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-   -   Can someone tell me why my low voltage lights are reading high current at the switch? (http://www.diychatroom.com/f18/can-someone-tell-me-why-my-low-voltage-lights-reading-high-current-switch-145960/)

kmc 06-04-2012 02:43 PM

Can someone tell me why my low voltage lights are reading high current at the switch?
 
I'm trying to figure out if the problem is the way I'm measuring or possibly the transformer. I compared three different lines that each have two bulbs. Each line has it's own switch.

First line: I measure voltage at the wall switch which reads about 120.9 volts. Then I measure voltage at the led light sockets and it reads 12.1 volts from each of the two sockets on the line. So all is good at this point. Then I turn on the switch and measure current with my clamp ammeter at the switch and get 0.59 amps. So if I multiply 0.59 by 120.9 I get 71 watts. These are supposed to be 6.5 watt bulbs x 2 = 13 watts total. I then measure the current at the socket of the bulbs and get 0.41 amps. O.41 x 12.1 = 5 watts for each bulb.

Second line: If I duplicate the process again, I get 0.12 amps at the switch for the two bulbs there making the power consumption of each bulb about 7 watts. This is in the ballpark of what I want.

Third line: I measure current at the switch and it is again around 0.59 amps for the two bulbs or 71 watts.

Any ideas?

mpoulton 06-04-2012 04:21 PM

First, your clamp-on meter is probably not very accurate at such low currents. Check the specs - it's probably good for something like +/-3% of full scale, which would likely be 40A or more. That's an allowable error of +/-1.2A.

But the real issue here is power factor. Measuring current and voltage separately allows you to calculate "apparent power" in volt-amps, but you do not know the phase relationship of the voltage and current waveforms. They are never perfectly aligned, and the phase shift between them results in less "real power" than the volt-amp calculation indicates. This difference is called the "power factor". Transformers that are under-loaded have very poor power factors, as do some electronic devices. So if the transformer for this lighting is rated for much more than the 13W load, you can expect the volt-amp measurement to be much higher than the real power being consumed. Similarly, if it's a cheap electronic lighting transformer, the power factor may be very low. To measure real power you need a wattmeter that accounts for the power factor.

curiousB 06-04-2012 05:41 PM

A trick to make clamp on current meters more accurate for lower readings is to wind 10 turns of wire around the clamp and splice that into the circuit. Then divide reading by 10. You can do any number of turns that make sense,20, 30, ..... Just divide reading by the number of turns.

brric 06-04-2012 07:34 PM

Quote:

Originally Posted by kmc (Post 936159)
I'm trying to figure out if the problem is the way I'm measuring or possibly the transformer. I compared three different lines that each have two bulbs. Each line has it's own switch.

First line: I measure voltage at the wall switch which reads about 120.9 volts. Then I measure voltage at the led light sockets and it reads 12.1 volts from each of the two sockets on the line. So all is good at this point. Then I turn on the switch and measure current with my clamp ammeter at the switch and get 0.59 amps. So if I multiply 0.59 by 120.9 I get 71 watts. These are supposed to be 6.5 watt bulbs x 2 = 13 watts total. I then measure the current at the socket of the bulbs and get 0.41 amps. O.41 x 12.1 = 5 watts for each bulb.

Second line: If I duplicate the process again, I get 0.12 amps at the switch for the two bulbs there making the power consumption of each bulb about 7 watts. This is in the ballpark of what I want.

Third line: I measure current at the switch and it is again around 0.59 amps for the two bulbs or 71 watts.

Any ideas?

What's the point?

dmxtothemax 06-04-2012 08:32 PM

Are you using electronic transformers or iron cored trannys ?

ddawg16 06-04-2012 09:38 PM

Just for clarification......

You have low voltage LED's that are being fed via a step down transformer? Or is it a solid state electronic transformer?

Are you sure that there is NOTHING else on the switches that feed the two lights? One way to confirm would be to measure the current going into the transformer....


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