|Oct 18, 2012, 12:43 AM|
Electrical: DIY Power Distribution Board
I am copying and pasting the following info from my entire build log just so to focus on the PDB alone.
The objective is to hard mount all the ESCs for the multirotors on a single plate of PDB, double sided. Since there is none in the market to handle the higher than expected current and power, the only way to test the idea is to build one, out of acrylic and copper sheet.
So here goes.
Now that the frame and motor mount and motor is done.. time to move up the base and think of how to mount the ESC.
My design believe in having a low CG and all components should concentrate to the middle as much as possible. Even my base plate is very small at 100x100mm for a 750mm Quad.
So to reduce wire and cable clutter, I want to formulate my own PCB to carry those power.
So the idea came from http://hackaday.com/2008/07/28/how-t...gle-sided-pcb/
Its interesting to learn that one can laser print onto practically any flat surface, literally.. But instead of meddling with pungent chemicals, i decided to adopt my own method..
Here's the PCb plan on drawing:
S that means, we can employ the same manner to transfer these lines onto a sheet of copper! Voila!
But which paper or foil to use? I suppose using the aluminum foil looks better as I can see how the print out will turn out.. But before I can do that, I think it may sense to reverse the color like this:
Reason is simple. the track should be clear of any printing so we do not need to clean it off later. The portion to be cut away will be printed instead.
Then off to print direct onto the aluminum foil.. I tried out both dull surface and shiny surface, the effect is the same..
Now cut away those tape that held the foil to the paper and remove slowly taking care not to crisp the foil. I realise to print it very nice, always use a super nice and flat foil without any lines.. try.. The foil i had in hand was a lousy one, it was thinner than usual..
Now slowly flip it over and lay the foil with the printed side, down to the copper sheet. You can see the print did almost an embossing on the foil.. this is before ironing..
Tips. Now, you gonna move that iron around so better tape down the foil too. But do not use plastic tape. I happen to have aluminum film tape so it came in useful..
Then I place the sheet on a flat surface like an acrylic sheet top with a thick but firm cardboard.. I lay a smooth cloth like a shirt over it so that my iron can slide smoothly..
Turn the iron to the hottest and lets do it!
After 3-4 minutes of moving the iron around making sure it covers all the area..
Remove the hot iron.. remember to turn it off.. wait for the copper sheet to cool.. you can put it in front of a running fan to cool it faster..
And once cool enough to work on, peel the foil away slowly.
Here's are the results:
This is with less than idea flat foil without lines..
This is from a nicer foil, almost perfect without lines.
You can see the importance..
I will cut the bull short, than i just trim the printed portion with a big strong pen knife..
|Oct 18, 2012, 12:44 AM|
Hole puncher came in useful for this work, exactly at 5mm.
You will know the use later.
Getting down to mixing the epoxy.. I got these from Carbon Mod UK as a kit set. Been useful.. You can get any other one of course.. After few mixes as it keeps drying up, I got the hang.. 2:1 as it is clearly written, one is a hardener.. surely you dun need 2 parts of hardener.. So its the other way around.
The acrylic sheet (2mm for this one) is usually wrapped with proper sticky paper on both side. So I peel off one side first without touching the other side. Gonna glue this +ve and -ve plate to the acrylic first as they are larger in contact area and so easier to work with.
Once the ratio is correct and stirred, make sure the bubbles are gotten rid of as much as possible. I use ice cream stick to stir and to get rid of the bubbles, simply keep pressing the mixture against the side of the cup..
Started pouring onto the acrylic now and get enough to work on.. more is ok.. just overflow the sheet.. sweep it down the side.
few minutes and its done. not too thick.
Then just place the copper sheet down. With the slippery epoxy, the sheet can be move around for sometime..
One tip, after cutting the copper sheet to shape, put it another a flat surface like a concrete floor, place piles of newspaper on it to flatten it.
If you have a glove on, just push the sheet everywhere to make sure it is flat on the acrylic. Mine was not totally flat.. so i have to find ways to keep the sheet pushed down.. until it is tacky enough then remove the weight.
After placing the sheet down, dip the brush with more epoxy and go through the top of the copper sheet again to get a layer on top of it. This will act as protection again short circuit.
Next come the use of the punched holes.. these will be used as contact surface to the other side of the PCB.
Tip: Just use a tweezer to move it around. I have the dimension and placing of the whole PCB design printed out in transparency. so use the transparency as out lay and shift the 'dots'. Accuracy is not all too important here as long as the bullet connectors touches a part of the copper, solder will stay onto the copper area to secure both top and bottom side.
Once it is dry enough, use a sharp wire cutter to snipe off withholding joints which is there for the purpose of making sure both stripes of copper stay in same gap through out.
