LED bulb question

[Ether]

Originally Posted by Mr V

Is there anything in the documentation that would indicate the tolerance is different at different parts of the range?

The max watts spec for the unit is 1800W.

So even if 0.2% means 0.2% of 1800, the unit is still way out of spec.

[Ether]

Originally Posted by FrankJ

Have you tried measuring the bulb resistance with a DVM?

The resistance of the bulb changes substantially as the filament gets white hot.

I don't have a DVM that uses 120 volts to measure resistance.

.

[Ether]

Originally Posted by Ether

I don't have a DVM that uses 120 volts to measure resistance.

... and the inexpensive DVMs I have access to don't support measuring AC current with 120V applied.

[Ether]

I just called the tech support number and the friendly fellow who answered wasn't able to shed any light on the situation.

.

[Ether]

Richard, If you are still following this thread, could you please take a peek at the label on the back of your Kill-A-Watt and compare it to this and flag any differences for me? Note especially the red arrows.

I'm wondering if your unit is an improved version.

I bought mine at Menard's, and they appeared to have been hanging on the rack for quite some time (packaging covered in fine dust). Also, the printing on the cardboard label inside the thermal-welded plastic packaging is copyright 2011.

[DonRotolo]

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).

[Richard Wallace]

Originally Posted by Ether

Richard, If you are still following this thread, could you please take a peek at the label on the back of your Kill-A-Watt and compare it to this and flag any differences for me? Note especially the red arrows.

I'm wondering if your unit is an improved version.

I bought mine at Menard's, and they appeared to have been hanging on the rack for quite some time (packaging covered in fine dust). Also, the printing on the cardboard label inside the thermal-welded plastic packaging is copyright 2011.

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

[GeeTwo]

Originally Posted by Ether

The resistance of the bulb changes substantially as the filament gets white hot.

I don't have a DVM that uses 120 volts to measure resistance.

.

Originally Posted by Ether

... and the inexpensive DVMs I have access to don't support measuring AC current with 120V applied.

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.

[Ether]

Originally Posted by Richard Wallace

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

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.

Originally Posted by DonRotolo

Data point: My Kill-a-watt is wildly inaccurate at very low (< 10W) power levels

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?

[FrankJ]

Originally Posted by Ether

The resistance of the bulb changes substantially as the filament gets white hot.

I don't have a DVM that uses 120 volts to measure resistance.

.

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.

[Ether]

Originally Posted by FrankJ

There is always the amp function of your DVM.

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) ?