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My daughter's IKEA reading light has/had a 12V 20W bulb with a transformer that listed that it output "11.5V ~ 10-20W 60Hz".

The transformer stopped working, and I can't find a replacement available online. In my electronics pile I found a transformer that outputs "15V ~ 1000mA" (no mention of frequency, and I've no oscilloscope here to measure it).

I spliced and soldered the light fixture to the new transformer, and it works. The transformer does not get hot with prolonged use, but I'm wondering:

Since 20W / 15V = 1.33 amps, is a Halogen bulb like a low-ohm speaker, where it's going to have such a low resistance that it's going to pull 1.3A through the transformer and it's skinny wires, possibly causing a fire hazard to it? Or do things not work like that with AC, and if anything the bulb is being over-volted but under-powered, and I should feel good about my hack?

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  • \$\begingroup\$ First I must say.. IKEA got their math wrong if they are feeding a 12V 20W bulb with that power supply.... If the transformer/wires are not getting hot and the bulb doesn't look OVERLY bright compared to before, your probably good to go. \$\endgroup\$ – Trevor_G Mar 4 '17 at 19:19
  • \$\begingroup\$ Halogen tungsten wire resistance rises 10x from cold to hot. so current drops same. Numbers dont add up though. NO the bulb will work the same on DC as AC as the inductance at 60Hz is negligible. \$\endgroup\$ – Sunnyskyguy EE75 Mar 4 '17 at 19:30
  • \$\begingroup\$ BTW.. your math is off a bit. 12V 20W bulb at 15V is 25W. but without knowing if they are all rated at a.c. or d.c. you may be comparing apples with oranges. \$\endgroup\$ – Trevor_G Mar 4 '17 at 19:34
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    \$\begingroup\$ A 12 V bulb should be operated with a 12 V source. If you use 15 V instead, the bulb would have a very short live only. A transformer rated with 1 A should be operated with no more than 1 A. The 20 W bulb needs 1.67 A at 12 V, that is a lot more than only 1 A. If operated with 15 V instead of 12 V, the bulb would draw more than 1.67 A. You overload both the bulb and the transformer. Forget it and use a transformer for 12 V and 20 W. \$\endgroup\$ – Uwe Mar 4 '17 at 22:39
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    \$\begingroup\$ it does NOT need AC but the DC supply must be able to drive 10x nominal current when cold without shutting down \$\endgroup\$ – Sunnyskyguy EE75 Mar 5 '17 at 0:39

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