Stepping down 12v to 6v DC

[Emmo]

I want to power my Peco turntable using an Expo Turntable Motorisation Kit, and do not really want the bother of using batteries as supplied. I have a 12v PSU which will be fine, but the Expo motor only runs on 6v. I learn there's something called a resistor bridge where two resistors in line are soldered across the PSU output and one output is taken from between the two resistors and the other can be taken from either side of the bridge.

 

Can someone tell me what value resistors I need to do this, and if possible the colour coding of them. I have a box of hundreds of resistors, and if also possible, I would like alternative values I could use if I don't have the exact ones required to do the job.

 

Many thanks in advance.

[John ks]

A resistor voltage divider is not the best way to get 6V from 12V & I wouldn't recommend it

2 equal value resistors connected in series 12V will give 6V across 1 of the resistors but as soon as you connect a load to the 6V  the voltage drop across the upper resistor will increase leaving little voltage to power the turntable (see top 2 drawings )

If your turntable ( represented by the 32ohm resistor on the RH side the circuit)  draws about 200mA at 6V then its equivalent resistance will be about 32 ohms 

 

If you were to setup your turntable to the bottom LH drawing it could work but as soon as you stop the turntable the voltage rises to 8V

If the turntable control has any electronics then when the motor stops the voltage will rise & may damage the electronics

I have also shown the power consumed (wasted ) by each resistor

 

 

 

Other ways to get 6V are to use a stepdown converter as shown in the bottom  drawing

They cost generally less than £10.00 & some come with an onboard  voltmeter

Another possibility is a USB phone charger, these are 5V which is probably enough to drive a 6V motor

 John

 

 

 

[NinOz]

As per post above, something like :

https://www.ebay.co.uk/itm/AC-DC-5-35V-to-3-3V-5V-9V-12V-24V-Buck-Step-Down-Converter-Linear-Regulator-S60/264491809274?hash=item3d94f0b9fa:g:L~sAAOSw7k9dnGw6

Edited April 3, 2020 by NinOz

[Junctionmad]

11 hours ago, Emmo said:

I want to power my Peco turntable using an Expo Turntable Motorisation Kit, and do not really want the bother of using batteries as supplied. I have a 12v PSU which will be fine, but the Expo motor only runs on 6v. I learn there's something called a resistor bridge where two resistors in line are soldered across the PSU output and one output is taken from between the two resistors and the other can be taken from either side of the bridge.

 

Can someone tell me what value resistors I need to do this, and if possible the colour coding of them. I have a box of hundreds of resistors, and if also possible, I would like alternative values I could use if I don't have the exact ones required to do the job.

 

Many thanks in advance.

I think you mean a potential divider, the issue is the current drawn has a big effect 

 

do you know the expected current 

 

I better solution would be a LM317 regulator a few resistors and capacitors 

[spamcan61]

7 minutes ago, Junctionmad said:

I better solution would be a LM317 regulator a few resistors and capacitors 

Assuming a 317 can supply sufficient current for the 6V motor that would be my solution; RK Education (usual disclaimers) make modules for this sort of thing that are so cheap I can't be bothered with stripboard and ebay.

 

In this case there's this for 3 quid:-

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---built-2918-p.asp

 

2 quid in kit form:-

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---self-build-kit-2919-p.asp

 

 

[Pete the Elaner]

Why don't you just create a track section for the turntable, then drive it from the controller like you would a loco?

When you don't want it, just switch it off.

[Junctionmad]

10 minutes ago, spamcan61 said:

Assuming a 317 can supply sufficient current for the 6V motor that would be my solution; RK Education (usual disclaimers) make modules for this sort of thing that are so cheap I can't be bothered with stripboard and ebay.

 

In this case there's this for 3 quid:-

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---built-2918-p.asp

 

2 quid in kit form:-

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---self-build-kit-2919-p.asp

 

 

Absolutely , great idea. 

[JimFin]

If you did not fancy the electronics option, an old mobile phone charger would probably do the job for you, typically 5V 1A rather than the 12v PSU you have.

[cctransuk]

35 minutes ago, spamcan61 said:

Assuming a 317 can supply sufficient current for the 6V motor that would be my solution; RK Education (usual disclaimers) make modules for this sort of thing that are so cheap I can't be bothered with stripboard and ebay.

 

In this case there's this for 3 quid:-

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---built-2918-p.asp

 

2 quid in kit form:-

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---self-build-kit-2919-p.asp

 

 

 

I have already done some experimentation in this area - see

 

Is there something similar to the RkPSU v1 Single Output PSU that could handle a current a little higher - this one seems to be rated at 1 amp.

 

Regards,

John Isherwood.

