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230VAC - 5V, 7.5V and 12V DC conversion

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May 27, 2010
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Hi all,

i need to make a power supply, with 3 DC output
5 V, will need max 0.56A,
7.5V, will need 2A (stepmotors)
12V will need 1A
i guess switching voltage regulators will be better.
i will select all currents a bit higher than max value.
I am confused about different variations :
- Should i first use 12V voltage regulator, and connect output of 12V regulator to 7.5V and 5V DC regulators? or should i use output of 7.5V for input of 5V?
- Should i use multiple output regulator? 12V and 5V output and 7.5V after 12V output? would inductive load effect rest of circuit negative?
What would you suggesst me to do, and why?

I just know that with voltage regulators - if you mean basic 78xx type devices (from messing up designing a power supply then having to start from scratch after reading up diligently on the subject) the lower the Vin compared to Vout, the better. Regulators are a grosso modo like a resistor/a funnel, so the higher the voltage/current you squeeze in when you need to get far less voltage out on the other side with pretty pathetic low current loads even, they'll heat up fast, unless you use a bypass transistor and heat-sink where necessary.
You may not find this an issue with a switching regulator. In my ignorance, if that's the design, personally I'd fan the output from one larger regulator to three different regulators (12, 7.5, 5V) so they can do dedicated supplying, to put it that way, and if I understand well a plus is that then you can double filter the switching noise before your respective loads (to a degree).
I'm suffering making a linear supply, and learning about transformers, regulators and variable voltage outputs, and I have to say I prefer measurement devices as they are way less difficult to put together without a lot of important complex issues you must deal with in my opinion, so maybe my advice is not much use, besides Vin - Vout x I = P.
Best of luck! :)

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Hi again.

TI, and many others, have interesting power solutions, besides some aimed at motor circuits if I remember well. I remembered this from the other day, maybe this type of device or something similar (with a higher current rating) and appropriate to your needs (I'm sure some provide multiple output voltages, like PC power supplies) could provide load current sufficient for the three regulator voltages and current draws you need:


The power management section might offer ideas or packaged solutions:

I know SMPS are supposed to kick back/inject noise into the mains supply, and apparently are uncomfortably noisy on the circuit side, but every problem/issue has a solution, I read that a linear regulator and decoupling techniques remove some of this switching noise (but not much according to a couple of articles); other people with experience in this department will know how to solve that issue with ease.
I stuck to a linear supply as I don't like noise, even if it's unavoidable, and ensuring there would be no mains supply line issues seemed beyond my skill level, but from your choice you are obviously aware that linear supplies have the contra of being cumbersome bricks...

(Disclaimer - despite the appearance, I don't get paid for sponsored TI links, but they do have some pretty nice, small packaged solutions!)

Can you use some 12V@ 3-4A adapter and MP2307 boards to make 5V and 7.5V@2A

If a load can change suddenly, then its supply should be independent of other supplies.

If a device requires a stable voltage, then its supply should be independent of other supplies.
The simplest solution would be to design for a 12V output and then use 2 separate DC-DC converter chips to get the 7.5V and 3V. The complete DC-DC converter circuits will probably cost you less than a $ each if you can find some chinese chips like the XL1410E from XLSEMI or similar. The XL1410E can take dc input upto 60V and give you upto 1.5A output. There are similar chips for higher outputs upto 3A. TPS54560 from TI can give you upto 5A output.
Seeing that you total power requirements are about 30W, a quick and not so dirty solution would be to use a laptop charger as your primary dc source and use 3 dc-dc converters :)

thank you for answers. There is something else confuses me,
there is some IC like L6566B, it takes so high DC as input, wouldnt it be so hot, because input is 230VDC then. Why they didnt use Transformer in the begining of circuit?
**broken link removed**

would you suggest me to use first 230AC/15VAC transformer, then go to 12V and 7.5V Switching power supplies?

L6566B is a switch mode controller, the HV input connecting directly to the DC bus capcitor is only used for µA currents during startup. The power flow is in fact through a (high frequency) transformer. Reading the IC datasheet thoroughly would reveal this.

Presuming you have little experience in circuit design and probably no tools to safely test a high voltage circuit, I would strongly recommend to make a conventional 50 Hz transformer supply.
thank you FvM,
would you use suggest me, linear or switching voltage regulator?
i am thinking to use get 15VDC to 12V voltage regulator and 7.5V voltage regulator. and after 12 i am thinking to add other regulator for 5V. Am i thinking correct?

hi BradtheRad ,
should i use a second transformer for inductive load, did u mean that?
or can i use same 15VDC for,
voltage regulator of normal load and voltage regulator of sudden changing load?
thanks again

Hi there,
Reading may not solve a lot of circuit problems, but it doesn't hurt to try to be as informed as possible where electricity is concerned, before trying to do something.

An extreme example is copying a circuit you find online that is supposed to be a long distance listening device, but in fact it is a microwave circuit and you "cook your face" - nasty.

Knowledge is power supplies, especially mains supplies. Some of the stuff in the rar folder won't help, some will, some isn't very good, it's all related to power supplies, feel free to have a little read.

View attachment Power supply.rar

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Linear and Switching Regulators:

View attachment SWITCHING REGULATORS snva559 TI.pdf

View attachment Basic Concepts of Linear Regulator and SMPS AN140fa Linear Technology.pdf

View attachment Linear and Switching Voltage Regulator Fundamental Part 1 snva558 TI.pdf

I read a lot before trying something, and still make a lot of mistakes, but it doesn't hurt to understand what I'm trying to make.
Good luck!
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or can i use same 15VDC for,
voltage regulator of normal load and voltage regulator of sudden changing load?
thanks again

I believe you can. The aim is to avoid a sudden drop in voltage when you drive a heavy load.

You can probably get by with one transformer, 14 or 15V. It should be rated 3.56 A minimum (taking the total of your Ampere specs). A higher Ampere rating is wise.

Attach a single full diode bridge (about 6A). Follow with a large filter capacitor (several thousand uF). Your output will be 18 or 19 V with no load. Your loads will pull it down to 14V (approximately).

Then attach 3 separate voltage regulators.

The 12V can be ordinary linear since you do not need to drop many volts. (This is the reason your transformer should be 14V, because your 12V regulator needs to be driven by a higher input voltage.

The other regulators can be smps.

It's a good idea to design so you'll have an easy time making changes to the real circuit.

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