Universial Ac Dc converter circuit for small projects

[Rutger]

Hi all, first of all thank you Dave Jones for start this excellent blog, I love it!
My background is very similar, but I ended up in the software side, but have always tinkered with electronics.
After viewing many of the video blogs I thought of a project I that I would be interested in doing but have trouble getting started and then I thought maybe other people might be also interested in this?

The project I would love to see a video blog is on is how to design a simple universal ac/dc power supply using switching technology for small projects or small applications, like input 85V to 260V ac & output in the 12 volt range and no more than 0.5 amps.

Anyhow if any of you have already done this or knows of an excellent chip to use for this project feel free to blog me back.

Thanks for reading my little message.

Rutger

[Simon]

hm for powers this low there are a good range of all in one solutions from most stockists like farnell and digikey.

Their datasheets often give full details including PCB for the finished circuit.

Dave did do a blog on using the MC34063 chip (or any of it's clones)

[Rutger]

Thanks Simon, but isn't that chip a DC to DC converter?

[Simon]

well any switching regulator is a DC/DC conveter, you just put a rectifier and smoothing capacitor in front. Designing switchers that run straight off the mains can be tricky and more delicate so from a hobbiest perspective often a standard unregulated power pack followed by a low voltage SMPS is easier

[Rutger]

Hi Simon, I guess I am rusty in electronics design, I get the rectifier and capacitor part, but what about the voltage input range of 85V to 260V AC, are you suggesting a small traditional transformer to step this down to within the chip specs of 3 V ~ 40 V?

[Simon]

yes, for hobbiest uses that is generally easier although if your really into things you may be able to design one that runs right off the mains DC voltage, basically the inductor is replaced by a small transformer, because of the higher frequency the transformer is much smaller and offers isolation like a normal 50Hz stepdown transformer would

[Rutger]

I have looked at the data sheet for the MC34063 but all the calculations are based on a fixed input voltage and I don't see any schematic examples were there is some kind of 'fly-back' or feedback of the possible changes in the input voltage, I really don't think you can use this chip for a universal power supply, but hey maybe I missed something? I was looking at the LNK302D chip of the following website

http://www.powerint.com/products/linkswitch-family/linkswitch-tn

and thought this was more the route to go?

[Simon]

I've not personally used the 34063 but it should cope with some input variation. I think the trick would be to increase the inductor as this is the part that is specs for a certain input and output. The formula's specify for the min input giving the min inductance and other minimal specs

The 34063 is an old one so there will be many more better ones. I'm just looking into this sort of thing myself too

[Rutger]

Thanks for your feedback Simon and let me know if you find a better solution!  

[Simon]

well if you post your requirements I'm sure someone will suggest a good IC for the job

[Zero999]

Hi Simon, I guess I am rusty in electronics design, I get the rectifier and capacitor part, but what about the voltage input range of 85V to 260V AC, are you suggesting a small traditional transformer to step this down to within the chip specs of 3 V ~ 40 V?

That's not normally how it's done.

The typical method involves rectifying 85V to 260VAC to give 120V to 368VDC and using an isolated DC-DC converter to convert it to a lower voltage. The DC-DC converter is an oscillator which converts the DC into a PWM AC waveform, passes it though a lightweight ferrite cored transformer and a rectifier to produce lower voltage DC. Feedback is taken from the low voltage DC side via an opto-coupler or another oscillator connected to a pulse transformer and the duty cycle of the PWM waveform is altered to compensate for and changes in voltage.

Very few hobbyists bother making mains powered SMPSes because they're difficult to build, can be dangerous if designed or assembled incorrectly and cheap off the shelf units are readily available.

[Rutger]

I though I did, but here is my specific spec:

Input: 85V - 260V AC
Output: 12V DC, 0.5 A

Rutger

[Simon]

why do you have such a wide input voltage requirement ? if this is a basic power circuit for hobby needs can't you assume that the mains is 230 V +/-10% ? that way you use a transformer to get a reasonable input voltage and the use the DC/DC converter

[Rutger]

Thanks Hero999, I understand there are units available, but I am looking at a bigger project/possibly commercial were there are other components that I want to put on one board, I could possibly put 2 boards in one box, but I rather keep it all on one.

