Capacitors on a Power supply (9v to 5v)

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bdickens
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Capacitors on a Power supply (9v to 5v)

Post by bdickens » Fri Sep 19, 2008 7:51 am

I am working with my son on a project and we had to build a voltage converter. Looking at 7805-based circuits, I noticed that they all look "architecturally" the same, but the capacitors are different sizes for each. We are using a 9V transitor battery so some of this is clearly the input but can anyone give me a simple explanation as to how the capacitors are sized ? What is the effect if they are larger / smaller than what is specified ?

Thanks

Robert Reed
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Post by Robert Reed » Fri Sep 19, 2008 12:50 pm

Assuming a normal maximum load current of 20 MA for your battery size, you probably only need a 0.33 mfd/ 50 volt ceramic on the input side to the 7805 and a 0.1 mfd/50v ceramic on the out put side of that device (50 V because this is a common lo voltage size). A 10 MFD/16V 'lytic on the output side again will improve transient response and load stability, but may not be needed depending on your application. The 0.33 MFD on the input should be tied close to the 7805 and in fact is not required if the battery to device leads are shorter than 6". Too small of a cap on the output side will not filter out load caused disturbances on the B+ line. Too big is a waste of money and space. Incidently with that small load current I would use a 78L05. this comes in the smaller TO-92 case as opposed to the larger TO-220 package of the 7805.

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philba
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Post by philba » Fri Sep 19, 2008 2:17 pm

It's likely that most of the designs you looked at were for supplies that take their input from rectified AC. In that case, the filter cap needs to be fairly big to filter out the ripple. There is even a formula to determine cap size. With a battery, you don't need much at all. In fact, you could probably get by without any cap on the input.

By the way, your 9V battery will run out really fast because they have almost no capacity. If you could find a low drop out regulator (instead of the 7805), you could use 4 AA cells for dramatically longer run time.

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MrAl
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Post by MrAl » Fri Sep 19, 2008 8:32 pm

Hi,

I have to agree that AA cells will last much longer, especially the
rechargeable types. 9v cells run down too fast.
LEDs vs Bulbs, LEDs are winning.

dyarker
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Post by dyarker » Fri Sep 19, 2008 9:51 pm

If you need the "9V" elsewhere in the circuit, or you want to stay with 7805, you can use 6 AAs. If 6 AAs take up too much space, then 6 AAAs would be a little bigger than a 9V; and would still last much longer.

Cheers,
Dale Y

bdickens
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Post by bdickens » Sat Sep 20, 2008 10:38 am

Thank you all. That cleared up a number of questions.

Points on battery life well taken. It did in fact drain it faster than I would have liked. But wondering a bit more, I traced the input from the battery to the processor and discovered that they had already embedded a 2 step conversion, first to 5v, then to 3.3. So I just tapped into the 5 and all was well.

But now I know more and that was the point. Thank you

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