The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

The New Hotness

  • AD9850 Module: Oscillator Thermal Time Constant

    With an LM75 atop the 125 MHz oscillator and the whole thing wrapped in foam:

    LM75A Temperature Sensor - installed
    LM75A Temperature Sensor – installed

    Let it cool overnight in the Basement Laboratory, fire it up, record the temperature every 30 seconds, and get the slightly chunky blue curve:

    125 MHz Oscillator - Heating Time Constant
    125 MHz Oscillator – Heating Time Constant

    Because we know this is one of those exponential-approach problems, the equation looks like:

    Temp(t) = Tfinal + (Tinit - Tfinal) × e-t/τ

    We can find the time constant by either going through the hassle of an RMS curve fit or just winging it by assuming:

    • The initial temperature, which is 22.5 °C = close to 22.7 °C ambient
    • The final temperature (call it 42 °C)
    • Any good data point will suffice

    The point at 480 s is a nice, round 40 °C, so plug ’em in:

    40.0 = 42.0 + (22.7 - 42.0) × e-480/τ

    Turning the crank produces τ = 212 s, which looks about right.

    Trying it again with the 36.125 °C point at 240 s pops out 200.0 °C.

    Time for a third opinion!

    Because we live in the future, the ever-so-smooth red curve comes from unleashing LibreOffice Calc’s Goal Seek to find a time constant that minimizes the RMS Error. After a moment, it suggests 199.4 s, which I’ll accept as definitive.

    The spreadsheet looks like this:

    T_init 22.5
    T_final 42.0
    Tau 199.4
    Time s Temp °C Exp App Error²
    0 22.250 22.500 0.063
    30 25.500 25.224 0.076
    60 28.000 27.567 0.187
    90 30.125 29.583 0.294
    120 31.750 31.317 0.187
    150 33.250 32.810 0.194
    180 34.375 34.093 0.079
    210 35.375 35.198 0.031
    240 36.125 36.148 0.001
    270 36.813 36.965 0.023
    300 37.500 37.668 0.028
    330 38.125 38.273 0.022
    360 38.500 38.794 0.086
    390 39.000 39.242 0.058
    420 39.500 39.627 0.016
    450 39.750 39.959 0.043
    480 40.000 40.244 0.059
    510 40.250 40.489 0.057
    540 40.500 40.700 0.040
    570 40.750 40.882 0.017
    600 41.000 41.038 0.001
    RMS Error 0.273

    The Exp App column is the exponential equation, assuming the three variables at the top, the Error² column is the squared error between the measurement and the equation, and the RMS Error cell contains the square root of the average of those squared errors.

    The Goal Seeker couldn’t push RMS Error to zero and gave up with Tau = 199.4. That’s sensitive to the initial and final temperatures, but close enough to my back of the envelope to remind me not to screw around with extensive calculations when “two minutes” will suffice.

    Basically, after five time constants = 1000 s = 15 minutes, the oscillator is stable enough to not worry about.