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Thread: Fooled again by assuming......

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    Senior Member nevetslab's Avatar
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    Fooled again by assuming......

    In an on-going recent project...calibrating the digital temperature meter on my old Weller EC2000 Soldering Station, one of the items called for in the procedure was a 13 ohm 40W power resistor with a 24V lamp wired across it. In one step of the calibration procedure, you're supposed to turn the Temp control fully CW, then back it down until the 24V lamp just stops flashing. I didn't have such a lamp at the time, and using a DMM to monitor the AC voltage across that resistor wasn't a good solution, as further steps were based on that control setting.

    So, I looked thru my Chicago Minature and JKL incandescent lamp lists, found a JKL 24V/50mA lamp with wire leads, ordered a couple from Mouser. Those came in yesterday, so, the first thing I did was see what the DCR of the lamp was. Figuring 24V/50mA would yield a 480 ohm load across that 13 ohm 40W resistor. DMM read 44 ohms. Huh?

    Lets see. DMM is using 1mA constant current source to read the resistance, and with this bulb, no markings, just the package showing JKL 2176, this would be a 24V 50mA lamp. When I connected my 12V output at 60Hz from a 600 ohm source generator, it loaded down to about 3V, seeing no glow. OK. Wrong source. Moved my HP 6227B power supply to the bench, connected it across the lamp terminals, and, monitoring both voltage and current with it's meters, slowly brought up the voltage. I half-expected I'd blow the lamp, thinking this bulb was the wrong one, since I found three lamps in the 2.7V thru 6.3V whose DCR at those rated currents were around 42-45 ohms. As I increased the voltage, the current remained really low, finally got it up to 24VDC with the lamp now glowing brightly, what I'd expect. Current reading was 50mA. Now, at this condition, I'm seeing 480 ohm effective resistance with 50mA current flowing thru it.

    I must have missed that lab experiment in college.

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    Supporting Member eschertron's Avatar
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    That's quite a temperature coefficient! Or else there's a big inductive component to the element?

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  3. #3
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    Quote Originally Posted by eschertron View Post
    That's quite a temperature coefficient!
    Assisted by a huge temperature rise (>2000 deg C ?)

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    A resistance increase between cold and hot by a factor of 10 - 15 is normal with LV incandescent lamps.

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    Last edited by Helmholtz; 10-04-2019 at 08:07 PM.
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