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Old 19th November 2011, 08:40 PM   #1
epilot is offline epilot  United States
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Default Ground pplane

Hi guys,

I would like to see if it is a good choice to use ground plane/polygon for the power supply units or it is better to do not use it?

PLZ,What is your idea about the dual power supply PCB I have designed based on 78XX and 79XX?

Thnks
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Old 19th November 2011, 09:47 PM   #2
kubeek is offline kubeek  Czech Republic
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For 78xx 79xx linear regulator the board is way too big.
It depends on what power youre wasting in those - how big heatsink you need, but anyway Im sure you can make the vertical dimension just the size of C2+C1.

Other than that, post a schmatic, because I cant see which connector is output and which is input. Anyway AFAIK these linear regulators need two small caps each, not one, If C1 and C2 are output caps then they seem pointless to me, if they are input caps then what do D1 and D2 do? Are they preregulation zeners? Then where are the series resistors?
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Old 19th November 2011, 10:18 PM   #3
epilot is offline epilot  United States
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Hi, and thanks a bunch for your reply.

Quote:
Originally Posted by kubeek View Post
For 78xx 79xx linear regulator the board is way too big.
It depends on what power youre wasting in those - how big heatsink you need, but anyway Im sure you can make the vertical dimension just the size of C2+C1.
I'll try to see if I can make it smaller than what it is for now..

Quote:
Other than that, post a schmatic, because I cant see which connector is output and which is input.
Actually the left connector is the input and the right one os the output.

Quote:
Anyway AFAIK these linear regulators need two small caps each, not one,
What do you mean 2 caps really plz? The 2 small caps are valued as 104...

Quote:
If C1 and C2 are output caps then they seem pointless to me,
C1 and C2 are pointless?!! but way are you thinking so? they actually are the output caps but plz why do you think that they should not be used there?!

Quote:
if they are input caps then what do D1 and D2 do? Are they preregulation zeners? Then where are the series resistors?
[/QUOTE]

As I said the c1 and c2 are the output caps. The diodes are 1N4001 and are used there to do not let the back EMF's and so on to harm the regulator chips...
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Old 19th November 2011, 10:35 PM   #4
kubeek is offline kubeek  Czech Republic
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Quote:
Originally Posted by epilot View Post
What do you mean 2 caps really plz? The 2 small caps are valued as 104...
See this for starters, page 9. http://www.national.com/profile/snip.cgi/openDS=LM78M05
These two caps per regulator should be placed close to the regulator.
The board traces from input should first go to the cap, then from the cap to the input of the regulator.

Quote:
C1 and C2 are pointless?!! but way are you thinking so? they actually are the output caps but plz why do you think that they should not be used there?!
I assume you have a bridge rectifier and large electrolytics prior to this board, these caps are the smoothing caps. Placing large capacitance like C1 and C2 on the OUTPUT of the regulator is just asking for problems, because you 1) temporarily short the regulator on startup, 2) hinder it's transient response (caps oppose voltage transients)
Quote:
As I said the c1 and c2 are the output caps. The diodes are 1N4001 and are used there to do not let the back EMF's and so on to harm the regulator chips...
Ok these diodes have their place there. You should not use any large caps on the output, just the recommended 100nF.
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Old 23rd November 2011, 10:38 PM   #5
epilot is offline epilot  United States
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Quote:
Originally Posted by kubeek View Post
Ok these diodes have their place there. You should not use any large caps on the output, just the recommended 100nF.
Thanks for your reply.

But most of people use 100/220uF caps at the output of the 78/79xx chips amd recomend to do it!
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Old 23rd November 2011, 11:24 PM   #6
gootee is offline gootee  United States
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A finer point would be that you should always keep the power and ground traces/conductors (and ALL conductor pairs) as close together as possible, to avoid creating "enclosed loop area", which acts as an antenna (receiving and/or transmitting) for time-varying magnetic and electromagnetic fields, of which there will be plenty. (See "Faraday's Law", or Maxwell's Equations.)
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Old 23rd November 2011, 11:47 PM   #7
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Quote:
Originally Posted by epilot View Post
Thanks for your reply.

But most of people use 100/220uF caps at the output of the 78/79xx chips amd recomend to do it!
The datasheet says 100nf on input and output. If there is something noisy in the circuit then that should be decoupled locally.
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Old 24th November 2011, 12:05 AM   #8
epilot is offline epilot  United States
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Quote:
Originally Posted by gootee View Post
A finer point would be that you should always keep the power and ground traces/conductors (and ALL conductor pairs) as close together as possible, to avoid creating "enclosed loop area", which acts as an antenna (receiving and/or transmitting) for time-varying magnetic and electromagnetic fields, of which there will be plenty. (See "Faraday's Law", or Maxwell's Equations.)
Hi, and thanks.

What do you really mean by "conductor pairs"? I though that the enclosed loop problem is just for ground or each trace itself, now you seem to say that it happens between pairs too (for instance between VCC trace and ground)!
Furthermore plz how it acts as an antenna!?

It would be ok if you could plz show me what you are meaning by a pic...

Thanks again

Last edited by epilot; 24th November 2011 at 12:09 AM.
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Old 24th November 2011, 12:06 AM   #9
epilot is offline epilot  United States
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Quote:
Originally Posted by nigelwright7557 View Post
The datasheet says 100nf on input and output. If there is something noisy in the circuit then that should be decoupled locally.
But plz google the schematics for the 78/79xx regulators. MOST of them have big caps at their outputs....
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Old 24th November 2011, 01:04 AM   #10
kubeek is offline kubeek  Czech Republic
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Like this? First 7805 supply on google. http://www.circuitstoday.com/5v-power-supply-using-7805
Big cap on input, small caps near regulator.
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