[Emmo]

33 minutes ago, Pete the Elaner said:

Why don't you just create a track section for the turntable, then drive it from the controller like you would a loco?

When you don't want it, just switch it off.

 

I did think of that, but the controller is outputting 12v, which would fry the TT motor surely? If I'm barking up the wrong tree, could you show me how with a diagram please?

Martin

[Emmo]

1 minute ago, cctransuk said:

 

I have already done some experimentation in this area - see

 

Is there something similar to the RkPSU v1 Single Output PSU that could handle a current a little higher - this one seems to be rated at 1 amp.

 

Regards,

John Isherwood.

 

Hi John. If I used one of these units, do I need a meter to determine 6v output with the potentiometer or are there calibrations on the board to tell me where to put it?

Thanks

Martin

 

[Nearholmer]

8 minutes ago, Emmo said:

did think of that, but the controller is outputting 12v, which would fry the TT motor surely

No, the controller for the track has a maximum output of 12V. 
 

As you turn up the ‘speed’ the voltage increases from 0 towards 12V. Speed of train is roughly proportional to voltage supplied to train motor.

 

Unless you turn it up too far, and have the turntable zooming round, it will be perfectly fine, and you may find that a realistic speed is achieved at, say, 3V, rather than the full 6V anyway.

 

Doing this is a far easier option than the others, if you are prepared to live with the slight inflexibility of not being able to run a train while turning a loco.

 

 

[cctransuk]

18 minutes ago, Emmo said:

 

Hi John. If I used one of these units, do I need a meter to determine 6v output with the potentiometer or are there calibrations on the board to tell me where to put it?

Thanks

Martin

 

 

Martin,

 

The variable voltage regulator unit actually has an onboard LED voltage output indicator.

 

I have to agree with other posters that, using your train controller on a separate track section would be by far the easiest solution - you'd just need to make sure that you used about half power on the controller dial.

 

Regards,

John Isherwood.

[Emmo]

2 minutes ago, Nearholmer said:

No, the controller for the track has a maximum output of 12V. 
 

As you turn up the ‘speed’ the voltage increases from 0 towards 12V. Speed of train is roughly proportional to voltage supplied to train motor.

 

Unless you turn it up too far, and have the turntable zooming round, it will be perfectly fine, and you may find that a realistic speed is achieved at, say, 3V, rather than the full 6V anyway.

 

Doing this is a far easier option than the others, if you are prepared to live with the slight inflexibility of not being able to run a train while turning a loco.

 

 

 

I have a spare controller actually which I could wire up to the TT motor, in fact it's a very small unit from China on a PCB about 25mm square and works very well, and, as you say, as long as I don't turn it round to it's full movement, should be ok. I didn't want to use the main controller for the layout as I want to have trains running as I'm operating the TT. I agree, a slower TT speed would be more realistic anyway and probably achieved at just 3v. The reason I wanted to step down from 12v to 6v was purely to use a totally separate PSU to control the TT and I only have a 12v one. Basically I'll use that one now as originally planned with my spare controller to operate the TT.

 

The motor by the way is an Expo, specifically designed to run TT's. It comes with a polarity reverser switch for rotating the TT clockwise or anticlockwise, and it's either on or off and a bit of a hit and miss affair to line up the tracks. Using a spare controller does give me facility to slow the TT bridge right down to line the rails up.

 

Reading through what I am planning now, do you think it will be ok?

Thanks

Martin

[Ian Morgan]

The voltage regulator circuits, such as those using the LM317, have a disadvantage in that they produce heat. Dropping 6 volts for, say, half an Amp, will generate 3Watt, which will quickly get pretty hot.

 

The electronic step down converters (often referred to as Buck Converters), are much more efficient, do not get warm, and often include a built in voltage display so you can set your desired output voltage with ease. They also include short circuit protection.

 

This one costs pennies more then a LM317:

 

https://www.ebay.co.uk/itm/LED-Display-DC-5V-12V-24v-3A-Step-down-Voltage-Regulator-buck-Module-Converter/282215589884?hash=item41b55c27fc:g:HJIAAOSwh-1W4j-p

 

[Pete the Elaner]

53 minutes ago, Nearholmer said:

No, the controller for the track has a maximum output of 12V. 
 

As you turn up the ‘speed’ the voltage increases from 0 towards 12V. Speed of train is roughly proportional to voltage supplied to train motor.

 

Unless you turn it up too far, and have the turntable zooming round, it will be perfectly fine, and you may find that a realistic speed is achieved at, say, 3V, rather than the full 6V anyway.

 

Doing this is a far easier option than the others, if you are prepared to live with the slight inflexibility of not being able to run a train while turning a loco.

