24 VAC model train voltage controller

Thread Starter

AJArena

Joined Feb 11, 2018
8
Hi Everyone, I have a hobby related question. I have used standard light dimmer switches on the front end of step down transformers to run my Lionel train. This has worked quite well when I set the train up for Christmas and run it all around the living room; one train on one power circuit. Now I've decided to run larger a larger set up with multiple trains and multiple power zones. It would be nice, and cost effective, to put dimmer switches on the 24 VAC side of the transformer. The standard dimmers don't work at this reduced voltage. What do I need to change on the 120VAC dimmer switch to have it work at 24 VAC?
 

MaxHeadRoom

Joined Jul 18, 2013
21,393
I have an design for a Model R.R. controller in Old design manual from GE , if interested I can copy & post it if needed.
I made a similar version for a hot wire cutter.
Max.
 

Thread Starter

AJArena

Joined Feb 11, 2018
8
Thanks Max,
That could be helpful. I am hoping to just swap out a couple of parts from a production Leviton dimmer. I need to control 10 power zones. The Leviton is a standard format device. If I can reduce my problem to just swapping out the pot or the triac, I simplify my life. Swapping out one or two parts on 10 existing units would be simpler than creating 10 units from scratch.
AJ
 

philba

Joined Aug 17, 2017
960
It's likely the trigger circuit in the dimmer uses a threshold higher than 24, maybe a 35V diac. It all depends on the dimmer circuit. I personally would just build a lower threshold TRIAC dimmer circuit rather than crack open a dimmer and try to figure it out. Lots of possible circuits out there.

Is your goal to drive multiple circuits off of one step down transformer? Is there anything special about the step down transformer?
 

MaxHeadRoom

Joined Jul 18, 2013
21,393
Well I think this version my be more 'fancier' than you need, it uses a full wave bridge and a SCR in place of a Triac, also the BJT needs replacing with a more modern diode or Diac, the rectifier/SCR circuit is slightly better than the Triac due to its non symmetrical firing.
Max.
 

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Thread Starter

AJArena

Joined Feb 11, 2018
8
Yes, the goal is to drive several power zones from one transformer. Two transformers, in all, will be required. I already have these; and they aren't cheap. So reworking multiple $6 dimmer switches is more attractive than buying 8 additional transformers. And yes the additional transformers would be smaller than the ones I already have but still a cost I wouldn't mind avoiding; not to mention the additional real estate the 10 versus 2 transformers will require.
 

cmartinez

Joined Jan 17, 2007
7,080
I personally would just build a lower threshold TRIAC dimmer circuit rather than crack open a dimmer and try to figure it out.
Me too. The use of a triac dimmer is probably simpler, but a PWM control would be better suited for this application. Triacs have a minimum amount of current that must be applied to their gates for them to work, and that gate suffers from hysteresis. This results in "jumpy" behavior when starting the device.
 

philba

Joined Aug 17, 2017
960
Me too. The use of a triac dimmer is probably simpler, but a PWM control would be better suited for this application. Triacs have a minimum amount of current that must be applied to their gates for them to work, and that gate suffers from hysteresis. This results in "jumpy" behavior when starting the device.
Interesting, even "logic level" triacs?
 

MaxHeadRoom

Joined Jul 18, 2013
21,393
Thanks Max,
That could be helpful. I am hoping to just swap out a couple of parts from a production Leviton dimmer. I need to control 10 power zones. The Leviton is a standard format device. If I can reduce my problem to just swapping out the pot or the triac, I simplify my life. Swapping out one or two parts on 10 existing units would be simpler than creating 10 units from scratch.
AJ
If the transformer is large enough you only need one and one large bridge.
It states extra smooth starting and stopping!.
Max.
 

Dodgydave

Joined Jun 22, 2012
9,413
Hi Everyone, I have a hobby related question. I have used standard light dimmer switches on the front end of step down transformers to run my Lionel train. This has worked quite well when I set the train up for Christmas and run it all around the living room; one train on one power circuit. Now I've decided to run larger a larger set up with multiple trains and multiple power zones. It would be nice, and cost effective, to put dimmer switches on the 24 VAC side of the transformer. The standard dimmers don't work at this reduced voltage. What do I need to change on the 120VAC dimmer switch to have it work at 24 VAC?
You can modify the standard mains dommer to run on 24V ac, the diac will need to be changed for a lower voltage.
 

Reloadron

Joined Jan 15, 2015
5,646
Since this is Lionel (or similar) O Gage running on 24 VAC (MAXIMUM) you could come off one large 24 VAC source (transformer) and using that 24VAC get some low voltage AC dimmers. Check out the Applications setup in the link.

Now this figures as the ZANE 2-Wire Low Voltage AC Dimmer, 10A, 12V (AC2-10AL-12V) is the 12 volt flavor on Amazon and Not Available so that sucks. Everywhere I look the 24 volt model is sold out, Guess we need another boat from China?

Ron
 

Thread Starter

AJArena

Joined Feb 11, 2018
8
That's been the problem. I can't seem to find 24 VAC dimmers. That's why I'm trying to inexpensively make about 10 of them.
 

philba

Joined Aug 17, 2017
960
The Leviton Dimmer does not use a DIAC. The device is pretty simple; 3 caps, 3 resistors, 1 pot, 1 choke, and 1 Triac
Take a close look at the resistors and caps, one of them could be a DIAC. If not, then the TRIAC is probably a Quadrac (integrated DIAC).

You got a decent picture of the PCB?
 

Thread Starter

AJArena

Joined Feb 11, 2018
8
Not being an electrical design engineer, you guys are teaching me a lot. Based on the online search following your input, it looks like I need a Quadrad with a VboMin of something in the 2 to 6 V range.
 
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