What do you think about the LNK302D chip from powerint.com (see previous post for link).

Rutger

[Simon]

interesting looking chip, I'm unclear as to why there are so many source pins but only 1 drain pin. If they provide all the building information on the datasheet and it meets your requirements go for it. usually non isolated supplies are not liked due to the potential shock hazard. If your building something commercial it might pay off to buy premade units that meet your rerquirements and insert it alongside your own board avoiding some EMC issues and having your own board having to cope with mains voltages

[Rutger]

Hi Simon, sorry that I did not explain this from the start, but I am researching a possible application where this application can be used world-wide, but thought this could also be of interest to other users as well.  I am based in the US right now but have a world wide customer base. I am actually in the coffee business and sell parts for commercial espresso machines.  If people are interested I can expand on the project more.

[Simon]

We are always interesting in what people are up to. Yes in your case you do need the wide input  range and it looks like that is what chip was designed for. If it meets your needs go for it. The easiest way to use an IC is follow the manufacturers recomendations

[Rutger]

Ok, so the project I am thinking about is to make what is called a 'level control board'.
It does a couple of small important things:

1) Control the water level in the steam boiler of an espresso machine, by opening a fill valve and activating a water pump
2) Protecting the heating element from being exposed to air via a low water level probe in some machines.

The manufacturer right now of these control boards make a different flavor for each machine, not just a 110V version and 220V version, but I am talking about at least 2 dozen different versions, its crazy!  So I want to make one that is universal! if it can be done without costing to much.

Rutger

[Simon]

well mass production is your key to cheaper production costs. But what current requirement do you have ? that part you found won't produce high currents.

[Zero999]

Thanks Hero999, I understand there are units available, but I am looking at a bigger project/possibly commercial were there are other components that I want to put on one board, I could possibly put 2 boards in one box, but I rather keep it all on one.

You can buy open frame PSUs which are just a circuit board with mounting holes.

For 12V at 500mA though, just buy a plug-top mains adaptor. You can even get them with different plugs so all the user needs to do is fit the one appropriate for their location.

What do you think about the LNK302D chip from powerint.com (see previous post for link).

The maximum output current is only 120mA, you need 500mA.

The circuit on the datasheet doesn't have any galvanic isolation so you have to treat the 12V side with the same precautions as the AC side which means your circuit could electrocute someone, if not assembled correctly. It also means you can't connect it to any external circuits without using some form of galvanic isolation.

http://bbs.dianyuan.com/bbs/u/24/1102589586.pdf

[Rutger]

yes, the power supply is there to supply the circuit to power the level sensors and to power either triacs or small relays in the range of 10A. What is your thoughts on relays versus triacs to switch on valve coils and turn on pump motors.

[Rutger]

Actually I am just guessing the power requirements, I think I need much less. All I have to power is to activate 2 relays at the same time at probably 50ma max each and the circuit to measure 2 level probes, I guess this won't me much more than 30 ma, so 120ma might work fine. If my design would use triacs the power consumption would be less. Do you have any suggestion on how to add the extra isolation from the high voltage? I don't want to shock anyone!

Rutger

PS: I have looked at the open frame PSU and they all are so big! I really need a very small board, probably no bigger than 5 x 5 cm (2"x2")

[Simon]

you need to acertain exactly what your amperage requirement is as otherwise you won't be able to select appropriate parts. SRC's will use less power than relays but are DC devices, you could use a triac I think, I'm no expert in these two though

[deephaven]

How about something like this: http://www.xppower.com/pdfs/SF_ECL05-30.pdf

I would highly recommend using a module/circuit board like this as there is an enormous amount of saftey and EMC requirements to deal with, and these boards have already gone though those approval processes.

[Zero999]

Actually I am just guessing the power requirements, I think I need much less. All I have to power is to activate 2 relays at the same time at probably 50ma max each and the circuit to measure 2 level probes, I guess this won't me much more than 30 ma, so 120ma might work fine. If my design would use triacs the power consumption would be less. Do you have any suggestion on how to add the extra isolation from the high voltage? I don't want to shock anyone!

Please post a schematic.

The kind of questions you're asking indicate a lack of experience required to build a safe mains powered project. I can see sparks flying and someone getting hurt/killed.