Model Railway Forum

Go Back   Model Railway Forum > Model Railway Construction > Wiring & Electronics > Other


Reply
 
Thread Tools Display Modes
Old 25-10-2017, 05:51 PM   #1
Chuffchuff
Member
 

Join Date: Nov 2016
Posts: 54
Chuffchuff is on a distinguished road
Default DC - DC converter 12v or 5v LEDs

I would be grateful for thoughts / advice on the following;

In my bits box, I have a couple old metal cased scalectrix 12v 2.5 amp DC power supplies. They both work ok, give about 13v on no load, so should stabilise to 12v in use. Generally in a clean and tidy condition.

In the grand plan, I propose to use them for led lighting and was thinking of using a DC - DC converter to output to 5 volts for all led circuits.

My thought process is that the lower voltage would be better to run the LEDs, and the output current would higher so I could run more LEDs.

Very roughly 12v x 2.5 amps = 30 watts. If I convert to 5 volt I should get 5 amps or so allowing for losses.

Does the idea have merit, ?
Or just tinkering for the sake of it ?.
Or could I light a model city with 12v and still have power for village ;-) or using a DC DC converter to stabilise the output from the power supply at 12v in case the old power supplies have a ripple voltage ?
or maybe just use a 3 amp rectifier to smooth the voltage ?

Rgds
Chuffchuff is offline   Reply With Quote
Old 25-10-2017, 06:04 PM   #2
cadman
Senior Member
 
cadman's Avatar
 

Join Date: Mar 2016
Location: Leighton Buzzard
Posts: 485
cadman is on a distinguished road
Default

This Buck converter may be just the job........input & output readings, fully adjustable......can buy on ebay......HB
Buck converter.jpg

Buck step down converter notes.jpg

Last edited by cadman; 25-10-2017 at 06:08 PM.
cadman is offline   Reply With Quote
Old 25-10-2017, 06:11 PM   #3
cadman
Senior Member
 
cadman's Avatar
 

Join Date: Mar 2016
Location: Leighton Buzzard
Posts: 485
cadman is on a distinguished road
Default

LEDs @ 7.7v.jpgThis is 12v reduced to 7.7v to drive led strip........HB
cadman is offline   Reply With Quote
Old 25-10-2017, 06:39 PM   #4
Flashbang
Administrator
 
Flashbang's Avatar
 

Join Date: Jul 2009
Location: SE UK
Posts: 8,364
Flashbang will become famous soon enough
Default

Why do you think you will get more current? The power supply is rated at 2.5Amps.

You should regardless of voltage always add a series resistor to each LED. Typically a 1K0 (1000) ohm resistor will give around 10milliamps per lit LED. So at 12v around 250 LEDs could be lit from the 12v 2.5A supply.

This web sites calculator is handy to bookmark... LED Calculator
__________________
Broken? It was working when I left it!
Flashbang is offline   Reply With Quote
Old 25-10-2017, 08:00 PM   #5
Tinker
Motive Power Supervisor
 

Join Date: Jan 2016
Location: N W Norfolk
Posts: 2,336
Tinker is on a distinguished road
Default

I use 13.8v 5amp regulated supply a bus runs round the layout and where i need LED lighting i run 5v regulators 7805 type rated at 1 amp the LED's are take from srips there in 3 led's per section with resistors built the signalboxs, warehouse lighting all work from this as well as under layout lighting, 6 regulators in use atm with a amp meter show less than 1/2 a amp being used.
Tinker is offline   Reply With Quote
Old 25-10-2017, 09:28 PM   #6
Flashbang
Administrator
 
Flashbang's Avatar
 

Join Date: Jul 2009
Location: SE UK
Posts: 8,364
Flashbang will become famous soon enough
Default

If a power supply is 'regulated' meaning its a constant voltage regardless of load, then there is IMO little point in reducing the voltage further. Doing so serves little or no additional benefits for LED lighting and can increase current due to losses. Whether 12, 13.5 or 5 volts you still need suitable ohm series resistors for LEDs. If the power supply has a large current output its very wise to divide the output into smaller current rated sub circuits. One suggestion would be to take a 5.0Amp supply and immediately divided it into 5 x 1.0Amp sub circuits. Each sub circuit being protected by its own fuse or self resetting circuit breaker each rated at 1.0Amp. Therefore a fault only cases one sub circuit to trip.
__________________
Broken? It was working when I left it!
Flashbang is offline   Reply With Quote
Old 25-10-2017, 09:30 PM   #7
Suzie
Senior Member
 
Suzie's Avatar
 

Join Date: Sep 2013
Posts: 399
Suzie is on a distinguished road
Default

You might just as well by a modern 30W 5V switch mode supply which will be more efficient, smaller, and probably cost no more than the converter.

Rapid have quite a good range of units starting at a tenner or so, and this 20W/4A one will probably be quite handy:-

https://www.rapidonline.com/stontron...2502st-90-2577
Suzie is offline   Reply With Quote
Old 25-10-2017, 10:40 PM   #8
cadman
Senior Member
 
cadman's Avatar
 

Join Date: Mar 2016
Location: Leighton Buzzard
Posts: 485
cadman is on a distinguished road
Default

The Buck converter was 2.16.........HB
cadman is offline   Reply With Quote
Old 26-10-2017, 08:51 AM   #9
Suzie
Senior Member
 
Suzie's Avatar
 

Join Date: Sep 2013
Posts: 399
Suzie is on a distinguished road
Default

Looks like the buck converter provides 3A (15W @5V) so you can run two of them from each of the Scalextric transformers.

Cheaper versions are available from eBay:-

https://www.ebay.co.uk/itm/Waterproo...-/351259407149


If you don't mind buying such things from eBay you can get a 3A mains PSU for only 4.06:-

https://www.ebay.co.uk/itm/DC-5V-12V...-/371790292804

Suzie x
Suzie is offline   Reply With Quote
Old 26-10-2017, 12:24 PM   #10
chris_db4
Senior Member
 

Join Date: Nov 2014
Posts: 559
chris_db4 is on a distinguished road
Default

Quote:
Originally Posted by cadman View Post
The Buck converter was 2.16.........HB
but he will need 4,

Advantage of buying a new 5V supply well its a nice supply and all in a nice box

advantage of using the old supplies, well you've got them all ready and you can then have a mix of 12 and 5 v supplies for a range of uses,

If you run the LED's from 12V then you are burning 200mW for each one and most of it is lost in the resistors at 5V that drops to 80mW so as long as the way you are reducing the voltage is reasonably efficient then you get a considerable increase in the number of led's you can run
chris_db4 is offline   Reply With Quote
Reply

Bookmarks



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump



All times are GMT +1. The time now is 06:08 AM.


Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.