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Old 23rd November 2011, 10:34 PM   #1
epilot is offline epilot  United States
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Default PCB track

Hello Guys.

I just would lik to know if a track having 1.5 millimeter width is good enough to carry the VCC or VEE of the output in a regulated power supply? the regulated voltage is +-12V.

Thanks
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Old 23rd November 2011, 10:49 PM   #2
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hi epilot,

You need to specify current and copper thinkness before anyone can give you a definitive answer.

regards
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Old 23rd November 2011, 11:04 PM   #3
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We need copper ounces and current.
I have a track width calculator in my software and will pop the numbers in for you.
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Old 23rd November 2011, 11:11 PM   #4
epilot is offline epilot  United States
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Quote:
Originally Posted by Greg Erskine View Post
hi epilot,

You need to specify current and copper thinkness before anyone can give you a definitive answer.

regards
Oh so sorry, I for got to add it. the maximum current is 300mA
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Old 23rd November 2011, 11:15 PM   #5
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Quote:
Originally Posted by epilot View Post
Oh so sorry, I for got to add it. the maximum current is 300mA
Yes 1.5mm will easily take 300mA.
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Old 24th November 2011, 12:22 PM   #6
marce is offline marce  United Kingdom
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Download this:
PCB Via Current | PCB Trace Width | Differential Pair Calculator | PCB Impedance
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Old 24th November 2011, 12:41 PM   #7
AndrewT is offline AndrewT  Scotland
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Marce,
After your tirade in another post, would you care to explain why Professionals, who you allege never need to know and never take account of trace and wiring resistances, would need to know that a via dissipates 2mW through 0r0013 in the example you posted?
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Old 24th November 2011, 01:10 PM   #8
marce is offline marce  United Kingdom
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Yawn Andrew, read what I put,
Quote:
A quick note on trace resistance, in over 25 years of laying PCB's I have never known an engineer consider trace resistance as critical, just the tolerence of resistors are going to have a bigger effect, as well as other component parasitics. When doing high current the copper wieght and trace sizes are chosen with regards to IPC-2221 for current carryibg capacity (use the Saturn Toolkit). Even for RF, the main consideration has been the inpedance of a trace can be critical, and often tuned by moveable capacitors.
But never have I known an engineer use trace resistances as illustrated above when doing a design, or worrying about them in feedback circuitry. For general design you dont realy have to worry about trace resistance, when high frequency or doing RF trace impedance (mainly inductance) can be important.
Now I dont know how much real world electronics you have done, but if you like I will send you my CV, as you seem to like to argue with everything I put up (including the drift velocity of electrons), now I know not everything I say agrees with audiophile beliefs, but surprisingly I still get paid big bucks to do PCB's for some quite interesting companies, and over the years tend to pick up the odd bit of data, as well as doing electronics at technical college many years ago. I also go to quite a few engineering seminars on PCB design and techniques, the most recent being on PDS (power delivery systems), and find I learn stuff their. Now my comments were regarding trace resistance were in response to youe assesment of the problem and you trying to use high brow technical blurb to discredit and belittle what I had put.
As to the example its to download the Saturn Toolkit which you may find useful, I know a lot of people who do. as to via power dissipation, again for power tracks we have to ensure that we dont burn out the tracks and thus us the toolkit to work out the safe working parameters, dependant on the temp range of the product being designed and the worse case operating conditions.
By the way that is Saturns page, not mine they chose the view.
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Old 24th November 2011, 02:23 PM   #9
macboy is offline macboy  Canada
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Don't mind Andrew. Try putting him on your ignore list as I did.
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Old 24th November 2011, 04:32 PM   #10
marce is offline marce  United Kingdom
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No, compared to life in general having a minor spat with some one is not to serious, and while having a gripe I do hope the discussions can continue, I used to love debating at school, that aside I have been pondering again trace resistance and have some more thoughts that I hope get some feedback...
Let's put this trace resistance into perspective, as I said before, most electronics you can get away with 1% resistors (0.1% are used in critical locations such as variable regulator feedback potential dividers and in some very critical analogue circuits). So say you have a chip amp with a feedback resistor of 1K 1%, the 1% variance of the resistor is 10R, this equates to a track length of 6.25m (20 foot) (based on a 0.3mm/0.012" track width 1oz copper, a pretty standard home design geometry), even if you use a 0.1% resistor, the tolerance equates to a trace 625mm (2 foot) long. So apart from very very exotic designs, of which there are very few, for general PCB layout trace resistance is not critical, and if it is or becomes so you are doing something wrong .
And before we jump on the feedback inputs, most regulators have a very high input impedance on the feedback pins so you must place the resistors for the potential divider next to the pin otherwise any noise can couple into this pin causing mayhem (you also take the feed for the voltage being measured from the DC side of the output capacitor for SMPS, and/or depending on the circuit topology near the load if required).
The other problem if you were using trace resistance is controlling the value, for home etched PCB's and most standard fabricated PCB's you would really have to use a tolerance of 20%. So in the world that I work in trace resistance (apart from the caveat I have mentioned such as IPC-2152, very specialised designs) is not considered (as a single entity) for designs, trace impedance is as we are quite often concerned with ac signals with a frequency content rather than dc.
So have fun and keep things simple, or as Albert Einstein is quoted as saying in the epigraph of 'Electromagnetic Compatibility Engineering':
"Everything should be made as simple as possible, but no simpler."
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