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I have a couple of obscure german “AT60” power supplies I scavenged from some designer lamps. They’re small, and don’t get hot even under ~30W load — but for my purposes, I need them to be always on, or at least switch ON at a load of 0.25W. A microcontroller needs to be always on and do some things, which may bring the load up to 10-15W, by my estimates.

Any clues would be greatly appreciated. I have of course already googled and asked a ham radio operator; neither knew anything.

I have no diagrams, only the images enclosed... There are two components related to (presumably) SMPS, these are named U1P220 (BULK 38D CRW). The large coil leads directly to the 12V terminals. foo

To clarify: I need this powersupply to either turn on at lower load, or be always on. It needs to power a microcontroller at ~0.2W, and this micro will in turn switch “things” on, up to a max load of ~15W.

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closed as unclear what you're asking by pipe, Elliot Alderson, Chupacabras, RoyC, Warren Hill Jan 13 at 10:39

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ If it requires a minimum load, then it isn't a simple transformer. It is a power supply. \$\endgroup\$ – JRE Jan 12 at 15:11
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    \$\begingroup\$ Allow me to hazard a guess, it’s a “halogen transformer”? \$\endgroup\$ – winny Jan 12 at 15:30
  • \$\begingroup\$ @JRE Shine your light on me, please. How can I reduce the required load? \$\endgroup\$ – user2497 Jan 12 at 15:32
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    \$\begingroup\$ If that large coil indeed directly connects to the bulbs from the lamps, then what is output there can't be DC, so you'll have to at least have a bridge rectifier, followed by stabilizing caps, and most likely followed by yet another regulator to get the microcontroller voltage. Are you sure this pays, and it wouldn't be much easier to just either add an old phone charger supply (got a stash of 5V 250 mA supplies from the chocolate-bar-sized-phones days), or to make your microcontroller run off e.g. a NiCd cell that you only charge when you enable the large load anyway? \$\endgroup\$ – Marcus Müller Jan 12 at 18:32
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    \$\begingroup\$ why would you remove the casing? If you keep the supply intact, you might not have to re-certify your product as grid voltage product! \$\endgroup\$ – Marcus Müller Jan 12 at 19:12
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As pointed out in the comments you have switched mode power supplies, not a simple transformer.

If they need a 0.25 W load to keep them awake then you can just add a permanent resistor load. From \$ P = \frac {V^2}{R} \$ we can calculate the required resistor value, \$ R = \frac {V^2}{P} = \frac {12^2}{0.25} = 576 \ \Omega; \$. 560 Ω would be the nearest standard value below this.

Use a 0.5 W version so that it runs cooler. You could possibly install the resistor in the PSU.

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  • \$\begingroup\$ Yes, I wanted to do this with a few power resistors — and it works perfectly. But each SMPS needs ~20W load to output 12V. That’s a lot of heat, and I theorize I can adjust one of the resistors (replace it with a potentiometer, and adjust until I see the 12V rail turn ON). But which resistor? Can’t upload more images right now, some invalid app request error:/ \$\endgroup\$ – user2497 Jan 12 at 15:42
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    \$\begingroup\$ That's rather different to what's asked in your question. \$\endgroup\$ – Transistor Jan 12 at 16:24
  • \$\begingroup\$ A photo of the back of the board would be useful, and part numbers/markings off the black rectangles. \$\endgroup\$ – Will Jan 12 at 17:47
  • \$\begingroup\$ @Will I’d love to, but am having some problem posting the images. Will update when it works. \$\endgroup\$ – user2497 Jan 12 at 18:56
  • \$\begingroup\$ you can upload them to imgur.com directly and add only the links to the images to your question. A good samaritan will then embed them. \$\endgroup\$ – Marcus Müller Jan 12 at 19:14

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