How would you simply and safely sync to AC (50/60Hz)?

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blueroomelectronics

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I got to thinking about a project for the Firefly, I wanted to sync to the mains (AC) and was wondering what would be safe and easy.
Examples a simple coil of wire around a power cord.
Or CDS or phototransistor to sync to fluorescent lighting.
A scan line on a tube TV?

Non contact with AC preferred...
 
Since you'd prefer a contactless signal, the coil idea sounds good to me. I've used it in the past.

Otherwise, you'd of course get excellent results with an opto as well.

Remember you will have to wind the coil around only one of the conductors in the mains cable. If you do it around the whole cable, you'd get no signal.

Alternately, why not just tap the signal off your power supply before rectification? Or use another small mains transformer?

Regards,

Anand
 
I take it it's battery operated, if not then use the unrectified side of the power suply.

You don't even need to have a direct connection to the mains, you can create a resonant aerial using a loop and capacitor to tune it to about 55Hz, it shoud then recieve the mains signal, you could also use a long piece of wire connected to a high impedance amplifier and band pass filter.
 
If you wind a coil around the whole mains cable you will get a huge signal. Unless the mains AC comes from an ungrounded alternator.
The neutral wire in a mains cable is supposed to be grounded, so the mains cables become very unbalanced and radiate hum all over the place.
 
It is battery operated, and designed for newbies, students and hobbiests.

I'll try the coil first, and see if I can get a usable signal on a comparator.

The light seems more fun, 60Hz is slow, I wonder if a CDS is fast enough.
 
The only problem with syncing to lights is that in most commercial premises they use 3 phase and alternate the lights therefore eliminating flicker. Schools (unless extremely small) would use this lighting system.

Mike.
 
Compact fluorescent light bulbs flicker at 40kHz. You don't want to sync to it.

Ordinary fluorescent tubes flicker at 120Hz, not 60Hz.
 
Filament bulbs flicker at 120hz as well, but it's just a dimming not an outright black out so no one ever notices it.
 
120Hz would work fine. I'm looking for a somewhat accurate timebase to calibrate a clock on. It's for learning more than any real practical purpose.
 
I haven't actually tried passing a mains cable through a coil, but if a current is induced in it, it implies that there is a return path that bypasses the cable. This would trip the breaker in a GFCI-protected circuit. In homes in the US, neutral is not supposed to be connected to ground except at the main breaker panel.
 
The coil is portable and its common is referenced to ground. The mains neutral wire is also grounded so the live wire radiates mains hum.
Maybe the radiation is electrostatic (voltage), not electromagnetic (current). Surely some electrostatic radiation will feed the coil through capacitive coupling.
 
I suspect you are right.
 
Capacitve coupling at the distance the insulation presents is going to be extremely weak compared to the inductive coupling, and it's not a high frequency. A zener diode and a high voltage high ohm resistor can safely feed logic level signals as long as the power is kept to within the zeners rating takeing into a count possible voltage spikes. Atmel has an appnote that uses an AVR's internal clamp diodes and a single 1 meg resistor rated at the mains voltage to do zero detection. The same thing should apply to any PICS with internal clamp diodes, or to a discrete zener.
At normal 120volt mains a 1 meg resistor will limit the max current to 170uamps and will not damage the AVR chips clamp diode at least if the current is kept bellow 1ma, which means it could sustain a few spikes of 1000 volts.
 
Sceadwian said:
Atmel has an appnote that uses an AVR's internal clamp diodes and a single 1 meg resistor rated at the mains voltage to do zero detection. The same thing should apply to any PICS with internal clamp diodes, or to a discrete zener.
Click to expand...

Yes, there's a very old MicroChip application note that does the same thing, presumably Atmel 'copied' a number of the MicroChip notes as they were aiming for the same market?.
 
A resonant LC circuit with the inductor having as larger crossectional area as possible will give the best results. You can find the formulae for winding inductors and resonance from Google.
 
Very likley Nigel, Atmel's not known for their highly detail oriented document people =) Half their PDF's are copy and pasted from earlier document releases and not very well controlled. If I were a professional embeded hardware engineer I probably wouldn't recommend them as often as I do. But as long as you do you research they're very solid chips, and on average I think the entire architecture is a little more streamlined than PICs are, but they are going through some rough times right now.
 
Sceadwian said:
Very likley Nigel, Atmel's not known for their highly detail oriented document people =) Half their PDF's are copy and pasted from earlier document releases and not very well controlled.
Click to expand...

Must be a common failing then?, MicroChip are well known for errors in their application notes and datasheets!

If I were a professional embeded hardware engineer I probably wouldn't recommend them as often as I do.
Click to expand...

There's nothing wrong with Atmel products, and you do see the occasional one used in commercial products.

but they are going through some rough times right now.
Click to expand...

I didn't realise that, but they have always struggled a bit, it's hard to try and take market share from a company with such a huge lead.
 
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