on board Switching power supply - Page 1
Forum section: electronics / projects

author: javi.ar
author_rank: Newbie
timestamp: November 09, 2012, 08:44:02 pm
content: 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

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 09, 2012, 09:05:52 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: November 10, 2012, 08:03:44 am
content: 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

author: Pak_Engineer
author_rank: Contributor
timestamp: November 10, 2012, 09:32:27 am
content: 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).

author: Psi
author_rank: Super Contributor
timestamp: November 10, 2012, 10:39:58 am
content: They even make switchmode modules in a 7805 footprint/style.

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 10, 2012, 10:47:21 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: November 10, 2012, 11:03:25 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: November 10, 2012, 12:12:08 pm
content: 100mA ? depends on the relays and how many

author: SeanB
author_rank: Super Contributor
timestamp: November 10, 2012, 12:32:33 pm
content: It is an oven, so likely one relay and 20ma for a SSR controlling the heater.