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m.marino
05-12-2012, 09:47 PM
Hello looking for a source of 2oz (70 micron) single sided copper clad PCB. I have looked at rs, farnell, and rapid and non of them list 70 micron thick copper PCB's. I am looking to finish the build on my power supply and it is this or wiring a bread board up and while I am much more familiar with bread board wiring I was hoping to learn a bit more in the process. So anyone know a source that I can get this from?

Michael

Jonathan
05-12-2012, 10:38 PM
I can send you a bit, else you'll end up paying silly money for postage. What size do you need?

Alternatively just make the tracks twice as wide, or solder some reasonable size copper wire on to the board so that the current is shared between the board and the wire.

ecat
05-12-2012, 10:53 PM
Use whatever you have to hand and bulk up the tracks with solder? This is a common enough trick seen in power supplies, amplifiers etc

EEVblog #317 - PCB Tinning Myth Busting - YouTube (http://www.youtube.com/watch?feature=player_embedded&v=L9q5vwCESEQ)

Jonathan
05-12-2012, 10:55 PM
Use whatever you have to hand and bulk up the tracks with solder?

That helps a little, but the resistivity of solder is substantially greater than copper, which I why I suggest soldering copper wire on to the board since that will make a much bigger difference.

ecat
05-12-2012, 11:33 PM
Use whatever you have to hand and bulk up the tracks with solder?
That helps a little, but the resistivity of solder is substantially greater than copper, which I why I suggest soldering copper wire on to the board since that will make a much bigger difference.


Time between posts, 3 minutes.
Duration of video, 15 minutes.
Conclusion of video: A 50% reduction in the resistance of a 1oz copper track, see video for dimensions, is easily achieved.

Is this good enough for m.marino's application? I have no idea.

Jonathan
05-12-2012, 11:53 PM
Time between posts, 3 minutes.
Duration of video, 15 minutes.

No need to spend 15 mins watching a video which just shows something that can be calculated in well under 3 mins.


Conclusion of video: A 50% reduction in the resistance of a 1oz copper track, see video for dimensions, is easily achieved.

Is this good enough for m.marino's application? I have no idea.

He can only find 1oz, but needs 2oz which implies the resistance needs to be halved. Since this is for a power supply space is not likely to be an issue, so doubling the trace width is the easiest option.

I should mention that in a demanding application where the current fluctuates a lot it's not sensible to add copper wire to the track since it expands at a different rate with changes in temperature compared to the solder, so it can cause cracks over time. A better way would be to solder a thick jumper wire. Wire is cheaper than solder...

HankMcSpank
06-12-2012, 12:03 AM
Another option (assuming all 'through hole' components are being used) ...use double sided 1oz copper board, just arrange it so that your bottom layer is a replica of the top layer (so it would need to be mirrored)....voila you've got 2oz copper board for all your tracks!

If you're worried about alignment...just make sure your pcb board design has some index holes so that when you flip the board, everything will be aligned perfectly on the 2nd isolation run (and presumably this being a powers supply, your tracks are going to be quite chunky, which will be more forgiving for alignment)

ecat
06-12-2012, 07:45 AM
Oh come on Jonathan, stop being such a tease. What is your 2 minute calculation and how exactly does it change the measured result shown in the video?

While you're at it, what is a 'demanding application', what is 'a lot', what is the difference in the heating effect of a fluctuating current and a steady current, and what evidence do you have for this 'cracking over time' in any reasonably well designed product?

Edit:
Almost forgot this useful resource, read the comments too.
http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

m.marino
06-12-2012, 08:03 AM
Ecat,

The application is for 69V DC @ 15A through 5 4700uf cap's to power 4 drivers with shop temp's going down into the negative numbers when shut down (shop is lightly heated and closed down in evening as not running over 10 hrs per day at present). Back EMF is a possibility (though not highly likely with the AM882 drivers). This is going to be inspected by a professional spark as it is part of a business and needs to fall under normal practices for safety and insurance purposes (which is why I am having to tidy everything up and rewire some safe but not clean pathways on the machine).

You idea has merit, that I will agree but those I have to please in the legal side of the real world are a different beast, they see things in nice boxes and you have to meet that when dealing with coverage and business practice issues. I won't go into my rant on this issue.

Michael

Jonathan
06-12-2012, 02:25 PM
Oh come on Jonathan, stop being such a tease. What is your 2 minute calculation and how exactly does it change the measured result shown in the video?

It's just resistors in parallel - it doesn't change the result, just shows it without having to watch the video which takes longer.


