How to make a 12 V DC computer fan work on 39 Volt AC in the old school way (schematic & demo)

Please read the description/textbox. Circuit of a classic 1 phase rectifier + a series regulator made with only 1 transistor (the BD 139, Hfe must be 110 – 140) that can supply a 12 Volt DC computer fan out of 39-40 Volt AC. Please note: there are smarter electronic ways to get to this result nowadays (2022).

That computer fan has some electronics inside (to drive the AC coils/DC motor). For that reason I had to be (we must be) careful not to overload the fan and not to reverse the (+) and (-).

Like I did that in a first test. Result: immediately defective within 0,01 second and some smell of burned electronics. Though sorry, not a big problem, because I had a second one on stock.

Corrections/remarks regarding the video:

1. I talk about the effect of a dropped down voltage on the ventilator, making the air stream to diminish, when the power supply gives out its maximum current. It is no real problem here, because we supply the 12 V fan with a stabilized voltage via that BD 139 series regulator. The problem only can occur when the output voltage of the power supply goes down to a level where the (40 V to 12 V) transistor stabilizer cannot work any longer in a proper way. Say when the collector voltage on the series transistor (here a BD 139, could also be a Darlington or whatever) drops down to 10 Volt or so, that is totally unreal here.

2. At the end of the video some smoke starts. Strange to say, but that is/was in a kind of way "normal" in this case. The reason is that the end-transistor (A2000) got too hot and the kit with which I had glued the 2 collectors heatsinks together got too hot. This is a common problem that I also experienced in the past with other power (transistor) circuits. Say (transparent) silicon kit starts to smoke forever in such a case (don’t use it). That is why I experimented with another type of kit in this case/this video, but, having seen this, that will also not work. Thus I will remove that kit, of course.

Earlier videos of the past year regarding this power supply:

Most recent video (1 March 2022, about how to discover and cure parasitic oscillations) is here https://youtu.be/dVLAwjEHgxo

First VLOG about this power supply on 5 may 2021 https://youtu.be/WMNrqqHwufo
Second VLOG about this Power supply on 5 may 2021 is here https://youtu.be/fusQIrGtALo
Part 3 basic setup of this power supply https://youtu.be/URpAQYbWHjQ
Another part about this power supply https://youtu.be/uVVH9h3MHQs
Voltage doubler of this power supply (can be used everywhere) https://youtu.be/PQ7Tamffv9w
Heat control tips of this power supply https://youtu.be/3N9dpRfPh0k
Pop out AC fuse detection circuit https://youtu.be/Pam963MnRX8

The S2000A (old school driver transistor in analog TV circuits, originally driving the HV coil of an analog TV set on say 16 KC) or the BUV 47 R (say the same application) and the BUT 11 AF (driver) are High Voltage Silicon NPN transistors that can withstand the heat/dissipation. Datasheets are on the www.

Their Hfe amplification factor is very low (!) say between 5 and 10 or 15. That is normal in these cases. Important: that was tested with a normal transistor tester, not one specified to test HV transistors…

The end-transistor must be mounted on a very large heatsink, visible in the video, because of its DC drive. The driver (BUT 11 AF) needs a moderate heatsink. Preferred: a separate one for the BUT 11 AF driver because the end-transistor must not heat-up the driver transistor. That is a kind of common law.

When you have questions about (certain analog) electronic circuits and how to make them: go to my Channel trailer (Radiofun232 on You Tube) and go to the “looking glass”. Type there the keywords that you want/need like: “power supply”, audio amplifier, capacitor tester, oscillator, radio, shortwave, shortwave radio, or any keyword you like to find the essential info & schematics on my YT channel.

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc; so the electronic subject you are interested in.

My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.


I keep all my YT videos actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot prevent them. Upload 3 March
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