Igbt vs triac dimmer

[aaronik19]

I am planning to construct a dimmer and on google i read that a transistor dimmer is much better than Triac especially when you have small load e.g <20w? Is this true? What kind of transistors? Igbt? I am asking since i read this on an forum page and the comment had no further replies.

[Mike Walne]

Depends on what you are trying to do.

Please enlighten us.

Mike

[temtronic]

MikeW is right, we need more information!

What voltage are you trying to control 12VAC,120,240,347 ???

Do you need zero crossing ?

How about opto-isolation?

Frequency ? 50Hz,60Hz,400Hz,user defined?

type of load? light bulb(incandescent),LED,solenoid valve?

range of control ? linear, log, antilog ?

feedback required ?

A 'simple' 8pin PIC can do the job....for 50C....

but we need more input from you !

hth
jay

[aaronik19]

Thanks for reply.

I am working on 240vac 50hz
With opto isolation.
The load is about 5A electronic halogen tansformers.

I am trying to find again the forum but for some reason i cant find it on google again.

[Ttelmah]

Er. Well 240v 5A, is 1.2KW. Hardly <20W. Unless you mean they are 12v 5A output (most halogen lamps are 12v). Still well over your 20W (60W).

However transistor dimming is commonly used at much higher powers than this.

There are though a lot of questions. Many 'electronic transformers', cannot be dimmed at the input. Trying to do so, can result in them overheating. You need to do some significant research on the units involved. The commonest designs are forms of capacitive switch mode power supplies, which in some cases will automatically increase their 'on' time to compensate for decreases in the incoming voltage, to regulate the output. Also, feeding switch mode circuits from thyristor/triac dimmers can result in very nasty current spikes, which is why a lot will carry warnings that they must not be fed from such dimmers.
You need to start by verifying that your units support input dimming. There are three types:

1) Non dimmable
2) Types described as 'dimmable with standard incandescent
dimmers'.
3) Types requiring special dimmers.

Now the problem with standard thyristor/triac designs, is that once the 'switch' is made, it can't be turned 'off' till the power disappears (there are exceptions, with GTO thyristors, but these get more complex). Hence dimming is done by changing 'when' the switch is turned on in the mains half cycle, and then switching it off at the next 'zero crossing point'. Turning the power on mid cycle, gives nasty RF spikes. So when dimming things that don't mind having power removed for a short time, you instead use 'zero crossing', and turn the switch 'on' for single half cycles. Problem then is that as you dim further, there are longer and longer 'gaps' with no power applied, and the tendency to flicker. Transistor switching allows you to instead use 'reverse phase' dimming, turning the power on at the zero crossing point, and leaving it on for as long as needed, then cutting it mid cycle. This reduces the tendency to RF interference, and the peak loads through the components.

Type 1 - forget it.
Type 2 - can be dimmed with a thyristor dimmer, but with very significant risk of buzzing, and flickering and interference.
Type 2/3 - generally dim with a reverse phase dimmer for best results.

Have a look at:
<http://www.epanorama.net/documents/lights/lightdimmer.html#reverse>

Which gives a comparison/history etc..
You will find example reverse phase circuits on the web, but remember 'think safety first'. Many web published designs are not fully competent.

The reverse phase designs are much more likely to work well, but you are going to need to do some _careful_ experimentation.

Best Wishes

[aaronik19]

Dear All,

I found this document published from ST Micro controller which might interesting you. http://www.st.com/web/en/resource/technical/document/application_note/CD00003820.pdf