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How much current can a PCB handle?

A

Andrew

Jan 1, 1970
0
How much current can different copper weight PCBs handle? Not so much
in terms of how much can traces handle, but a PCB in general. I'm
talking strictly DC current, relatively low voltage (+24VDC).

If traces are more or less avoided and copper pours are used for the
high current nets, what would be a rough estimate of the maximum
current that could be run through 1/2 oz. PCB? How about 1oz. or 2oz.?

I'm really just curious because one of the things I am thinking of
designing in the future is a motor driver, where the current could
reach 15A + continuous and could easily spike to 100A or more, in some
cases. I know there are motor driver pcbs out there now that are very
small size and can handle these types of current outputs, but I'm not
sure what guidelines are used to determine what is needed (even copper
pour area size) for high current in terms of a PCB.

I am just a newbie, but when I hear about 20 to 30 amps being passed
through a PCB, I get nervous... even if there was an entire plane
dedicated to the high current power and ground. Am I worried for no
reason? Can an entire plane of, say, 1/2 oz. weight, board size, say,
9sqin or how about 25sqin handle 15-20 amps continuous (or at least for
a minute or two)? How about a copper pour area of 1/2oz. that is sized
at about 1sqin? Could that handle this type of current?

Any info on high current PCB design or any general high current design
notes would be greatly appreciated, or any pointers to where I could
get further information or maybe even calculations for amount of copper
needed to safely run certain current levels.
 
A

Ancient_Hacker

Jan 1, 1970
0
Try looking at this on-line calculator:

http://tinyurl.com/o8gf4

I suggest using a safety factor of at least THREE times above the
expected peak, and of course use fuses, circuit breakers, or thermal
fuses/cutouts..

You may want to go to an even lower resistance if the board is
particularly long, to minimize voltage drops and heating.

And of course there's always dual-sided boards or ones with thicker
copper. I recall looking at a street-light controller PCB (with
Tubes!), which had particularly heavy copper as it had to carry many
amps.
 
P

PeteS

Jan 1, 1970
0
Mike said:

I successfully did a 30A switching power supply on a small (2.5 inch by
1.2 inch) board. That had 2 oz copper for each plane, however.

A larger backplane I did had to distribute 70A, and I used 2 x 2 oz
planes for that (because of the distances involved and the fact there
were thousands of holes for the plug-in board connectors ;)

In both cases I used the PCBTemp program which yu can still
occasionally find floating around.

Cheers

PeteS
 
D

David L. Jones

Jan 1, 1970
0
Andrew said:
How much current can different copper weight PCBs handle? Not so much
in terms of how much can traces handle, but a PCB in general. I'm
talking strictly DC current, relatively low voltage (+24VDC).

If traces are more or less avoided and copper pours are used for the
high current nets, what would be a rough estimate of the maximum
current that could be run through 1/2 oz. PCB? How about 1oz. or 2oz.?

I'm really just curious because one of the things I am thinking of
designing in the future is a motor driver, where the current could
reach 15A + continuous and could easily spike to 100A or more, in some
cases. I know there are motor driver pcbs out there now that are very
small size and can handle these types of current outputs, but I'm not
sure what guidelines are used to determine what is needed (even copper
pour area size) for high current in terms of a PCB.

I am just a newbie, but when I hear about 20 to 30 amps being passed
through a PCB, I get nervous... even if there was an entire plane
dedicated to the high current power and ground. Am I worried for no
reason? Can an entire plane of, say, 1/2 oz. weight, board size, say,
9sqin or how about 25sqin handle 15-20 amps continuous (or at least for
a minute or two)? How about a copper pour area of 1/2oz. that is sized
at about 1sqin? Could that handle this type of current?

Any info on high current PCB design or any general high current design
notes would be greatly appreciated, or any pointers to where I could
get further information or maybe even calculations for amount of copper
needed to safely run certain current levels.

I posted some tips a while back on this topic here:
http://groups.google.com.au/group/a...frm/thread/e54194623cfedb7d/e890c37873e5c21c?

Regards
Dave :)
 
A

Andrew

Jan 1, 1970
0
Mike said:

I've seen these calculators before but I thought they were primarily
for circuit traces (tracks) instead of just copper areas (pours).
Also, they max out at 35A and 0.4" (the ones I've seen). At any rate,
is the "copper area" they produce as part of the results the same as
the copper area that would be needed for a copper pour/area? i.e. if a
particular result on these calculators says "4000 mils copper area"
does that mean a 2" x 2" copper pour would produce the results it says?
I always thought the copper area result they gave was some sort of
interim result that was needed for the calculator to find the remainder
of the results. I never considered that it might mean the actual
copper area (answer to my question).
 
