Help with an AC choke/inductor

[dc11]

I'm experimenting with simple security timer light switches for 12v AC bulbs.

I only have access to the 12v hot line (not neutral) and want to get about 10ma current flowing to power the timer circuit which will switch a triac on to let the full current flow to light the 12v bulb.

My understanding is that I can use an inductor and capacitor to 'choke' the current and then the triac can open a parallel circuit but I can't find any circuits or formulas with more information.

Can anyone help me with the concept here?

Thanks,
D.

 

[(*steve*)]

You'll have to explain what you want to do. Your explanation of how you're going to do it sounds like you misunderstand capacitors, inductors and triacs.

What will turn the lights on?

What will turn the lights off (e.g. time delay)?

 

[dc11]

Attached is a simple back-of-napkin concept. As mentioned, I don't have access to the neutral so I need to 'leak' 10ma from the live line in series with the bulb, This small amount of current won't light the bulb.

The timer ciruit is unimportant but it will probably be PIC based and will turn the TRIAC on at specified intervals. I'm happy with the PIC part, the timer part and driving the TRIAC.

I see this as 2 parallel branches: one for the PIC drawing 10ma in series with the 12v bulb. The other is the TRIAC in series with the bulb allowing sufficient current through to light the bulb when the timer circuit is driving the triac.

How can I draw this small current from a 12v AC line in series with an incandescent bulb?

Thanks,
D.

 

[(*steve*)]

The simple approach would be to place a bridge rectifier across the triac and use this via a current limiting resistor (if required) to charge a capacitor.

You need enough charge to operate the timing circuit for as long as the lamp stays ON and enough OFF time on the lamp to recharge the capacitor.

A current limiting resistor may not be necessary if the slightly slow turn off of the lamp or the initial flash when power is applied can be ignored.

You will probably have to optically trigger the triac unless you can place an SCR across the output of the bridge rectifier (you will get a greater voltage drop though).

 

[Resqueline]

You have to include more information about the timer circuit. What is its operational voltage range for example? And if not AC does it have provisions for rectifying?
You can use an inductor, perhaps not a capacitor in this case, but the current & voltage is so small that I'd just use a resistor to avoid complications & issues.
R=U/I where U is the voltage difference and I is the current. But then there's the question of half- or full-wave rectification.
But, one problem with your circuit is that as soon as the triac is triggered the timer circuit looses its power supply.
That is unless you use phase controlled trigging to leave a voltage (spike) across the triac at all times.
If not (and with no battery backup) then you'll have to use the attached circuit solution.

 

[dc11]

Thank you for your suggestions. I'm going to try to prototype a working resistor-based solution with a rechargeable battery to keep the PIC alive whilst the bulb is switched on.