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Driving a led with mains.

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radiotech

Sun Sept 22 2013, 08:03PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Of course you could make a very nice project by connecting
electroluminescent panels in series with the LEDs Those things
present as somewhat lossy capacitors. This way even the capacitor
D will yield up some light. You have to careful with 1/2 wave on
50 Hz.It makes LEDs very annoying to look at.

Ash Small

Mon Sept 23 2013, 05:41PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

OK, I've now realized the mistake I made at the beginning of the thread.

I had the diode in the wrong place, basically.

In fact I had two diodes, both in the wrong place. I now see that what I was trying to do requires that the diode be placed between the capacitors. (I knew there was a problem with 'current flow' in my first circuits, I just couldn't see it) This then produces the 'basic' circuit that Bjorn linked to. I can now also see that, due to the reactance of the capacitors, a zener or similar over-voltage protection is required. It seems that the value of the series resistance depends on the reactance of the capacitor, C1, and that if the reactance of C1 is sufficient, the series resistor is redundant. Are my assumptions correct?

Thanks Mattski for the tip on custom components, I'll try it later. I'll do some more simulations before I wire anything up.

Radiotech, it sounds like you could do away with the LED's completely

radiotech

Mon Sept 23 2013, 06:39PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

There may be a ratio that would relate the illuminance (lux) of the LED to the electroluminescent
panel.

It may turn out that the latter is a more efficient light, for equivalent wavelengths.

Those panels were extensively used to provide lighted signs in the past, albeit ones
seen at night.

Ash Small

Mon Sept 23 2013, 07:31PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've done a few more simulations, and realise I need to know the reactance of the capacitors, so I'm going to try measuring the resonant frequency of the capacitors, and then try to work out the inductance.

I'll post again after I dig out my 'scope and signal generator.

Bored Chemist

Mon Sept 23 2013, 08:42PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I think I now understand why they still use neon lamps.

Ash Small

Mon Sept 23 2013, 09:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Bored Chemist wrote ...

I think I now understand why they still use neon lamps.

I agree, but I'm wondering if this could be useful for phone chargers, and the like.

I've also learned a lot already.

Mattski

Tue Sept 24 2013, 02:46AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Ash Small wrote ...

I've done a few more simulations, and realise I need to know the reactance of the capacitors, so I'm going to try measuring the resonant frequency of the capacitors, and then try to work out the inductance.

I'll post again after I dig out my 'scope and signal generator.

I would think that at 50/60Hz you're operating well below the self-resonant frequency, and don't really need to take the inductance of the caps into account.

Ash Small

Tue Sept 24 2013, 06:08AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Mattski wrote ...

I would think that at 50/60Hz you're operating well below the self-resonant frequency, and don't really need to take the inductance of the caps into account.

Well, if I start a simulation @240V (ie mid-way through a half cycle), with some inductance added to the capacitors, I get ringing, and too much voltage at the LED's. I've tried various values of inductance, with similar results. I've got too far into this now to risk blowing a couple of LED's for the sake of a couple of 'back to back' zeners, or whatever. Also, I'm curious.

Steve Conner

Tue Sept 24 2013, 08:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

While it might work in theory as a phone charger, in practice it would be forbidden on electrical safety grounds as it's not isolated from the mains.

Ash Small

Tue Sept 24 2013, 09:37AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

While it might work in theory as a phone charger, in practice it would be forbidden on electrical safety grounds as it's not isolated from the mains.

Thanks for the tip, Steve. I assumed that if the capacitors complied with the relevant standards, and it was fused, it would be ok,

What can these circuits be used for?......Bjorn's link implies that they do have uses, and can be used to supply uC's.

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