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Thread: Doorbell wiring

  1. #41
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    180505-2351 EDT

    minesh21:

    I ran some experiments on two different small transformers. Continued from earlier post.

    The second one is a, within last 20 years, Signal Transformer 341--6-20 120 V in and 20 V out rated at 30 VA. Single primary coil and center tapped secondary designed for DC power supplies. The iron core is smaller than the NuTone yet VA is 1.5 times that of the NuTone. Also a split bobbin coil is used. The EI core on the outside reads 2.28" x 1.88" x 1.05" vs 2.63" x 2.21" x 0.94" for the NuTone.

    21.6 vs 57.2 ohms is primary DC resistance.
    1.0 vs 3.0 ohms is secondary DC resistance. 25.4 is estimate of secondary reflected to primary. Thus, expect about 47 ohms as total seen at primary with secondary shorted.

    Using a Kill-A-Watt EZ and a Fluke 27 the measurements were:

    Open secondary with no load,
    120.1 V, 0.05 A, 2.0 W, 6.7 VA, 0.29 PF, secondary 23.8 V, compared to
    119.8 V, 0.02 A, 1.4 W, 2.6 VA, 0.53 PF, secondary 26.4 V.

    Shorted secondary with reduced input voltage. 70 V is about lowest with EZ.
    Internal impedance of this transformer is too low to run at 70 V with secondary shorted.
    The NuTone was
    70.0 V, 0.55 A, 38 W, 38 VA, 1.00 PF. Apparrent input resistance 127 ohms.

    With nominal 120 V input and various secondary loads:
    15.4 ohms ---
    120.0 V, 0.28 A, 34.1 W, 34.5 VA, 0.98 PF, secondary 21.5 V.
    ............... --- load power 30.0 W, eff is 88%.
    ............... --- Approx source impedance of sec 2.3/(21.5/15.4) = 1.65 ohms.
    ............... --- Reflected to input about 41.8 ohms.

    12.1 ohms ---
    120.0 V, 0.34 A, 40.9 W, 40.9 VA, 0.98 PF, secondary 20.9 V.
    ............... --- load power 36.1 W, eff is 88%.
    ............... --- Approx source impedance of sec 2.9/(20.9/12.1) = 1.68 ohms.
    ............... --- Reflected to input about 42.7 ohms.

    This transformer was designed to have a much lower internal impedance. as it should be, because one wants minimum output voltage variation with load in a DC voltage power supply.

    .

  2. #42
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    180506-0957 EDT

    minesh21:

    In an ordinary iron cored power transformer with one primary and one secondary coil why is the secondary DC resistance approximately equal to the primary DC resistance divided by the square of the primary to secondary turns ratio?

    .

  3. #43
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    Quote Originally Posted by steve_p View Post
    I had one job where I needed to install a 30va transformer for the Ring to work.
    Yup I installed the ring and it can't power up with only 10VA (my existing transformer). So I need to upgrade to the 30VA.

  4. #44
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    Quote Originally Posted by gar View Post
    180505-2351 EDT

    minesh21:

    I ran some experiments on two different small transformers. Continued from earlier post.

    The second one is a, within last 20 years, Signal Transformer 341--6-20 120 V in and 20 V out rated at 30 VA. Single primary coil and center tapped secondary designed for DC power supplies. The iron core is smaller than the NuTone yet VA is 1.5 times that of the NuTone. Also a split bobbin coil is used. The EI core on the outside reads 2.28" x 1.88" x 1.05" vs 2.63" x 2.21" x 0.94" for the NuTone.

    21.6 vs 57.2 ohms is primary DC resistance.
    1.0 vs 3.0 ohms is secondary DC resistance. 25.4 is estimate of secondary reflected to primary. Thus, expect about 47 ohms as total seen at primary with secondary shorted.

    Using a Kill-A-Watt EZ and a Fluke 27 the measurements were:

    Open secondary with no load,
    120.1 V, 0.05 A, 2.0 W, 6.7 VA, 0.29 PF, secondary 23.8 V, compared to
    119.8 V, 0.02 A, 1.4 W, 2.6 VA, 0.53 PF, secondary 26.4 V.

    Shorted secondary with reduced input voltage. 70 V is about lowest with EZ.
    Internal impedance of this transformer is too low to run at 70 V with secondary shorted.
    The NuTone was
    70.0 V, 0.55 A, 38 W, 38 VA, 1.00 PF. Apparrent input resistance 127 ohms.

    With nominal 120 V input and various secondary loads:
    15.4 ohms ---
    120.0 V, 0.28 A, 34.1 W, 34.5 VA, 0.98 PF, secondary 21.5 V.
    ............... --- load power 30.0 W, eff is 88%.
    ............... --- Approx source impedance of sec 2.3/(21.5/15.4) = 1.65 ohms.
    ............... --- Reflected to input about 41.8 ohms.

    12.1 ohms ---
    120.0 V, 0.34 A, 40.9 W, 40.9 VA, 0.98 PF, secondary 20.9 V.
    ............... --- load power 36.1 W, eff is 88%.
    ............... --- Approx source impedance of sec 2.9/(20.9/12.1) = 1.68 ohms.
    ............... --- Reflected to input about 42.7 ohms.

    This transformer was designed to have a much lower internal impedance. as it should be, because one wants minimum output voltage variation with load in a DC voltage power supply.

    .
    Its fine I am just replacing the transformer because it is 10VA and I need 30VA.

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