Some ideas for a 24VAC dimmer?

[fish2005]

All dimmer is designed to use on main power {eg 220VAC} and is imposible to control a 24VAC load. I found on net manny DC dimmers for low power but no for AC. I need a schematic for a simple 24VAC dimmer. I mention that i intend to use this for a 24VAC 48W 2Aheater element.
Please help.

 

[audioguru2]

A heater doesn't care if the power is AC or DC. Use a 24VDC light dimmer or motor speed control circuit.

 

[Sukhbinder]

i agree with audioguru that the heater element can work out with both AC or DC.
use any simple dc dimmer or motor controller circuit as suggested by audioguru. u can use a PWM based curcuit for it.

 

[Alun]

True, but it is possible to build a 24VAC dimmer with a triac, I've done it before but I can't remember what resistor values I used.

You can build a 24VDC dimmer with a 555 timer or op-amp.

 

[ante1]

Do you remember what kind of diac you used?

 

[Alun]

I didn't use one, you don't need a diac for such low voltages as it'll only trigger on the peaks so the controller will never achieve anywhere near full power.

 

[ante1]

I know that’s why I asked because I like to see how you managed the trigging! ;D

 

[Alun]

I've just got out my old college notes and we didn't use a triac we used an SCR so it was only half wave, we use a normal 1n4001 rectifier at the gate for trigering, you could use a triac and two 1N001s in reverse paralell.

Here's the schematic redrawn for a triac, I can't promise that it'll work because I haven't tested it and you may need to change a few values.

 

[audioguru2]

My son got a timer-switch for the light in his room to wake him each morning. The amplifier for his alarm clock I made for him didn't wake him (it woke everyone else in the neighbourhood) and this timer-light didn't either. He got a shock trying to install it himself.

Now he has a remote dimmer for his light. It doesn't help wake him either but he thought it would save electricity.
I had to replace the low-power compact florescent bulbs with high-power incandescent ones to allow dimming, but they glow dimmly 24/7 all the time they are "off".
I wonder how much power the cold filaments use since when cold their resistance is very low.

The dimmer doesn't turn the lamps completely off in order to gradually increase the brightness. Ordinary dimmers turn off the lights but need to be turned up pretty far before the lights begin to shine. ;D

 

[fish2005]

Thank you guys for your answers. From youre debate I understand some important things.

1. A DC power supply is fine for this job. This is verry good because I can use my 12VAC transformer whith a voltage regulator and a NPN current booster. If i use a ajustable voltage regulator, the problem of dimming is resolved. Please corect me if I am wrong.
Question: In my knowlege, 24VDC is not equivalent whith 24VAC. What is the equivalent DC power for my iron specification.

2. This thread is a backup solution because I dont know if my project for a temperature controller will be a succes. This is my first major project.
I orderred (today or tomorrow is delivery)this iron.

This tipe of iron (24VAC, 48W, 2A) is sell as a spare part for a astronomical priced soldering station and have an integrated thermocouple. I intend to make a controller whith UAA1016 and a triac for this iron and save money.
In UAA2016 datasheet is a simple circuit for this, but this circuit is designed for NTC tipe sensor.
http://www.onsemi.com/pub/Collateral/UAA2016-D.PDF

I find on net this circuits related to a homemade soldering station:

This is the tested schematic for a NTC tipe sensor:
http://tinypic.com/view/?pic=ekm6mv

And this is the adaptation for a K thermocouple tipe iron made by same person and tested whith succes by others:
http://tinypic.com/view/?pic=ekm6o4

My schematic is the combination of these

...but I'm not sure if everything is correct conected. If you guys wiew any errors please corect me. I repeat: this is my first major electronic project.
I dont know the wattage for resistors. This iron comes in 2 diferent models, whith 2 specification for K thermocouple (7-8 mV at soldering themperature and 20-25 mV) , so require an adaptation of opamp resistors.

So guys, if enybody is interested or want to help a novice....
I post new thread for this subject: TEMPERATURE CONTROLLER

 

[Alun]

I think this is way too complex, at 24V you don't need zero crossing as the power is to low to generate significant RF harmonics which the transformer with absorb to some extent anyway. Your best bet is to power the comparator from the transformer using a half wave rectifier with a 1000uf capacitor and posibly a voltage regulator, connect the output of the comparator to the gate of the triac via a diode and it should work.

 

[fish2005]

I have allready the UAA2016, but is logic to try the simple solution first. I dont know...