 

 

Well described.

This just seems like a really easy way to set it up, while being far more flexible & realistic than any fixed voltage supply.

After all, the controller is designed to drive a motor, which is exactly what is being used.

[cctransuk]

30 minutes ago, Ian Morgan said:

The voltage regulator circuits, such as those using the LM317, have a disadvantage in that they produce heat. Dropping 6 volts for, say, half an Amp, will generate 3Watt, which will quickly get pretty hot.

 

The electronic step down converters (often referred to as Buck Converters), are much more efficient, do not get warm, and often include a built in voltage display so you can set your desired output voltage with ease. They also include short circuit protection.

 

This one costs pennies more then a LM317:

 

https://www.ebay.co.uk/itm/LED-Display-DC-5V-12V-24v-3A-Step-down-Voltage-Regulator-buck-Module-Converter/282215589884?hash=item41b55c27fc:g:HJIAAOSwh-1W4j-p

 

 

The advantage of the LM317 is that it has a full size pot, that can easily be used for train control purposes.

 

The Buck Coverters seem to have a tiny, screw controlled variable output.

 

Regards,

John Isherwood.

[sharris]

On 03/04/2020 at 14:27, cctransuk said:

 

The advantage of the LM317 is that it has a full size pot, that can easily be used for train control purposes.

 

The Buck Coverters seem to have a tiny, screw controlled variable output.

 

Regards,

John Isherwood.

 

You are referring to pre-made modules using the LM317 or buck converters,

 

The LM317 and buck-converters are single chips which are 'programmed' with a few discrete components for the output you want - the size of the pot or presence of a display are niceties of the module manufacturer - you could knock up your own with the appropriate regulator chip and resistors and capacitors on a bit of veroboard for a couple of pounds, with your preferred size of knob.

[cctransuk]

20 minutes ago, sharris said:

 

You are referring to pre-made modules using the LM317 or buck converters,

 

The LM317 and buck-converters are single chips which are 'programmed' with a few discrete components for the output you want - the size of the pot or presence of a display are niceties of the module manufacturer - you could knock up your own with the appropriate regulator chip and resistors and capacitors on a bit of veroboard for a couple of pounds, with your preferred size of knob.

 

For the electronically savvy, I am sure that it's a doddle. For someone with O-level Physics (just) from 1965, whose recollection of those days extends to V=IR and W=VA, it's a complete no-go area.

 

I managed, with considerable assistance here, to modify the LM317 by substituting a different pot, and the home-brewed controller that I built using it works very well; (except that first 90 degrees of pot rotation produces no dicernable movement). Running is exceptionally smooth and motor noise is virtually absent.

 

 

Thanks to Junctionmad here, I have purchased and had delivered a  https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---built-2918-p.asp , which would seem to be ideal for powering layout lighting or a turntable; however, I wonder whether something similar is available with a rating of, say, 2 amps, which would make it suitable for locomotive control?

 

John Isherwood.

[dasatcopthorne]

I use 2 Buck converters in a self made Desktop Power supply. 12v dc for controllers ans 3.5vdc for LED testing.

 

From a UK eBay seller. Very useful.

 

Dave

[Brian]

I agree using a Buck converter is IMO the best way, especially one with an on board voltage display, so you can set the output exactly to what is needed.   Example link

In addition you will need a switch after the converters output both to start and stop the TT motor and even to reverse its direction of travel.  I would use a DPDT Centre Off switch wired on the output side of the Buck converter.   Like this example... SW312  Link to example switch

 

 

Edited April 10, 2020 by Brian

[cctransuk]

1 hour ago, Brian said:

In addition you will need a switch after the converters output both to start and stop the TT motor ....

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---built-2918-p.asp

 

This one, (I have bought a sample), seems to have an ON / OFF switch incorporated.

 

John Isherwood.

Edited April 10, 2020 by cctransuk

[PenrithBeacon]

Isn't this all a bit complicated? Surely all that's needed is a resistor

[letterspider]

I was always told - in a situation like this where you want to divide up the voltage with a resistor - use an (incandescent) light bulb instead

[Brian]

6 hours ago, cctransuk said:

 

https://www.rkeducation.co.uk/rkpsu-v1-single-output-psu-for-model-railways---built-2918-p.asp

 

This one, (I have bought a sample), seems to have an ON / OFF switch incorporated.

 

John Isherwood.

Does the switch allow reversing of the TT motor?  Doubt it.

How are you going to set the output volts? You will need a meter across the output to be able to set the volts correctly. Hence my suggestion of a built in volt meter. 

But if you have purchased that one already then that's fine,

Edited April 10, 2020 by Brian