While you're at it, what is a 'demanding application', what is 'a lot',

It depends...One where you are trying to conduct a higher current through the track than reccomended, and one where that current changes from a high to low value frequently but at a low frequency.


what is the difference in the heating effect of a fluctuating current and a steady current,

The peak temperature will be the same, but if the current fluctuates at a low frequency (say the device is turned on for 5 mins then switched of, then turned on after 10 mins) the temperature of the solder and copper wire will rise and fall, and since they expand at different rates this results in metal fatigue, so the material can fail over time.

I know linking to another forum is not a great reference, but here it is anyway:
DipTrace Forum • View topic - How do i make certain tracks soldered rather than painted (http://www.diptrace.com/forum/viewtopic.php?f=16&t=95&start=10)

There's some good information on the topic in this thread:
current capacity of a solder loaded track - Page 2 (http://www.electro-tech-online.com/general-electronics-chat/105347-current-capacity-solder-loaded-track-2.html#post861034)

Post #23 in this thread says that adding solder to the track does not comply with CE regulations, although I've seen it in plenty of ATX PSUs I've dismantled.

This discussion is a bit over the top though for a PSU powering stepper motors. Although the transformer is presumably capable of delivering 15A constant current, it never will in this application since at 69V that is well over the power rating of 4 Nema 24 stepper motors. From m.marino's build log we can see that he's using the standard Nema 24 3Nm motors, so from the datasheet the motor is rated for 4.2A and 5.46V, so the power rating is 4.2*5.46=23W per phase, two phases so 46W. Four motors so 184W. If the average power drawn from the PSU is any more than that then the motors are being run outside their ratings and should overheat. Therefore the rms current drawn from the PSU is 184/69=2.7A which is far less than the 15A we were previously designing for. It cannot be higher for a significant amount of time, otherwise you are exceeding the motor's rating. Clearly there will be current spikes well above 2.7A, but the duration of these is small so they are not important for thermal considerations

ecat
06-12-2012, 04:46 PM
Aha, Michael
So for those of us playing at home:

The peak voltage determines the minimum track separation, theoretically. In practice the minimum track separation may be more about the safety standard your product is trying to attain. Something like PCB Trace Spacing Calculation for Voltage Levels (http://www.smps.us/pcbtracespacing.html) .

Minimum track width is determined by the acceptable rise in temperature above your operating ambient and the acceptable voltage drop across the length of the track. This in turn is primarily determined by the track thickness, the (average?) current and the track length. The temperature rise will be limited by the choice of PCB material, if you are willing to push things that far, which is probably not wise.

Once you've achieved your minimums everything else is gravy.

I am curious. Is the word of a professional spark sufficient? On the one hand you have the external connections which are a sparks domain, on the other hand you have risk of fire and electrocution due to circuit design which is an area not usually covered by sparks. I think for non Baseffa applications my old company used to get away with 'due diligence' and a 50 million insurance policy but I'm sure lip service to CE certification must have featured somewhere along the line.


Now stick a big question mark at the end of all of the above. I'm not preaching, I am asking if my understanding agrees with yours while hopefully providing the rest of internet with sufficient information to draw their own conclusions. And I apologise if I've missed some previous threads that were relevant.


All ideas have merit if they are backed by evidence. Any idea expressed without evidence is opinion and if I don't know the person expressing the opinion I'm sure as hell not going to take their word on faith, I'll either question them or ignore them. It's good to see Jonathan take this on board.

Edit:
I had this post in limbo for a while, at least 2 hours by the looks of things.

I think Jonathan's final paragraph of post #10 lays pretty much all of this discussion to rest. If you're expecting 2.7A design for 5A (?) and suddenly 1oz copper is looking quite good ?


There's some good information on the topic in this thread:
current capacity of a solder loaded track - Page 2 (http://www.electro-tech-online.com/general-electronics-chat/105347-current-capacity-solder-loaded-track-2.html#post861034)

Not that much information really. More build it, test it, see what it does. The guy in post #18 is speaking from 3 years experience and has never looked inside a PC power supply. And the comment on thermal dissipation in post #19 sounds 'out there' to me. I still say if your basic design is sound and safe there is an extra 10% to 40% capacity/safety/luxury available for next to nothing.


I know linking to another forum is not a great reference, but here it is anyway:
DipTrace Forum • View topic - How do i make certain tracks soldered rather than painted (http://www.diptrace.com/forum/viewtopic.php?f=16&t=95&start=10)

No, that's great. If the basic design is sound then there should be no problem with any solution. The guy does fail to mention that the most likely place for a fracture to occur is right at the component lead, track boundary - in my opinion.


It depends...

Exactly. It all depends on...
:sneakiness: