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power supply design

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yohanevindra

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i want to know how to design my own power supply for projects and how to use regulators in them and to know how to choose suitable regulators...

i have a project with a PIC18 microcontroller and which involves 2 servo motors, a ultrasound sensor and transmitter, IR sensor and transmitter, LCD and keypad... i made up a power and current table and worked out that 5V would work for each of these...however, when it comes to current, there is a requirement of about 3A when the ultrasound transmitter is sending out a pulse and about 1A when it is not.

How do i get about building my own power supply?there will be the possibility of obtaining a DC power supply from mains electricity which means i jus need a DC-DC conversion..

any online resources i can use?or any relevant chapters in books or anything?

Thanks!
 

EN0

Member
Hey,

You could use the LM338 voltage regulator which can provide up to 5A of current; that should be plenty if your doing electronic projects. First, you need a transformer to step the voltage down from 120V to probably 30V or so (it will depend on the LM338 datasheets input voltage required). Then you put that 30V into a rectifying bridge circuit and then to a filter capacitor. This will smooth out the ripple. After that, all you need to do is simply connect that output to the voltage regulator. The LM338 datasheet has some applications you can consider and so you can choose one of them. They also have the LM317 voltage regulator if you only need 1.5A of current.

Good luck!
 

yohanevindra

New Member
Thanks giftiger wunsch and electronerdy!

But when i hav a regulator, the devices will jus draw the current that they need right?and I jus hav to make sure that I dont overload the regulator with too high a current?
 

giftiger_wunsch

New Member
Current is the quotient of voltage and resistance. The current will always be the voltage divided by resistance.

If the regulator can handle up to 5A of current, you could either use a current-limiting circuit or a fuse / circuit breaker to ensure that you don't draw too much current and start destroying components. This will help minimise damage if a short occurs.
 

yohanevindra

New Member
so if i hav suitable resistors across each terminal of each device, they will draw only the current supplied by the resistor/regulator?

also, how would i get about supplyin 2A to the ultrasound device every second and the rest of the time supply it only about 100mA?
 

giftiger_wunsch

New Member
so if i hav suitable resistors across each terminal of each device, they will draw only the current supplied by the resistor/regulator?

Doing this you'll most likely dissipate too much power in the resistors; your best bet is just to make sure the current requirements of whatever is connected at any one time doesn't exceed 5A. Use a circuit-breaker or fuse so that if this does happen, or if a short occurs, the circuit is broken safely.

yohanevindra said:
also, how would i get about supplyin 2A to the ultrasound device every second and the rest of the time supply it only about 100mA?

As I said, current just depends on voltage and resistance. Provide the device with the recommended voltage and you're fine. When a device requires more current, its resistance drops, thereby increasing current. Of course this is a very simplistic explanation though. The circuitry in an ultrasound device is likely to be very complex.
 

BrownOut

Banned
also, what is the difference between buck converters, LDOs, switchin mode power supplies and stuff like that?

LDO's and linenar supplies (like the aforementioned LM883) use a DC sampled voltage, amplified and fed back to a "pass transistor" who's tranconductance changes with the voltage at the output in such a way as to keep the output voltage constant within tight tolerances. Buck and switchers use a self generated, fast changing signal derived from the input fed into magnetic elements to produce an output voltage in a specified range.
 
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yohanevindra

New Member
ok... so we chose a power regulator, i tihnk the LM1084IT. It is rated for max output of 5V and 5A.does the input voltage we put to the regulator have to be greater than 5A?or do we only hav to make sure that the required input voltage is matched?
 
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