid=28

You still need to come up with a positive/negative 5V power supply but that part is wicked easy. 

[VonRyan]

http://www.circuit-projects.com/index2.php?option=com_content&task=view&id=147&pop=1&page=0&Itemid=28

Above my pay-grade for sure. I'll have to dig up the link to the 9v throttle I built to give a better idea of the level of experience I have.

-Cody F.

[VonRyan]

Here it is:



It uses a TIP 120 Darlington Transistor.


-Cody F.

[peteski]

Your circuit should work. Just replace the 9V battery with 3 AA (or similar) cells connected in series. That will give you a 4.5V to start with. The transistor drops some voltage across the Emitter/Collector so that will make it just about right.  Or just use 2 AA batteries in series for a 3V input voltage.

[VonRyan]

Your circuit should work. Just replace the 9V battery with 3 AA (or similar) cells connected in series. That will give you a 4.5V to start with. The transistor drops some voltage across the Emitter/Collector so that will make it just about right.  Or just use 2 AA batteries in series for a 3V input voltage.

I see, so there really is no change in components just the input voltage. For this throttle i'm planning on using a constant power-supply (the plug-in type) so as to make the throttle smaller, and since the next biggest component is the 1k ohm pot, would it be possible to replace it with a much smaller trimmer pot (are pots even universal save for the ohms?) or is the type of pot being used at present all that can be used?

The type being used at present (looks the same, except mine came with a very long shaft, and the specs might be different)
http://www.parts-express.com/pe/showdetl.cfm?partnumber=023-510&utm_source=google&utm_medium=cpc&utm_campaign=pla


-Cody F.

[peteski]

Cody,
potentiometers come in lots of shapes and sizes.  Large, small, rotary (with several shaft diameters and lengths), or with linear slider. From the electrical side you need to consider the voltage across them and the current passing through them (which also leads to the power dissipation). In your circuit (using a 1000 ohm pot) you don't have to worry about the voltage and the current (and power dissipated) will be very small. So pretty much any size 1000 ohm (or 1K ohm) pot you find will work. Same goes for the other resistor - you should be able to use the smallest one you can find.

Here is a selection of small 1K pots from Digikey (my regular parts supplier):
http://www.digikey.com/scripts/dksearch/dksus.dll?FV=fff40004%2Cfff80335&k=potentiometer&vendor=0&mnonly=0&newproducts=0&ptm=0&fid=0&quantity=0&PV1=2358&PV2=981&PV2=799&PV2=246&PV2=982&PV2=1732&PV2=2&PV2=3&PV2=556&PV2=4&PV23=2&PV24=1&stock=1

While I haven't done any specific calculations, due to the voltages required to get the transistor to conduct, you should try the circuit with a 4.5V power supply.  I also suspect that the usable control range on the pot will be a small portion of the rotation.  If that is the case, you can add a series connected resistors on one or both ends on the pot to increase the control range of that pot.  Build the circuit as-shown first then we can tweak it later.

[VonRyan]

When I originally built my 9v battery throttle, I wanted to use a linear sliding pot, but couldn't find one rated at 1k ohms.

I'll have to dig up my bits and such tonight, fire up the iron, and once my computer is fixed, order the pot.


-Cody F.