24 Volt Dimmer

[Matt Durkin]

Hi All,

I'm about to buy a 24V AC transformer to run a low voltage supply into my
back garden. This will need to supply 24V AC to a pond pump, pond lights,
pond mister maybe, and additionally I want to run 4 x 24Volt, 25Watt bulbs
from the supply with some kind of dimmer so as not to blind everyone and to
give the garden a nice atmosphere!

Does anyone have any suggetions how I can take the 24V AC feed from a
torrodial and create a variable voltage ~4Amp supply for the lights?

I posted this on .basics but have had no response. Can anyone here help me?

 

[Tim Mitchell]

I posted this on .basics but have had no response. Can anyone here help me?

Use a separate "dimmable electronic transformer" for the lights, then
use a standard wall dimmer to feed the mains power to it.

Make sure that the dimmer is compatible with low voltage electronic
transformers and that the transformer itself is dimmable, some aren't.

 

[John Woodgate]

I read in sci.electronics.design that Matt Durkin

Does anyone have any suggetions how I can take the 24V AC feed from a

An ordinary light dimmer circuit will work, except that you can't use a
35 V diac for triggering the triac. Use a low-threshold trigger circuit.
Obviously, the triac needs a current rating much higher than 4 A to
allow for the inrush current of the lamps, but 20 A triacs are freely
available and cheap. Just check that 20 A is enough.

 

[CFoley1064]

Subject: 24 Volt Dimmer

From: [email protected] (Matt Durkin)
Date: 7/6/2004 5:45 AM Central Daylight Time

I posted this on .basics but have had no response. Can anyone here help me?

Hi, Matt. There are two basic problems with using a standard lamp dimmer with
24VAC. The first is that the triggering devices that are still made have
trigger voltages starting at 27V, which means tour AC waveform will be at
around 60 degrees before the diac triggers.

http://www.teccor.com/data/Data_Sheets/E8Diac.pdf

The second problem is that most of these simple dimmer circuits have turn-on
hysteresis, which will drive you crazy with 24VAC. You will have to turn the
dimmer control up to get turn-on, and then turn the dimmer control down to get
to the lower voltage. If there's a line voltage sag, the dimmer turns off.
Don't go there.

On another s.e.d. question, someone mentioned the Velleman K8003 kit, which
provides opto-isolated voltage input for a dimmer circuit. They're about $25
USD each from various hobbyist sources. If you look in the manual for the kit
(assembly required), you see that it has components for 120VAC as well as 24VAC
operation. All you will then need is a voltage source for the input, which you
can do like this (view in fixed font or M$ Notepad):

Zener 10V Power Supply
.--------. 1K Ohm 1 Watt
| | ___
o----o ~ +o---o----|___|----o---o
| | | | |
| | +| 1N4740| .-. 1K pot +
24VAC | | --- Vz = 10V/-/ | |<-------o
| | --- ^ | | Input to
| | |470uF 50V | '-' Velleman K8003
| | | | |
o----o ~ - o---o-------------o---o---------o
| | -
| |
'--------'

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

http://www.velleman.be/Downloads/0/Manual_K8003.pdf

Good luck
Chris