A

Andrew

Jan 1, 1970
0
PeteS said:
I successfully did a 30A switching power supply on a small (2.5 inch by
1.2 inch) board. That had 2 oz copper for each plane, however.

A larger backplane I did had to distribute 70A, and I used 2 x 2 oz
planes for that (because of the distances involved and the fact there
were thousands of holes for the plug-in board connectors ;)

In both cases I used the PCBTemp program which yu can still
occasionally find floating around.

Cheers

PeteS

I've found the pcbtemp software but I'm not sure how to handle copper
pours/areas or planes with it? As far as I can tell, I must enter a
track width? What do I enter for a width so that I get the proper
results? It seems that I could enter just (the bigger) of two
dimensions of a rectangular copper pour/area/plane? Length doesn't
affect the current, it is only used for voltage drop. It seems that
this calculator has a flaw? For example, if I enter a width of, say, 1
inch (1000 mil) and a length of 0.1" (100 mil), 10 deg C rise, it says
I can handle 35 amps with 1mV drop. If I enter 100 mil wide, 1 inch
long, 10 deg C rise, it says 7.5 amps, 20mV drop. Quite a big
difference for the exact same area of copper...

This just makes me not trust it 100%, because I'm not sure how to use
it. Any pointers for using these calculators/PCBtemp software for
copper pours/areas/traces?
 
E

Eeyore

Jan 1, 1970
0
Andrew said:
How much current can different copper weight PCBs handle? Not so much
in terms of how much can traces handle, but a PCB in general. I'm
talking strictly DC current, relatively low voltage (+24VDC).

If traces are more or less avoided and copper pours are used for the
high current nets, what would be a rough estimate of the maximum
current that could be run through 1/2 oz. PCB? How about 1oz. or 2oz.?

I'm really just curious because one of the things I am thinking of
designing in the future is a motor driver, where the current could
reach 15A + continuous and could easily spike to 100A or more, in some
cases. I know there are motor driver pcbs out there now that are very
small size and can handle these types of current outputs, but I'm not
sure what guidelines are used to determine what is needed (even copper
pour area size) for high current in terms of a PCB.

I am just a newbie, but when I hear about 20 to 30 amps being passed
through a PCB, I get nervous... even if there was an entire plane
dedicated to the high current power and ground. Am I worried for no
reason? Can an entire plane of, say, 1/2 oz. weight, board size, say,
9sqin or how about 25sqin handle 15-20 amps continuous (or at least for
a minute or two)? How about a copper pour area of 1/2oz. that is sized
at about 1sqin? Could that handle this type of current?

Any info on high current PCB design or any general high current design
notes would be greatly appreciated, or any pointers to where I could
get further information or maybe even calculations for amount of copper
needed to safely run certain current levels.

Here's a table of suggested values for a 10C temperature rise. It's very
conservative. There's a little formula for other temp rises. I'd be quite happy
with 30C for non-continuous duty.
http://www.vutrax.co.uk/trackwidth.htm

I've designed many high power audio amplifiers btw and the kind of currents you
mention are quite routine in that application. 2 oz ( 70 um ) copper is a good
idea btw.

Graham
 
D

DJ Delorie

Jan 1, 1970
0
Andrew said:
For example, if I enter a width of, say, 1 inch (1000 mil) and a
length of 0.1" (100 mil), 10 deg C rise, it says I can handle 35
amps with 1mV drop. If I enter 100 mil wide, 1 inch long, 10 deg C
rise, it says 7.5 amps, 20mV drop. Quite a big difference for the
exact same area of copper...

Current capacity depends on cross-section area, not surface area. In
your first example, you have a cross-section area of about 1500 mil^2,
and your second is 150 mil^2. That's a ten times difference. The
rest of the math is beyond my experience, but I'm not surprized that
the program gives you different numbers. I'd be suspicious if it
didn't.

For example, a trace 1 mil wide and 100 inches long will not have the
same current capacity as a 100 mil trace 1 inch long.
 
E

Eeyore

Jan 1, 1970
0
DJ said:
Current capacity depends on cross-section area, not surface area. In
your first example, you have a cross-section area of about 1500 mil^2,
and your second is 150 mil^2. That's a ten times difference. The
rest of the math is beyond my experience, but I'm not surprized that
the program gives you different numbers. I'd be suspicious if it
didn't.

