Last year in Dec. I built a LED lamp that is 12 of the star-type LEDs in series powered by house current (mains to you Europeans). Now I want to measure efficiency of the lamp, that is, Lumens per watt of the lamp.
Unfortunately I don't have a watt-meter and don't want to buy one as I would probably only use it rarely. But I do have a dual trace CRO (old style cathode ray oscilloscope).
In order to obtain the actual power dissipated by the lamp, I measure the apparent power at input to the lamp, V times A, and then multiply that by the cosine of the phase angle.
See the attached figure of my lamp. To get the phase angle, I connected the CH1 probe across the secondary winding of isolation transformer T1. The Ch2 probes I connected across the leads of resistor R2. The voltage drop across R2 is in phase with input current. Thus the extent to which the CH2 voltage leads the CH1 voltage tells me (in milliseconds) the extent to which current is leading voltage.
The CH2 voltage waveform crosses the 0 V horizontal axis 3 ms. before the CH 1 voltage. Actually the CH2 wave goes to 0V and is flat at 0V for about 2 ms. before going above/ below 0V. So I'm taking the 0V crossing time of the CH2 voltage to be when it starts to rise or fall after staying flat at 0V.
The frequency of the house current is 60 Hz which is a period of 16.7 mS. Therefore the phase angle of my lamp I take as being 3 ms divided by 16.7 ms times 360 degrees equal to 64.7 degrees.
Using a scope, would you say that this is a correct way to determine the phase angle?
Thanks,
Pete
Unfortunately I don't have a watt-meter and don't want to buy one as I would probably only use it rarely. But I do have a dual trace CRO (old style cathode ray oscilloscope).
In order to obtain the actual power dissipated by the lamp, I measure the apparent power at input to the lamp, V times A, and then multiply that by the cosine of the phase angle.
See the attached figure of my lamp. To get the phase angle, I connected the CH1 probe across the secondary winding of isolation transformer T1. The Ch2 probes I connected across the leads of resistor R2. The voltage drop across R2 is in phase with input current. Thus the extent to which the CH2 voltage leads the CH1 voltage tells me (in milliseconds) the extent to which current is leading voltage.
The CH2 voltage waveform crosses the 0 V horizontal axis 3 ms. before the CH 1 voltage. Actually the CH2 wave goes to 0V and is flat at 0V for about 2 ms. before going above/ below 0V. So I'm taking the 0V crossing time of the CH2 voltage to be when it starts to rise or fall after staying flat at 0V.
The frequency of the house current is 60 Hz which is a period of 16.7 mS. Therefore the phase angle of my lamp I take as being 3 ms divided by 16.7 ms times 360 degrees equal to 64.7 degrees.
Using a scope, would you say that this is a correct way to determine the phase angle?
Thanks,
Pete