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Schematic and PCB trace width help

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strokedmaro

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couple of questions I hope you experts can help with :)

1. Attached is a piece of a schematic....Ive seen dual diodes like this in several schematics but have no idea why you would want that. can someone explain?

2. Ive finished etching out a PCB for my latest project but I had a thought while laying in bed last night....is there a formula to know how much current can flow through a copper trace before it fails? The trace in question is 1mm wide, 1/2 oz copper which needs to carry approx 1 amp at 12vdc. Just doesn't sound reliable to me.

THANKS!!!
 

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ericgibbs

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strokedmaro said:
couple of questions I hope you experts can help with :)

1. Attached is a piece of a schematic....Ive seen dual diodes like this in several schematics but have no idea why you would want that. can someone explain?

2. Ive finished etching out a PCB for my latest project but I had a thought while laying in bed last night....is there a formula to know how much current can flow through a copper trace before it fails? The trace in question is 1mm wide, 1/2 oz copper which needs to carry approx 1 amp at 12vdc. Just doesn't sound reliable to me.

THANKS!!!
Hi,
Look here and down the free 'pcb design tutorial', excellent tutorial, I would recommend it for any level of pcb layout designer.

http://www.alternatezone.com/electronics/pcbdesign.htm

Diodes can be used for a number of functions.

Very basically, they conduct current in only one direction.

Need a little more circuit on the picture...;)
 

ronsimpson

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http://www.electro-tech-online.com/custompdfs/2008/04/PCBDesignTutorialRevA.pdf
Here is a tutorial for PCB design. Page 7 shows a chart of trace width and current. One amp will create a 10C rise in temperature with 30mils of trace. There is a big difference between 10C and failing.

Why two diodes? You did not include enough information to say what is happening in you example. The two diodes can block more voltage in the reverse direction. They will also drop 1.4 volts forward.
 

ronsimpson

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In the second circuit the MPSA14 can not pull ( C ) all the way to ground. I think the A14 is a Darlington and with 10M on the (B) it is lucky to bring ( C ) to 1 volt. The voltage on VR2 needs to go to zero volts. The two 1N4007s will drop 1.4 volts. SO: If MPSA14 is on but with 1 volt across C-E then with the drop in the two diodes the voltage on VR2 is zero.

Circuit one has the same thing happening. Starting on the left side. The first 3 transistors can pull their outputs very close to ground. (.2 volts) Then there is a logic made of diodes. (A=top, B=top & bottom, C=bottom) By the time you get through the diodes there is 0.7 volt loss. (.2+.7=.9) Now a low voltage is 0.9 volts. But we need to turn off the TIP41 where anything above 0.6 volts is on. The diode pair loose 1.2 volts ensuring that the base of the TIP41s will be at 0 volts when off.
 

strokedmaro

New Member
So, after looking at the table of current vs track width I should be more than good. 1 millimeter = 39.37 thous. My 1/2oz traces at 1mm wide are roughly double what is necessary to carry 1 amp! This should also give me about half the temp rise also...about 5deg C. THANKS!!!

I'm glad I don't have to re-etch this board...thankfully even though this was an afterthought, everything should work as designed. Attached is a pic of the board.
 

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ericgibbs

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hi,
My pictorial explanation is a little late, but what the heck!.:)
 
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ericgibbs

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strokedmaro said:
So, after looking at the table of current vs track width I should be more than good. 1 millimeter = 39.37 thous. My 1/2oz traces at 1mm wide are roughly double what is necessary to carry 1 amp! This should also give me about half the temp rise also...about 5deg C. THANKS!!!

I'm glad I don't have to re-etch this board...thankfully even though this was an afterthought, everything should work as designed. Attached is a pic of the board.
hi,
As you read thru the tutorials you will see that track width and current capacity are not the only criteria for a 'good' pcb.

You should consider the layout of tracks relative to each other for the signals the track carrys, screening. etc
 

Boncuk

New Member
.... also 90 degrees change of tracks will give less mechanical stability in case the board becomes hotter than assumed. Twice 45 degrees results in the same direction, but with split stress on the trace.

That way traces become shorter, too --> shorter trace --> less resistance --> less heating --> looks nice too. :)
 
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