convert 24vac to 12vac

[musastan]

I'm really rusty in remembering my basic electronic class info., but I'd like to run a 11W 12vac malibu outdoor light on a 24vac 650mA "watering the lawn" timer output.

I appreciate any info to help me step down the voltage from 24vac to 12vac? Thanks much!!!

 

[Dr.EM]

As the outdoor light requires 11W at 12v, that is nearly 1A of current. This means you need a high efficiency conversion of the 24vac 650ma source. This rules out using a restistive divider or similar arrangement where significant power will be lost. A 2:1 transformer with suitable current rating is most obvious, but may prove hard to source. Alternatively, I expect the light won't mind 12dc instead (is it incandescent?). So long as this is the case, you can turn the 24vac into DC and use a DC-DC convertor to give 12vdc at it's output with good efficiency.

 

[musastan]

Thanks, DrEm,
If I use a 4W bulb to lower the amp needs, is there somewhere that I can look up regarding a proper resistant divider arrangement/setup (yes, incandescent bulb)? Thanks

 

[Nigel Goodwin]

Thanks, DrEm,
If I use a 4W bulb to lower the amp needs, is there somewhere that I can look up regarding a proper resistant divider arrangement/setup (yes, incandescent bulb)? Thanks

You don't need a resistive divider a single series resistor would do (consult ohms law for the value!), or simply place two 4W bulbs in series - giving 8W at 24V.

 

[musastan]

Thanks much, two bulbs will work for me!!!

 

[Nigel Goodwin]

Thanks much, two bulbs will work for me!!!

The simple ideas are often the best!.

 

[musastan]

So true, Nigel. And the two bulb solution is best for my application. It's funny how so often a team/discussion approach to a problem (or a smarter mind than myself) will bubble up to a better solution. Thanks again!!!

 

[Hero999]

I know it's not needed here but for future reference the easiest way to get a transfomer with an unusual primary voltage is to buy a torroidial mains transformer for the appropriet power and if possible with the same secondary voltage as the /primary/secondary you want and rewind it. If you're really lucky then the secondary will be wound on the inner core so you can just use it as is.

I have an old 12V ct torroidial transformer lying around somewhere, the mains primay over heated. I though about throwing it away but I removed primary and just left the 12V secondary, it might come in handy if I need a tranformer with an odd primary or just a large value inductor.

 

[musastan]

That's an interesting thought. I do have a spare 120v to 12v transformer, but I've never cracked it open to see if it's toroidal. I may do that some day, for fun, rewind some turns, and see what output voltage I get. Thanks, Hero999

 

[Nigel Goodwin]

If you have a transformer you can use it as an auto-transfomer, using just the secondary. With a 12-0-12 transformer feed the 24V in across the two 12V wires, and output 12V between 0V and either of the 12V wires. You need to be VERY aware though that mains voltages will be present on the unused primary winding!.

 

[musastan]

In my particular application, I'll need to stick to my 24vac output transformer so that the timer/controller can power the lawn sprinkler solenoids (needs 24vac) on different time slots/frames. It's just that on one time slot/frame, I want the timer/controller to power this "12v wedge type incandescent bulb". Since it would be nice to be lighted at two locations about 10 ft apart, then I'll just use the two bulb solution running at 24vac that you suggested.

Thanks, Stan.