TRIAC circuit to control PW

John

I thought dimmers used DIACs. Any off the shelf dimmers that work on low voltage from the transformer secondary?

Dale H

Or fan/motor speed controls?

That would be wire wound potentiometers

But anyone use a variac for voltage control? I think if you pay attention to the maximum current allowed, a variac would work on low ac voltage outputs.

The DIAC is just the trigger for the Triac in the dimmer.

Here's a page that describes how they work: DIACs and TRIACs

It is but I think he's implying that it takes a bit of voltage, maybe 12 vac for the diac to trigger the gate. So the circuit wouldn't work on a low ac voltage secondary.

I was just answering Dale's question, not commenting on the viability of the DIAC for low voltage control.

Years ago I looked into this and seem to remember it was concluded that a dimmer circuit would not function if the input was off the 24vac secondary.  Sorry I have no more detail than that.

i am really learning a lot about not only this specific application but other tertiary ones as well. 

I am am grateful for the participation and everyone sharing ideas.

i can see where this can help everyone!

A Variac would work fine.

I used one on my original layout to limit the top speed.  Plugged a CW-80 into the variac, set the variac for about 80 volts, and the CW80 had a lower top speed.

A variac is a sine wave transformer that can feed another transformer with no issues.  No electronics involved, just a "variable" transformer.  So, a 24V transformer plugged into a variac set at 75%, would deliver 18V.

Ed

In one of my designs I used FETs so I could turn them on and off anytime during the sine wave unlike triacs which can only turn off on a zero voltage. I would start the power on around the center or peak of the sine wave alternation at 4.2 ms. So the pulse  would come at let's say 4ms and go off at 4.5ms. The power would be a half ms but since it's around the peak it's applying 24 volts. Increasing power would add time to each side of the pulse, ie 3.9 to 4.6 ms. The pulse shape is not as severe as a chopped wave as it is always centered around the rounded peak. And the higher voltage produces more motor torque than a chopped wave which helps at low speeds. 

I like that idea.

Hi all

Here is something from another area of the forum

It is a demo of a PSX1 circuit protector

note the use of a fixed output transformer

I just want to duplicate this with a potentiometer control rather than TMCC

forgot the link:

https://ogrforum.com/t...t-circuit-protection

Here is an interesting circuit... any comments??? If this works for higher voltage I assume it can be adapted for 24vac input.

http://www.electroschematics.c...egulator-with-triac/

Hi Ed

My understanding is that transformers designed for 120v invputs dont work well when the input is cut dramatically.  I am pone to comments on this but thought that hysteresis losses would cause overheating.

but it looks like lionel is designing with this in mind

See this video:

http://www.youtube.com/watch?v=1Ilj3p-Ov3c

Stan... here is a link explaining exactly about what you mentioned!

http://www.youtube.com/watch?v=1Ilj3p-Ov3c

This probably applies more for modern stuff, but it clearly is applicable

Lionel is saying... Yes.. we chop up the sine wave and yes we intend to.

thank you so much for sharing this.. I would love to learn more if you can help explain how I could do something like this.

I appreciate all you are sharing.

Chuck pointed out earlier that the DIAC requires significant voltage to trigger, perhaps 12 volts?  That might limit the usefulness of this circuit for train voltages.

In my circuits I use PICs which are microcomputer chips. For this circuit I used an 8 pin device (cost under a buck) that only requires gnd, 2-6 volt power, and up to 6 inputs/outputs. It has an A/D convertor for the pot input. I know this is beyond what the average hobbyist can handle but if you can write software this is the way to go. The only cost is for the programmer but there's info on the net on how to build one or buy one cheap -$25? The MPLab development system is downloaded for free. You can use the assembler language or download free CLite (C++).

As far as a smoke unit delivering more smoke I think that falls under the I2R law (I squared R). A smoke resistor at a certain voltage would have a certain amount of current flowing thru it according to Ohms law E=IR and a certain amount of power P=EI or P=I2R. If you double the voltage you have double the current but then the power is quadrupled. 

I think some of us "might" be able to handle it.   I used a uP to generate 400hz synchro signals for air data computer outputs, so varying a voltage with a triac doesn't seem all that difficult.

Koan,

Do you have a reference regarding 120 volt transformers not working properly with lower input voltages?  I drive Lionel 180 watt bricks with Variacs to get 12 volts, among other voltages and it seems to work fine, but maybe I'm missing something.  Also, trying to think through the role of hysteresis in this.

A 180 watt brick puts out 10 amps at 18 volts.  If you lower the input voltage to get 12 volts out, then at 10 amps you now have a 120 watt brick, which I don't think will cause overheating.  If you try and get 180 watts out of it at 12 volts by going over 10 amps out, then you may have a problem.  Not trying to argue here, but I really want to be sure I'm not missing something.

 I'll see if I can dig it up... Thank you for contributing... I could easily be incorrect .. Or maybe i was thinking originally of driving the variac off the secondary of the larger fixed transformer.

Thanks, will appreciate it if you come up with something.  Raising the voltage is clearly a no no, because the core will become over saturated, among other things, but I keep coming to the conclusion that lowering the voltage simply turns the transformer into an over designed device, with lower hysteresis losses.

You won't find a Lutron product rated for less than 120VAC input.  I can't confirm the reliability or performance at lower inputs.  Most of the digital ones have power supplies inside that run the micro.  The charging cap for those power supplies needs much more than 24VAC to run the supply.

I'm going to do some bench experiments with some MLV dimmers and cheap 24-volt transformers on the output of the dimmer.  I'll report back.  It won't be the cheapest solution, but the dimmers I want to use have RF input and I can create cool "scenes" with my layout. Imagine a keypad like this with buttons named "morning commute" and "workday freight"