I recently bought a house that has low-voltage outdoor Malibu lighting (by Intermatic). Currently it has a 200w power supply to power 10 tier lights and 5 20w flood lights. The setup is as follows, power supply ->
3 spot lights -> 10 tier lights -> 2 spot lights. The strange thing is the first 3 spot lights in the circuit are much brighter than the last two spotlights (in relation to the the power supply). All the tier lights seem the same brightness. My question is, why are the last two spotlights so dim? I would assume that if the power supply was not strong enough, all the lights would be dim. Do halogen bulbs dim as they are burning out? Could they be different bulbs that the brighter flood lights (the fixtures appear identical). Any suggestions?
The wire connecting the lights of adequate size? No matter how many watts the power supply is capable of delivering, if the wire is sized too small the resistance of the wire will cause a voltage drop and the lights at the end of the line will not reach full potential.
Malibu makes bulbs of different wattage for different sets. They look exactly the same. Only the small number printed on the bulb can indicate the difference. Yours may or may not be readable at this point in time. You may in fact have a lower wattage bulb at the fixtures on the end of the run.
As for your power supply, I think an 80% load is the recommended. I do recall that there is a minimum and a maximum and you should be somewhere between the two. Even if I am wrong about the % I do recall that all the bulb wattages can not exceed the % of transformer rating. I'm not sure you are correct about an over wattage reducing the brightness of all the bulbs. It might only affect the ones on the end. I just can't recall.
You didn't mention wire length, but that's probably what's causing the voltage drop (=brightness). Use a 15V AC voltmeter and read the voltage at the lamp closest to the transformer, and at the lamp farthest.
Could be several volts difference (drop) if it's 100 feet apart.
Since some lights are ok, and others are dim, the transformer is doing fine. Check the bulbs by putting a dim bulb in a socket that had a bright bulb, if it stays dim, its the bulb, if it gets bright, its a wire problem.
solutions: put the transformer near the middle of the string, use larger gauge wire, put every other light from the old wire on a new wire that also goes back to the trasformer.
Glad that helped, and I remember waiting for dark after installing a string of 15 lamps with 4 watt bulbs along my parents home walkways. (Gettysburg Pa area)
The problem they had was the bulbs only lasted a month. I placed one leg of a 20 amp bridge rectifier in series with one of the wires connected to the lamps from the transformer. The lamps drop to about
1/3 the brightness, but they last a year now. At night they still do a great job lighting the walkways. The real surprise was the first snowfall, they looked cute glowing with the snow around and on them.
The last two spot lights at the end of the line arent' getting 12 volts because of the power draw from the other lamps on the circuit and because the maximum resistance of the wire is between them and the power supply. You can check this by measuring the voltage at the power supply and then measuring the voltage on each lamp down the line. The voltage available to each lamp down the line will be lower.
Your system is designed to operate on 12 volts for several reasons including safety. But by going to a low voltage the current in the wires has to go up to supply the same power to a light bulb. For example, a 20watt floodlight draws only 0.2amps. on 120 volts; but a 20watt 12 volt bulb would have to draw 1.2 amps for the same light output.
A simple solution would be to get a length of liine equal to what you have now. Connect one end to the power supply, then run it along the existing line to the end and connect the ends together. Be sure to connect like wires to each other or, in other words, observe polarity. This is called a "ring" or "loop" circuit. It probably won't bring your last two flood lights to full brightness; but again check the voltage at the lamps. The extra wire should raise the voltage available to the lamps at the end of the line considerably.
Halogen bulbs do not dim as they burn out; in fact, halogen bulbs keep their brightness better than conventional incandescent bulbs over time.
It's certainly possible that different wattage bulbs were put into different sockets along the line as bulbs burned out; but that's easy to check. The wattage is marked on the bulb -- usually on the back in the case of a flood light bulb.
Makes complete sense. You are describing a classic voltage divider configuration. More resistance on the close leg means higher voltage drop relative to the final leg.
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