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I'm a beginner in Electronics Engineering. Any suggestions on simple power supplies with PCB designs which are run by a 24VAC transformer? Thanks!

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    \$\begingroup\$ Bridge rectifier, filter capacitor, regulator, done. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 30 '14 at 3:30
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24VAC is an inconveniently high voltage for many applications, with high line voltage and lightly loaded (the transformer regulation comes into okay) its quite possible to get 45V or more at the filter output (input to the regulator). Not many inexpensive switching of linear series regulators (definitely not LM78xx) are rated to safely operate with that high an inout voltage.

But if you do a parametric search for such regulators, they can be found, the details really depend on what output voltage and current you want.

You would generally use a bridge rectifier and an electrolytic filter capacitor between the transformer and regulator. It's easy to calculate the capacitor size and voltage rating from requirements.

For a switching regulator, something like an LT1767 would work, but there may be more appropriate parts for your requirements.

BTW if your 24VAC transformer happens to be center-tapped, the rectifier can use just two diodes and you get a nicer voltage to work with. If you use a bridge rectifier you can get both positive and negative voltages from a single transformer. Very nice for analog projects.

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  • \$\begingroup\$ 24VAC isn't a voltage at all, let alone an inconveniently high one. \$\endgroup\$ – Marquis of Lorne Nov 29 '14 at 9:11
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    \$\begingroup\$ That's a particularly weird comment. \$\endgroup\$ – Spehro Pefhany Nov 29 '14 at 12:17
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How much current to you need for your low-voltage power supplies?

24 Vac is the standard voltage used for HVAC applications in North America and parts of Europe. That's what you have available, so that's what you use.

I have several power supply designs that I use for our various HVAC designs. All use half-wave rectifiers so that the 24 Vac common line can also be the common line for the DC-powered logic stuff. This makes interfacing to thermostats and solenoid valves easier.

The simplest designs use a two-stage power supply: the initial supply produces about 24 Vdc for driving relay coils, the subsequent supply is a simple resistor - Zener supply for the +5 Vdc rail.

The 24 Vdc rail is good for several tens of milliamps, the 5V Zener supply is good for 20 mA or so. There is a separate power-up reset circuit that I haven't shown - this also quickly discharges the main bulk storage capacitor when power is removed.

Neither of these DC supplies uses a SMPS because it simply isn't needed. In addition, simple power supplies like this are far more reliable - this matters when you are building equipment that has an expected lifetime of decades.

But these techniques are NOT suitable if you need something like 5V @ 1 or more Amps. We get around that by designing our controller circuits to use only 5 or 10 mA from the +5V rail.

schematic

simulate this circuit – Schematic created using CircuitLab

I'll check the component values when I get into work but I'm pretty sure these are correct.

The 24 Vdc rail reliably supplies power for up to 8- JS1 relays or 3- AZ2150 / AZ2150A relays (or a mixture of both). Obviously, the relays have 24 Vdc coils.

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You can use a bridge rectifier and a big capacitor to void most of the ripple, there you have about 32vdc.

If you need 5v or 3.3v you can use a switching regulator like the R-78 family from Recom. As the unregulated voltage is over 28v, you cannot use the R78E family, I reccomend the R78C.

These are better than linear regulators because power dissipation is much much lower (no need to heatsink)

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