For example, a trace 1 mil wide and 100 inches long will not have the
same current capacity as a 100 mil trace 1 inch long.

The areas is actually important for radiating and conducting the heat away
though.

Graham
 
K

Keith

Jan 1, 1970
0
How much current can different copper weight PCBs handle? Not so much
in terms of how much can traces handle, but a PCB in general. I'm
talking strictly DC current, relatively low voltage (+24VDC).

If traces are more or less avoided and copper pours are used for the
high current nets, what would be a rough estimate of the maximum
current that could be run through 1/2 oz. PCB? How about 1oz. or 2oz.?

Take a look at PC motherboards. Some processors draw upwards of
75A, two of 'em on a board...
I'm really just curious because one of the things I am thinking of
designing in the future is a motor driver, where the current could
reach 15A + continuous and could easily spike to 100A or more, in some
cases. I know there are motor driver pcbs out there now that are very
small size and can handle these types of current outputs, but I'm not
sure what guidelines are used to determine what is needed (even copper
pour area size) for high current in terms of a PCB.

I am just a newbie, but when I hear about 20 to 30 amps being passed
through a PCB, I get nervous... even if there was an entire plane
dedicated to the high current power and ground. Am I worried for no
reason? Can an entire plane of, say, 1/2 oz. weight, board size, say,
9sqin or how about 25sqin handle 15-20 amps continuous (or at least for
a minute or two)? How about a copper pour area of 1/2oz. that is sized
at about 1sqin? Could that handle this type of current?

Also think about the connectors.
 
A

Andrew

Jan 1, 1970
0
DJ said:
Current capacity depends on cross-section area, not surface area. In
your first example, you have a cross-section area of about 1500 mil^2,
and your second is 150 mil^2. That's a ten times difference. The
rest of the math is beyond my experience, but I'm not surprized that
the program gives you different numbers. I'd be suspicious if it
didn't.

For example, a trace 1 mil wide and 100 inches long will not have the
same current capacity as a 100 mil trace 1 inch long.

Are you saying it depends on where the connections are, within the
trace?

The way I described what is puzzling me, the 0.1" wide by 1" trace VS
the 1" wide by 0.1" long trace, they would look exactly the same on a
board (copper amount). If you are looking at it one way, and rotate
the board 90 degrees, it looks like the second way I describe. The
same exact copper pour, yet the calculator gives two VERY different
results depending on which one you consider to the be "width" and which
you consider to be the "length" (basically, which way you are orienting
the board when viewing it, in the case I described). I assume this
either means there is a bug in the calculators, or that the placement
of your via/hole/pin within the trace matters greatly.

I'll try to draw it quick with some text below.

<-------------------------1 inch long------------------->
================================

* 100mil wide *

================================

Where the * is a via/hole/pin. This would be the "1 inch long by
100mil wide" version (above) that I described, and the "100 mil long by
1 inch wide" version (below). The below version is a little
impractical, but you can see how it is the same copper area, just a
different via/hole/pin placement which changes the wording of width and
length. Is this the important fact that I'm missing? That
via/hole/pin placement matters?

<------------------------1 inch long-------------------->
================================ -
*
100
mil long
*
================================ -

See how the copper is the same amount? I can definitely see how the
via placement would/could affect the current capacity (I guess), but I
am asking if it does? This is an especially important question when I
am trying to figure out the current capacity of a particular power or
ground plane, or copper pour/area. If the via locations do in fact
matter as much as (or more than) the size of the copper which they are
connected to, than this will greatly affect how much current a
particular pour/area/plane can handle, right? i.e. If I have an
entire plane (say 3" x 3" board) dedicated to power, if I place a via
in the top center and bottom center, it is much different current
handling capacity than if I place them both in the center, 0.125"
apart? Am I all screwed up? Am I thinking logically? I'm confusing
even myself now :(
 
J

John Woodgate

Jan 1, 1970
0
dated said:
Are you saying it depends on where the connections are, within the
trace?

I think when you get the message, you will kick yourself.

Think about wire instead of copper foil. I have this copper wire which
is 0.5 inches square and four inches long. It has a resistance from one
end to the other (4 inches) of X ohms.

Now I have this copper billet, which is 4 inches wide by 0.5 inches
thick and 0.5 inches long. It has a resistance from one end to the other
(0.5 inches) of Y ohms. Y is not equal to X!

Yes, unless the board conductor is a geometrically-square sheet, the
resistance does depend on where on the edges you connect to it. Even
with a square, it does vary a bit if you have only point contacts.
 
