Low Voltage Landscape Lighting

Less cost for the adapter and the LEds do not care, they just go out for 1/2 cycle, which lets them cool too.

You can also switch the source from AC to + or to -DC and have a selected set of diodes on each leg light up. With AC they all light with 50% duty cycle...they eye does not see it.

well the problem i faced when i used Low Voltage DC for land scape lighting was Voltage droops and cable sizing.ultimately i was forced to change over to Low voltage AC.

Ultimately an AC power soure is cheaper than a DC power source. Many of those are running transformer mounted on an outside location with a timer and a photo cell to a relay for switching and quite expensive. Any money that can be saved is a plus. The few dollars per unit on a 1000 units is 3000$. AC has better charateristics when running the lines underground anyway. It's a high current setup and DC is going to give you a little loss but it doesn't take much with 12V. It's low voltage because it's safe and it's 12V because there are may bulbs to choose from.

Brian

Brian

are you still trying to sell that capacitor bank supply, let me know i will offer $1500 if it is in working condition magnets_ieee.org

Jim

I would say the answer to your question is one of pure economics. For AC all you need is a transformer. For DC you have to add a rectifier. Depending on the current, you may also need a heat sink.

It comes down to manufacturing cost... You have line voltage AC, you have to reduce voltage for safety, and all you need is a transformer.

There is also the reliability of just a transformer as opposed to a transformer and recitfiers...

Plus of course a transformer alone can withstand a bulb blowing i.e. taking a short term overload as the filament melts and shorts etc... Whereas with silicon rectifiers they will blow very quickly under those conditions!

If the lights are used outdoors as you say, I'm sure that water ingress into the lamps is inevitable, so having DC would cause eletrolytic corrosion whereas with AC this is avoided...

John.

electrolytic corrosion still occurs with AC and can be worse.

How can that be Aurizon?

Surely with DC and water as an electrolyte, there will be an anode and a cathode to remove material and to plate onto...

With AC there is no anode and cathode to move material between?

John.

Cooper wires and connectors + misture + AC (or DC) current = blue sulphat in contacts. I dont know why. But is real.

if there is water between two electrodes and you have a DC potential between them above a certain threshold(which varies as per electromotive series) you will have the potential for corrosion. If you have platinum parts, you will get electrolysis of water with stray contaminants providing conductivity.

With assorted metals you will get deplating of one to the other, again with stray ions providing the electrolyte at first. There may also be some electrolysis of water(at 12 volts there will be)These reactions are not fully reversible, so if you use AC you will get the reaction going one way and then the other, and any electrolysis products will escape and not reverse. The mix of nascent oxygen alternately at one electrode and then the other will oxidize most metals = corrosion

Hmmmmmmmm not sure I agree with that...

Using AC will certainly result in less electrolytic action than using DC.

Also, besides the ability of a transformer to withstand a filament overloading and blowing in a light bulb (which I think is the most important reason to use AC). There is also the longer bulb lifetime when using an alternating voltage.

I'm sure we are all aware that a bulb's life is considerably shortened using DC aren't we?

John.

well the current and temp spikes are more severe with AC, at the peak this leads to excess mobility of the metal at the surface = more evaporation. At the ebb it leads to a cooling and shrinking cycle, so the life on AC may well be smaller than DC?

I will hunt for that

OK divergent answers. It depends on how close to the melting point you run the lamp. Higher temp AC is better because the unidirectional electron bombardment causes 'steps' to form. Valleys in the steps evaporate faster = earlier failure.

A lower temperatures the steps do not form and the repeated mechanical cool up/down stress causes earlier failure on AC.

ON halide lamps with the rebuild deposition taking place on the hottest spots, I expect this to mitigate the step formation, and this will lead to DC lasting longer.

Hopefully the DC LED 1156 Red,Green, Blue, s l o w color changing bulbs we sold will last longer than the 18 watt incandescent that were existing... If the moisture doesn't get them, and the power supply lasts well. :o)

Hey folks,

I REALLY do appreciate all the astute comments offered here!

Don Campbell

Campbell Lighting

There's one more point, Filament (incandescent) lamps live longer on AC than DC due to different type of emission, and better (lower) filament oxidation index.

I don't think this is the reason, but some more info can't hurt...

Wangito.

Thank you wangito, that's what I said....

I don't think it was agreed on though, but in my light bulb catalogues the manufacturers do state that for the equivalent lifetime when using DC the voltage should be reduced to 90% of rating or even less etc...

When I use filament bulbs in an instrument I always try to keep a 'keep alive' current flowing through the filament to extend the life by 2 to 3 times....

John.