PWM low voltage AC

[RustyH]

Some time ago, I investigated (via this forum) the possibility of PWM controlling a 240v LED bulb, in the end I decide to scrap the idea as I was not competent in messing with 240v

So Im not thinking of just using a 12v AC bulb instead. My question is, is it difficult to control 12v AC via PWM? Would Pulse out work better for AC? I assume it doesn't need to involve Triac circuitry (trying to sound like I know what Im on about there!!)

 

[SAborn]

Perhaps you can show me what a 12v AC bulb is, and why it wont work on DC.

I did not think a bulb was a polorized device, other than a led bulb which is a DC device anyway.

 

[Paix]

Can you post a basic diagram to indicate what you are thinking about RustyH? PWM is effectively AC with a variable frequency and a variable duty cycle which varies the effective brightness of the LED or lamp.

If you are thinking of somehow supplying a 12V AC and then applying PWM to it, then you either need your bumps read or to explain quite what you are thinking. You would be producing a modulation of some sort but not something that is likely to be particularly useful. Don't keep the brains trust guessing, a diagram is worth around 1000 words and can cut to the chase by exposing misconceptions or oversights quite quickly.

 

[hippy]

Don't keep the brains trust guessing

What you want to achieve in controlling the lamp would be a good starting point. Do you simply want on and off control or are you wanting brightness control ?

DC control would be easier than AC as with AC you will likely need phase control ( PWM synchronised with mains crossover ) and probably need a triac or similar. AC control would usually be the same whether 240V AC or 12V AC, just that 12V is safer to play with.

 

[SAborn]

Back to your original thought of driving a 240v led bulb, if the demand of the led light is low enough, and can cope with a square wave 240v, then simply driving a 240 to 12 volt transformer backwards, off a 12VDC supply with 50 Hz PWM should work. ( dont think PWM goes that low)

If you feel uncomfortable working with higher voltage circuits (240 V) than best dont try it. (it can kill you!!)

 

[RustyH]

Thank you for all your replies.

I'm looking for a basic on and off, BUT I want a fade on and off, so hence thinking about PWM.

The bulb is a standard MR16 LED bulb - http://www.sourcing4u.co.uk/shop/product_info.php?products_id=132

However, I think I may have made an error that has led to this thread. I had bought a LED driver that converted 240v AC to 12v AC, and that's why I was asking if I could PWM 12v AC. But I have just come across this which says it transforms from 240v AC to 12v DC which would make like alot simpler - http://www.amazon.co.uk/MR16-MR11-G4-Long-Life-Lamp-Company/dp/B003DV8QBS

I guess I had assumed that while LEDs and DC devices, the house hold bulb systems required AC. If this is not the case, I can simply buy a standard regulated power transformer??