On Fri, Oct 12, 2018 at 05:24:10PM -0400, Mike Gordon via olympus wrote:
> The transformers seem cheap enough and most seem not to rectify the current.
> There appear to be a few electronic low-voltage transformers (ELVT) which
> are new.
>
> I always thought LED's prefer DC to avoid flicker whereas doesn't matter a
> wit for incandescent. LED's would also rectify the current?--- and perhaps
> has implications for daisy chaining the lights. I don't have many, so voltage
> drop should be low--2 zones, max 2-3 lights on each.
>
> I am not sure why the plain old 12 AC output is so common. I thought this
> gizmo looked like the ticket:
>
> http://www.kichler.com/professional/landscape-lighting/new-landscape-lighting-products/design-pro-led-controller.aspx
>
> It can easily be run by one of those Vera gizmos to control with phone
> too--though others perhaps can as well.
> Not being an engineer, thought I'd better check here.
>
> In the dark, Mike
Mike,
Are you rolling your own with LEDs and wire, tapes, or are you buying
strings (or fittings that crimp onto a wire) or a ready made solution?
LEDs run on AC (or unfiltered DC) will flicker because they only shine
whilst there is energy available to excite the electrons, Incandescents
have an energy buffer (the hot wire of the filament) so tend not to
flicker visibly.
As you suggest, LEDs only pass current in one direction but like all
diodes, they do have a reverse breakdown voltage. In the case of LEDs it
is fairly low and the reverse avalanche can destroy the LED chip, so
convertional wisdom says if using an AC supply either put a diode in
series, or pair LEDs in inverse parallel:
---+--->|---+---
+---|<---+
If you're connecting in series you need to be sure to hook them up the
right way or none will work.
Forward voltage drop across a lit LED could be 1.8V to around 3V (or
more if internal series resistors are used) depending on the chemistry
and how they are configured. in general terms LEDs will take as much
current as you can supply once they are lit up to self-destruction so
series resistors or constant-current supplies are used (lots more can be
said here if in need of an insomnia cure :) ).
As for power supply, if you're rolling your own, I'd strongly recommend
that a commercially-made authority-approved (UL, C-tick etc.) supply be
obtained for plugging into the mains- just one less question to be
answered if anything happens. A good isolated switchmode or a proper
transformer supply, especially given your application would be prudent,
it would a little antisocial to have your garden LEDs having any direct
reference to the mains hot side. A chat with a local electrician may be
in order!
The security industry used to supply really nice internally protected
16VAC/1A transformer wall warts - I used those a lot 20 years ago for my
own projects.
The thing you linked to looks nice :)
LED suppliers these days sell electronic 'ballasts'. They are usually
sold for a specific string voltage with a nominal "watts" rating.
Beware, these things can be anything from nicely designed and safe
switchmode power supplies, through to horrible capacitive droppers,
that essentially connect the LED to the mains wires without isolation,
have a ratsh*t power factor and sometimes explode.
hope this helps.
davidt
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