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Discussion in 'Landscape Lighting' started by n2h20, Jun 8, 2005.
Here's a neat website I found that may help explain things.
Use a multimeter.Check the voltage output of the transformer by hooking the negative lead to the negative side of the txformer and the positive to the positive side.Next,check the current by reversing the leads(negative to positive,positive to negative) and setting your meter to measure amps.(Because current flows negative to positive,you must complete the circuit using your multimeter this way.It's the only way to measure current.)Once you have these two readings,use the P=I*E formula to calculate the power(watts).Now,compare that to what the ouput power(watts) of the transformer is labeled as.If it coincides with what the transformer is supposed to be,then the transfomer is ok and you have another problem.(Make sure to disconnect the wires coming from the transformer to the lights while doing this.)
Like Noel said, check your voltage at the fixture. The voltage rating of the bulb is probably less than the voltage being provided at the fixture.
Twenty 7 watt bulbs designed to run at the transformer rated voltage will theoretically operate with a 500 watt transformer as long as they will with a 175 watt transformer with no ill effect. Actually, the 500 watt transformer would probably run cooler.
Not to stray from the subject, but I would actually probably prefer the 500 watt transformer because this would give you the flexability to add more lights at a later time. The 175 watt transformer would already be at 80% of the rated power, and it's a good idea to leave 20% for overhead.
A couple good things to keep in mind are current takes the path of least resistance, and the smaller the load on the circuit (light bulbs in this case) the lower the consumption of power. Twenty 7 watt bulbs = (7 watts)x(20)=140 watts. There might be a tad bit of of heat expelled due to the resistance of the circuit wire, but that would be very negligible regarding the affect on power consumption.
Resistance does not increase in a parallel circuit.
Please explain then, why the manufacturers recomend keeping the wattage rating of the transformer closer to the actual intended usage so as not to decrease bulb life?
"Resistance does not increase in a parallel circuit."
You are correct.In a paralllel circuit,the resistance will DECREASE with each resistor installed,and in turn your current will increase.
Don't know what I was thinking.
By the way...........Good info guys.................
Reading through this thread I see a lot of unnecessary confusion about low voltage. Here's some comments in response to previous posts:
1. There's no minimum lamp load requirement for quality low voltage transformers. A 1500 watt transformer can be run 24/7 powering only a 20 watt lamp. However, if a poor quality transformer is used then the manufacturer may have boosted the voltage at the taps to compensate for the low transformer efficiency; in other words, the 12 volt tap may actually be delivering 14 volts under no load when it's loaded up with lamp wattage the actual voltage can drop 2 volts to the required 12 volts. All EI type transformers (cheaper types) are low efficiency; torroidal transformers have higher efficiency and will drop only about 0.5 volts under full load. A voltmeter will tell the story by measuring voltage at the fixture.
2. Contrary to one post, the only accurate way to measure amps on a low voltage circuit is with a clamp-on type ampmeter. This can be tested for wire runs by testing one wire at the common or the voltage tap or the entire transformer can be measured at the photocell loop.
3. Contrary to one post, the resistance of the wire is very important to include in calculations since low voltage loses voltage very rapidly according to wire gauge and distance.
4. Keep in mind that when checking voltage at the fixture, you need to have all lamps lit (even at the socket you're testing). If one lamp is out then the voltage reading could be off by as much as 0.5 volts. CAST has pigtails that give you an extra socket at fixtures for testing while the lamp is lit.