Model train 5v 100mA driving 12v 2A

[silve]

I have a question for any DIY electronics hobbyists. Sorry please hold the flames if this is off topic as I realise most of the forum is about mains installations/electrics. But I guess there's a chance someone might be into this.

Background:

I have an old 1940s train set running off a 12V dc transformer rated 2A. I'm trying to control this with a computer.

I have soldered up a Vellman USB board from Maplin. It can supply 5v at up to 100ma of current. I have program turning this digital output on/off.

The 5v activates a relay to turn the 12V track power on/off, all working fine.

Problem:

Now I'm thinking about being able to vary the train voltage from 0-12V using the analog output of the board. I.e. have a slider control the speed of the train. For this, the board outputs 0-5v again up to 100ma.

My question is, how can I get this varying 0-5v low current signal for the computer, to vary the train power from 0-12v also with its significantly higher current draw.

(I can't do this with the relay, as this just turns on/off and doesnt enable the continuous variation of the power.)

Apologies again if this is a long shot. Do any electronics enthusiasts out there have any ideas? Maybe someone has come across a sort of dimmer circuit or something that might do this for the voltage ranges indicated?

 

[TicklyT]

My first thought is a motor speed controller used for radio controlled models. Although the usual input is a PWM (pulse width modulated) signal rather than a varying DC voltage, some will operate in voltage following mode.

The power supply, unsmoothed rectified AC, instead of the DC battery supply they are designed for may also be problematic.

 

[davelx]

My first thought is a motor speed controller used for radio controlled models. Although the usual input is a PWM (pulse width modulated) signal rather than a varying DC voltage, some will operate in voltage following mode.

The power supply, unsmoothed rectified AC, instead of the DC battery supply they are designed for may also be problematic.

You really dont want to use variable voltage on the motor - that will give you very low torque at low speeds, which is not what you want as you won't get a smooth pull-away or braking action. All modern train speed controllers use PWM with the full voltage applied to the motor, which gives much better low speed torque - just like the speed controllers in cordless drills and RC racing cars.

You can get USB servo drivers for PC's which will be able to control that type of speed controller.

eg

http://www.active-robots.com/products/motorcon/servo-phidgets.shtml

and in fact that same site has USB motor speed controllers, but they are not cheap.

This looks promising too:

http://www.pc-control.co.uk/osc/index.php?cPath=21

You will need a smooth DC supply, but that's not difficult.

 

[ban-all-sheds]

1st question - does the train actually work OK at reduced voltage?

By how much can you vary the voltage and still have it work OK without the motor overheating or stalling? (The latter might do irreparable damage).

Do you need full analogue control over the speed, or would just low and high be OK? Those would be easy to do with a couple of relays and a dropper resistor.

DC motors can be speed controlled with pulse wave modulation, but I think you'd need to monitor the current being drawn for overcurrent/stall conditions.

Your best place for practical advice would almost certainly be a model railway and/or a robotics hobby fora. Ask on the former and you may find an OTS product.

EDIT.

Doh. Doing too many things at once - there'd been no replies when I started.

 

[silve]

Thanks for all the tips, and those links. If I can't get my current board working in some kind of setup I will look into those.

Yes the motor does run at lower voltages, the tech is so old (40s) I'm not sure they had pulsing DC to change motor speed then - even if it is a better way to control speed. Anyway the current controller doesnt seem to use a pulsed approach.

The resistor idea is a good one. I think with 3 relays switching a bank of resistors say 4x, 2x 1x some value, then 8 speeds will be possible which is good enough.

Will bring this thread to a close now as I don't want to swamp the board with off-topic posts.

 

[Mursal]

A really interesting problem if you don't mind me saying.
Sorry for the reply as I know the thread is now closed.

The resistors in series with the motor, as you probably know are used in the heater motor control on most cars. If you cant get large wattage resistors of the correct value off the shelf a good car breaker will have loads.

If you wanted to go the "elegant solution" route. A 555 8 pin timer chip can be used to PWM a circuit. As far as I can remember Maplin have 555 project boards that may be converted to your application.

 

[clubb699]

Hi silve,

Oooo cool another nerd

Just to say my model railway runs of the Pc by means DCC digital i have had to install i mini DCC chip into the loco then this as its own address that is controlled by the pc software i can configure so much regarding speed settings etc. Last year well those was the days playing chuff chuff in the shed instead of doing all this damn DIY

If your still stuck hope onto this site sign up and you will get all your answers

http://www.newrailwaymodellers.co.uk/Forums/

Ps take a look at one of the posts Sunshine in the lakes is such a lovely chap that did this model

Enjoy

Lee