# LED bulb question

Data point: My Kill-a-watt is wildly inaccurate at very low (< 10W) power levels: Some of my light bulbs don’t even register any reading. (This is based on measurements more than a year ago. I should try to repeat them now).

Mine is older than yours. Model 4400, yours is Model 4400.01

Mine is also marked Conforms to UL STD. UL 3111-1, yours is 61010-1

Mine is marked Voltage: 125VAC Power: 1875VA, yours is 120VAC, 1800W

Is your DVM sensitive enough to measure the resistance of a power cord? If so, you could measure the resistance (without 120V) and the voltage drop (with 120V through it and the lamp, before the power cord heats notably) to calculate the current flow. It will not tell you anything about the relative phase of current and voltage through the lamp, however.

I’m wondering how long ago you bought yours and and how much it cost. I paid \$20 at Menard’s.

Here’s my working hypothesis: to get the price down and appeal to a larger market, the newer model (4400.01) uses cheaper components with larger tolerances.

Don, would you please compare the label on the back of your unit to this and note any differences?

Also, could you try your unit with a 60W incandescent bulb and tell us what the Watt readout says?

There is always the amp function of your DVM. :]

The cold resistance gives you a reference to tell if things are really strange. You can also make some assumptions and do a little math and see if a simple physics model matches reality. Tungsten (assuming your filament is really tungsten) has a resistance temperature Cef of 4.5E-3 at 20C. (Assuming it is linear all the way to white hot is a big assumption at least for me.) Using 120 VAC for the rated voltage 2500K as the bulb temp and a little excel math I get about 22 ohm cold. Am I close? Of course with a 240ish ohm hot resistance the cold resistance doesn’t tell you that much without a tested model.

None of my 8 (inexpensive) various models of digital or analog VMs measure AC amps in the desired range.

The cold resistance of 60W soft white incandescent bulb measures approx 18 ohms.

Don, if you’re still reading this thread: does the label on the back of your Kill-A-Watt say model P4400 (like Richard’s) or P4400.01 (like mine) ?