# Converting 24 V ac to 12 V DC and 3.3 V DC

#### donaldm444

Joined Jul 16, 2010
23
Hi,
I need to take a 24 V ac signal and convert it to both 12 V DC and 3.3 V DC (to power a microprocessor and various ICs on a board I am designing). I want to use two LM 350 voltage regulator circuits in parallel or series with each other (see attached image), but I wanted to see which configuration would be better.

Based on my understanding, I would be wasting more power if I put them in parallel, but I am concerned about interference between the two circuits if I put them in series.

Any advice would be greatly appreciated.

Thanks,
Don

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#### thatoneguy

Joined Feb 19, 2009
6,359
At the voltages you are trying to regulate, linear regulators will get quite hot.

Here are some examples of switching regulators Ready to use, 300mA with 3.3 or 5V output selectable from up to a 42V input source. $8-$15 each, you can't build them cheaper than that.

The page has other options as well, if you can put a resistor in line with the power to drop a bit more voltage first.

#### SgtWookie

Joined Jul 17, 2007
22,230
Here's a DC-DC converter that accepts a wide range of input and puts out 3.3v @ up to 1.5A, for ~\$4.30 from Mouser:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtwaiKVUtQsNa9RSQZ1iZ/UnO1fxRs35eY=

Once you rectify and filter the 24VAC in, you're going to wind up with ~32v. That means for every 1W of power you dissipate at 3.3v, you dissipate nearly 9W power in a linear regulator. That's a heck of a waste. It's bad enough going from 32 to 12v; there you're wasting ~63% of your power input.

#### crutschow

Joined Mar 14, 2008
33,331
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Based on my understanding, I would be wasting more power if I put them in parallel, but I am concerned about interference between the two circuits if I put them in series.
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With linear regulators, putting them in series or parallel makes no difference to the overall efficiency. The only difference is that the total 3.3V power is dissipated in the 3.3V regulator for the parallel configuration, but is distributed between the two regulators in series connection. The total dissipation however, is the same in both configurations for a given 3.3V current.

But as noted, any significant current drawn from the linear regulators will waste a lot of power as heat dissipated in the regulators.