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Old 6th August 2012, 09:12 PM   #1
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Default 5V Linear power supply design

Hi,

I am designing a 5V linear power supply for an audio-controller with a 7805 regulator and do need some final design tips. The circuit is supplied with 12V DC from an external supply. The 5V are used for driving an Atmega microcontroller with multiple LEDS and transistors and a MAX232 interface. Approx. Continuous current draw is 200mA, worst case.

The current design is without any protection circuits. Two caps in parallel on the input of the 7805. 330nF and 47uF. Two caps in parallel on the output side, 100nF and 47uF.

However I do need some tips for the protection of the regulator:

Should i add a protection diode from the output of the regulator to the input to protect the regulator from having higher output voltage than input voltage with these capacitor values? Some do this, and some do not. What is the correct way?

Would it be a good idea to add a zenerclamp on the input to protect the regulator from transients? What is the best way of doing this? Any documentation would be great!

Thanks in advance
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Old 6th August 2012, 09:56 PM   #2
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If there is a possibility of shorting the input, a diode across the regulator is a good idea. I don't think clamping the input is necessary; I'd prefer to deal with any such transients at the source. Unless it's a poor 12V source, I don't think they will be any problem.
Your concerns are addressed in Texas Instruments document SNOSBT0H, which is the NatSemi LM340/LM78xx datasheet.
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Old 7th August 2012, 07:38 AM   #3
Mooly is offline Mooly  United Kingdom
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You add protection to suit the intended use and user/s.

If you know you will only connect it to a 12v supply then there is no need to protect against other scenarios. If you know you will never short or abuse the supply then short circuit protection needs to be basic and safety related only. ie a fuse to protect the transformer.
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Old 7th August 2012, 08:04 PM   #4
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Thanks for good advices and documents.

The input is not likely to short, so i think the regulator will do fine without in that case.

The 12V supply is a standard Mean Well supply, so i assume it have transient suppression built in. I was just curios if this could be useful in other psu circuitry as well.

About the 47uF capacitor values. What is the best way of calculating these values? I have found many ways of doing this with unregulated rectified DC, but there are less info about regulated DC. Are there any rule of thumbs in a design process to make this calculations quick and reliable?
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Old 8th August 2012, 07:28 AM   #5
Mooly is offline Mooly  United Kingdom
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I suppose the best answer is to consult the data sheet for manufacturer of the regulator used.

In practice its not critical. A small cap ensures stability of the regulator (again it's regulator specific). 1uF, 47uF or 100uF. That wouldn't matter. A larger cap could be said to improve the transient response of the supply within limits.

Remember any decoupling is only really effective where it's applied on a PCB. Add a few cm of wire or PCB print and things can start to look very different.

So don't get hung up on caps and sizing. Small caps close to the reg in and out pins and some local decoupling on the main PCB it feeds is all that's normally needed.
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Old 2nd May 2014, 10:54 AM   #6
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Just found a interesting bread board linear power supply on ebay for just over a 5 delivered from HK wondered it could be used with a dac build - the capacitor probably isn't low ESR but maybe could substitute with one that is some pics are available from the page and a data sheet below

Click the image to open in full size.

Click the image to open in full size.

Product data sheet http://netdrive.envell.com/pic/ebay/..._Datasheet.pdf

Any thoughts ? is this suitable to be used once the breadboard pins are removed & maybe replace the cap ?

Last edited by laughingbuddha; 2nd May 2014 at 11:20 AM.
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Old 2nd May 2014, 11:41 AM   #7
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Looks like a 47UF 35V TANTALUM CAPACITOR ?
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