I "hack" cheap "electronic transformers" for use as filament supply for my tube amplifier projects. They operate at about 50KHz so there is no audible hum (a particularly annoying problem of directly-heated tubes). For this purpose I rewire the secondary to get the 6.3V required by the tubes. Works wonders :-)

Now I want to experiment rewiring the secondaries to provide the high voltage for the vacuum tubes (B+). Something in the 150-250V range, a few tens milliamps current. Below is a pic of my first attempt, with stock unit at left).

My first attempt, with stock unit at left

As a precaution I wrapped the stock primary with mylar tape before winding the new secondary on top. I simply counted the stock primary turns and made a rule of three for the number of secondary turns. After winding, measured inductances seems in the ball park: my secondary has twice the primary inductance, which is 1.4142 voltage ratio.

Does everything seems ok to your eagle eyes? Now comes the light bulb part of the question: In a youtube tutorial on DIY transformer winding the guy puts a regular incandescent light bulb in series with the winding and hook it to the mains. If the winding is ok the light bulb does NOT light.

I guess the theory behind this test is that if the winding is good all the power goes into the magnetic field hence no glow on the bulb. Is that correct? And perhaps more importantly, is that a valid test?

For reference, here is the video URL at the test's start: https://youtu.be/Q6GkSNfAEx4?t=954 (The light bulb tests are between 15:50 and 17:30.)

Thanks for any insights! -Joe

  • \$\begingroup\$ You’ve not modified the primary side, so why are you worried about it blowing up? The lightbulb is an old trick to minimise the risk of a faulty power supply blowing up. \$\endgroup\$
    – Kartman
    Jun 17, 2022 at 0:30
  • \$\begingroup\$ The test you refer to is for low frequency transformers (50/60 Hz). If the series connected bulb glows, there is a shortcut in the transformer. The unloaded good transformers impedance is too high to allow enough current for the bulb. This is not so easy to translate to SMPS transformers. For the same test idea you would need a high frequency high voltage sine wave generator. You could measure the inductance before and after the modification, should not change much. \$\endgroup\$
    – Jens
    Jun 17, 2022 at 1:48
  • \$\begingroup\$ @Jens Actually I thought doing the test on mains with a smaller light bulb for the reduced current but thinking of it now I guess the small core would not develop enough field at 60Hz. I'm rather new to magnetic field theory, please bear with me :-) \$\endgroup\$
    – Joe
    Jun 17, 2022 at 2:10
  • \$\begingroup\$ The few windings of this transformer would just be a piece of wire for 60 Hz, the inductance is far to low. You are on the right track with "not enough field at 60Hz". The bulb trick is only valid if you use a frequency where the transformer is "made for", has a significant impedance. Let's assume the bulb has 400 ohm. It would glow if the impedance of the transformer in series is much lower. Your transformer looks like 500 µH, the impedance Z at 60 Hz is 2 * pi * f * L = 0.19 ohm (= a piece of wire). So this test at 60 Hz is useless. \$\endgroup\$
    – Jens
    Jun 17, 2022 at 2:33
  • \$\begingroup\$ At 100 kHz Z is 314 ohm, your bulb has a dim light then. If you have a short circuit in a winding the impedance would fall below 10% of that, your bulb would be bright. \$\endgroup\$
    – Jens
    Jun 17, 2022 at 2:33


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