Need help understanding multimeter results for AC

[friendly_jacek]

Here is a deal, I bought different cheap 110V AC LED bulbs from ebay to try them out. I'm too cheap to purchase the $10+ ones from the stores.
However, some ebay sellers lie about power, so I'm testing them by my trusted multimeter (GDT-293A). I check the AC current in the 200 mA range.

I calculate the power by current times voltage. That ignores the power factor, but seems to work fine. For example, I take a "reference" 7W CFL and the meter gives me 64mA. Times 125V gives me 8W, close to the rated 7W.

Some LED are close to the stated power, especially if ones rounds the numbers up. Some bulbs, especially the spot light type, blatantly lie about power (ie 9W vs 4W).

Here is a surprise. One (only one) small 3W bulb gives 2 different results when I change polarity of meter leads (21mA vs 27mA). How would that matter with AC?

Another surprise: the "reference" 7W CFL measures 64mA, but lists 115 mA.

Is the power factor issue confusing my meter?

 

[Donald]

Have you check the AC voltage. 125V is a little high. Normally 117V,

Are you sure your amp meter is AC amps not DC?

Some of the cheap LEDs maybe have the color temp. off and or may not last as long as claimed.

I would not go below Walmart house brand for LEDs.

A couple things I have noticed, a 7W of one brand LED may give more or less light output than another brand. LED are not much more efficient than CFL. For CFL and old incandescent they just made it slightly differently to give out 40W vs 60W. For LED they have to include more LEDs to make it give off more light. Thus some of the 120W flood lights get to be very expensive.

 

[friendly_jacek]

My AC is about 125V. If you select DC range for AC current, you get 0.

 

[redbone3]

You must put the multimeter in series to measure the current. The meter has some resistance, which will reduce the measured current.

 

[JHZR2]

I would imagine these do some pwm and perhaps how your meter samples and averages over time causes some difference?

 

[lugNutz]

Originally Posted By: friendly_jacek
Here is a surprise. One (only one) small 3W bulb gives 2 different results when I change polarity of meter leads (21mA vs 27mA). How would that matter with AC?

Is this a CFL or LED bulb?

 

[Carbon]

Quirk of the multimeter. I am not surprised that an LED has different amounts of current during each half of the cycle. While the ideal AC meter would not care, yours does.

 

[JetStar]

I can't explain your difference in polarity changing the magnitude of the current you measured.
As far as measuring the absolute value, your meter is probably an average responding meter, which assumes sinusoidal current. A true RMS meter may give you more accurate results. I don't know anything about LED light bulbs so I don't have a clue of what the current waveforms look like as far as frequency so that may be a factor too.

 

[lugNutz]

If it is the LED bulb that has the difference in current, would it make sense that there would be a transistor somewhere in the bulb? A pull-up resistor? Depending on the design of the bulb, there is a chance that changing the polarity could have a different reference to ground? That difference in current is hardly relevant in its application. What do you guys think?