Powering small devices from 24Vac

[AussieBruce]

I am involved with applications where the standard supply voltage is 24Vac, supplied (still I believe) from iron core mains transformers. In fact, the output from these transformers in operation can vary, I’ve seen it up as high as 30 volts occasionally, depending on the load and the state of the mains. Converting this 24 volts compactly, reliably and simply down to 12/5/3v3 DC for electronic functions isn’t impossible, but seems a bit awkward whichever way you try. So far I’ve done these…

•   Custom 24v to 9/9ct transformers. Not many custom small transformer makers around these days.

•   Find a switcher module that will reliably handle FW rectified and filtered 24v. Just about all of them seem to have their top rated input voltage around or below the possible peak input DC that may be encountered. So marginal.

•   Simple preregulator, eg. npn bipolar, followed by the usual 78xx devices. Currently my method of choice, but doesn't provide isolation, and can require some heatsinking, not the best design for  small sealed enclosures and high ambient environments such as retic sometimes encounters, and if a transient damages the preregulator device, it will blow the whole device.

Any thoughts on how best to do this? Typically, the output needs to be isolated 12v or 5Vdc at about 50mA.

[T3sl4co1l]

Plenty of commercial or automotive targeted parts with somewhat higher voltage ratings -- 60V is enough to handle 12V load dump for example.  Have seen at least one manufacturer using similar circuits inside their products (24VAC signaling / control power) and MP2456GJ for logic supplies.

If you're not looking for design advice but just something to throw together, yeah, you might not find a ready-made module.

You can combine #2 and #3, which greatly lessens the disadvantage of #3 alone.  The switching reg draws far less current at 24 or 30V than it delivers at 12 or 5V, so you only dissipate the difference (up to the peak voltage) at that lighter current flow.  Voltage stability is N/A, too, the switching reg is fine with whatever.  A typical use case is a depletion MOSFET source follower plus zener shunt regulator; which are commonly (to the extent that depl MOS are common at all..) available in fairly high voltage ratings, which can be used to make an extremely surge-immune system; a MOV should then be used to limit absolute maximum input voltage, when that's a concern (e.g. automotive surge).

If you need isolation (e.g. running a TRIAC referenced to 24VAC, connecting to other equipment that might not be isolated from 24VAC as well, etc.), and you can't isolate the signals (use optos or etc.), then for low isolation requirements (~100V), a split-winding 24V transformer might be an option -- use at least double the VA rating required, and use only the pair of secondaries (leave the primaries disconnected, and well insulated!).  Otherwise, any isolated SMPS suitable for the voltage range will do (but mind EMI, especially for crap Amazon/Ali/eBay specials; CM filtering is highly recommended).

Tim

[Shonky]

There are plenty of switching regulators that can handle a >30V input voltage e.g.

>50V input
https://www.analog.com/en/parametricsearch/11001#/p5573=min|20&p5574=50|max&p5347=min|12&p5357=12|max&p5349=0.5|1.13&qsfv=vinmin|20_vinmax|50_vout|12_iout|500m&p5362=Buck

Two of your solutions including your current method don't have isolation, but you say isolation is a requirement? How much isolation?

What about this sort of all in one?
https://au.mouser.com/c/power/dc-dc-converters/isolated-dc-dc-converters/?input%20voltage%2C%20nominal=24%20V~~48%20V&output%20voltage-channel%201=12%20V&rp=power%2Fdc-dc-converters%2Fisolated-dc-dc-converters%7C~Input%20Voltage%2C%20Nominal

They can be exy but you don't mention volume or cost.

[PCB.Wiz]

•   Find a switcher module that will reliably handle FW rectified and filtered 24v. Just about all of them seem to have their top rated input voltage around or below the possible peak input DC that may be encountered. So marginal.

Digikey has many higher voltage modules

examples like
K78U05-500R3   Vin is 9V~90V   Vout 5V and 500mA

Addit: If you are doing a SMD PCB design, and can handle QFN style packages, TI have a number of integrated inductor voltage regulator parts.
They show 21 over 42V Vin with 65 Vin looking like the new standard.