D

DJ Delorie

Jan 1, 1970
0
Eeyore said:
The areas is actually important for radiating and conducting the heat away
though.

Yeah, hence the "beyond my experience" comment. I know it matters, I
just don't know how. I figured *one* obvious discrepancy was enough
to discourage the "they should be the same" theory.
 
D

DJ Delorie

Jan 1, 1970
0
Andrew said:
The way I described what is puzzling me, the 0.1" wide by 1" trace VS
the 1" wide by 0.1" long trace, they would look exactly the same on a
board (copper amount). If you are looking at it one way, and rotate
the board 90 degrees,

If you put three 1k resistors in series, does it have the same total
current capacity as three 1k resistors in parallel? It's a similar
question to what you ask.
 
A

Andrew

Jan 1, 1970
0
John said:
I think when you get the message, you will kick yourself.

Think about wire instead of copper foil. I have this copper wire which
is 0.5 inches square and four inches long. It has a resistance from one
end to the other (4 inches) of X ohms.

Now I have this copper billet, which is 4 inches wide by 0.5 inches
thick and 0.5 inches long. It has a resistance from one end to the other
(0.5 inches) of Y ohms. Y is not equal to X!

Yes, unless the board conductor is a geometrically-square sheet, the
resistance does depend on where on the edges you connect to it. Even
with a square, it does vary a bit if you have only point contacts.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

So where the connections are matter *alot*. I get it now, I think.

The "from one end to the other" phrase you list is the key component.
The ends meaning where the connections are made. You could take the
same copper billet and place the connections (ends) the "long" way and
it would be the same as the copper wire, right? If so, I understand
completely.

I just didn't realize how important the closeness of the connections
was for current capability, when talking about an entire copper area or
plane.

Would this be correct to say as well: That the resistance is
determined by the closeness of the connections. The copper that
expands outward (width) is for heat sinking.. ?

If so, is there a point where the width stops affecting thte
resistance, at least significantly enough to consider?

(Sorry if these are ridiculous questions, but I've never considered how
resistance is affected by shapes and where connections are, etc)
 
J

John Woodgate

Jan 1, 1970
0
dated said:
I just didn't realize how important the closeness of the connections
was for current capability, when talking about an entire copper area or
plane.
Would this be correct to say as well: That the resistance is
determined by the closeness of the connections.

Yes. There is a formula, which usually uses Greek letters, but I spare
you that.

resistance = resistivity x length/area

resistivity is something you find from tables of material properties.
It's measured in funny units - ohm.metres. Don't bother why, until your
studies have progressed a bit.
The copper that expands outward (width) is for heat sinking.. ?

Maybe. It doesn't work well as a heat sink if it's expanded a lot.
If so, is there a point where the width stops affecting thte
resistance, at least significantly enough to consider?

Yes, but it's difficult to quantify.
(Sorry if these are ridiculous questions, but I've never considered how
resistance is affected by shapes and where connections are, etc)
Don't worry, but you may be happier posting to sci.electroncis.basics,
where the constitution says that there are NO stupid questions. There
may, however, be stupid answers, such a 'use a PIC'. (;-)
 
D

DJ Delorie

Jan 1, 1970
0
John Woodgate said:
There may, however, be stupid answers, such a 'use a PIC'. (;-)

Especially if you're trying to answer the question in $subject :)

Q: How much current can a PCB handle?
A: Use a PIC.
 
E

Eeyore

Jan 1, 1970
0
John said:
Yes. There is a formula, which usually uses Greek letters, but I spare
you that.

resistance = resistivity x length/area

resistivity is something you find from tables of material properties.
It's measured in funny units - ohm.metres. Don't bother why, until your
studies have progressed a bit.


Maybe. It doesn't work well as a heat sink if it's expanded a lot.

Yes, but it's difficult to quantify.
Don't worry, but you may be happier posting to sci.electroncis.basics,
where the constitution says that there are NO stupid questions. There
may, however, be stupid answers, such a 'use a PIC'. (;-)

John, surely you have IEC/EN 60065 or 60950 to hand ? Isn't there a formula in
there too ? Or is it just track spacing and I'm imagining it ?

Graham
 
J

John Woodgate

Jan 1, 1970
0
dated Tue said:
John, surely you have IEC/EN 60065 or 60950 to hand ? Isn't there a
formula in there too ? Or is it just track spacing and I'm imagining it

Spacing; nothing about current capacity of traces and sheets.
 
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