on board Switching power supply

[javi.ar]

Hi everybody, I was asked to build an electrical oven temp/time control, and I do need a small switching power supply for powering uC. I was reading a lot lately how to design one, but what I see more complex is to dimensioning the high frequency ferrite transformer. I am looking for help, some docs, sites, or info about ferrite transformers I have seen so many, I dont know how and what to choose.  Thanks a lot . Basic requirementes are 12v for relay (4 relays) , 5v for uC and 3x7 segment temp display and 4x7 segment display for timer.

Thanks in advance

[jeroen74]

Try to find an off-the-shelf one. Even better, just buy a module that fits your requirements if you only need to make a few.

[Simon]

most people don't go about making smps transformers and they are not readily available in too many options. Your best bet is an off the shelf unit. If you really want to build something you need to get an off the shelf supply that provides the power you need and the highest voltage you need. You can then use single inductor SMPS circuits to derive the lower voltages

[Pak_Engineer]

Is it absolutely necessary to use switch mode regulator? You might get away with using just a linear one (7805 or 317) as the current requirement won't be much (<100mA?). Besides you already have 12v on board (for relay) so heat dissipation won't be a major problem either. Infact for that low a current, 24v input could also be used with some sort of pre-regulator (a simple zener with resistor or putting two regulation stages in series).

[Psi]

They even make switchmode modules in a 7805 footprint/style.

[jeroen74]

I assumed because it's for an oven, the OP is looking for a switcher that works from the mains. There are plenty of off-the-shelf modules you can buy from Farnell, RS and others, in all kinds of power ratings, from a few watts to hunderds of watts, from one output to quad outputs.

Advantages are that you are certain they work so you can concentrate on your actual problem instead of reinventing the wheel, have all the required approvals, and they are EMC compliant too.

[SeanB]

What about a mains transformer with a 24VAC 100mA secondary? Enough power to drive relays for mains switching, and easy enough to drop to 5V for the micro as well. Small, low cost and approved as well.

[Simon]

100mA ? depends on the relays and how many

[SeanB]

It is an oven, so likely one relay and 20ma for a SSR controlling the heater.