Here's how it look after sniped off.
Don't worry about the unevenness. Have to 'paint' over another layer later.. Also, the portion sniped off will be ugly. Have a tin or a cup or a bowl of lacquer thinner ready on hand for washing of application brush and that stirring ice cream stick.. dip that ice cream stick into the thinner and lightly thrown a few time to make sure it is not dripping. tap the stick onto the surface that was sniped. Let the thinner melt away part of the the epoxy and let it set together. The pox surface will be smooth again..
Here's the dried progress.
|Oct 18, 2012, 12:45 AM|
While waiting for the epoxy to dry, started to work on the ESC mod.
I removed all the cables that came with it. This is the Turnigy TRUST 55A SBEC.
The bullet I replaced with is Zero Loss 5mm from Astro Flight. They are expensive stuff because Mil grade UAV uses them too.
You can find the link here:
After finishing the soldering, place it on the template to check the height balance.
Here's why this connectors are so expensive.
Double wall female socket to make sure the slide in and out is so precise and smooth that no sanding, no powdering is required.
|Oct 18, 2012, 12:46 AM|
2nd layer done.
Left in in my DIY enclosure which can double as oven.
I actually set the temperature at 90degC.. I'm not sure if that is the right or best but no harm actually. The epoxy hardens in an hour to 2..
After I epoxy down the ESC track layer, I relaid another thicker layer on the power track layer. I realized later that due to the sheet not flat enough, the copper actually lifted high enough (the eye can't tell exactly) to cause unevenness.
After 1 thick layers, and spot drop epoxy the 3rd time on some dip spots, i finally sand it down with Perma Grit Tungsten Carbide file.
Then turn to fine paper to smooth it out. This should do for now and will do polish after soldering the connectors.
That's what happen when I flatten the whole surface.. Not to worry, will spray conformal coating over once more after all is done.
Get down to open up a hole on each phase's track.
Did this using a rotary Dremel on small grinding wheel.
Then apply flux on these opening. I use this white one. Got it from a component shop. Not sure if this is good stuff or maybe just normal.. I have seen and used one which is brownish and came in bigger tub. I do not know what are their differences.
Done with laying and filling up the exposed copper..
And then the first connector. This is a 4mm.
With a little tilt to give easy access and insertion from the motor end.
All 3 done.
|Oct 18, 2012, 12:47 AM|
Now we proceed to solder all 4mm bullets around the concentric location.
That done, progress to create bore hole to expose just enough copper to solder the zero loss bullet.
Another view of the bullet inserted. The hole above need to be increase to 4.3+mm so lucky I have imperial sized drill bit set..
[IMG] http://i41.photobucket.com/albums/e2...t/IMAG1809.jpg [/img]
Here's when all of one ESC connection is done up.
The 3phases bullets have their soldering done on the underside. Remember where there was suppose to be the punched out copper sheet dots? The plan was to bore a hole on top of it too, and let enough copper exposed so the top part of the bullets can also have solder flow over them..
Alas, its only a 4mm dot, there wasn't much left when we drilled a 11/64 hole through it although not on the centre of it, just a tiny wee bits of left over copper dot. The moment if we bore a 6mm square shoulder hole over it, I'm afraid the copper dot will be spun off and come out. So we ditched that idea and left it alone. The solder on the underside should be good enough and strong enough. We ate anyway, going to flow another layer of epoxy over the power track to insulate the exposed thinned out portion.
Here's another look on the underside where the solders are.
As for the +ve and -ve bullets, we did the boring too, just deep enough to expose the copper.
There is no need to be precise, just eye balling will do since there was no need to drill a through hole for this. So anywhere close to the bullet's diameter will do. The bore drill size is 6mm, so there's a little room for off centre.
You can see the 2legs dropping down from the ESC are the power connectors. The one closer to the edge of the plate is the -ve one.
See the off centre causing the solder to bob out on one side rather than a nice shape which ideally should look like Mount Fuji! Lol.. anyway, that was too much solder in a haste.. will correct it if have time.
|Oct 18, 2012, 12:47 AM|
After seeing some portion of the epoxy thinning out from the surface sanding and heat from the soldering causing it to soften further, I decide to lay another layer of epoxy on the Power track side.
But I have already drilled some holes and if i did the layering, the epoxy will flow down and clog the hole again. Well I could just redrill them again but it won't be nice again.
Came out with an ingenious way to use the slippery backing from thin double sided tape, roll into a tube small enough to slip in the hole then block it.
Also, this time I wanted to pour a thicker layer to make sure eveything gets covered up. But the side will flow over and drip down. Using the same method, except that I stick that double sided tape onto masking tape so that the slippery side faces inner side within the perimeter of the square.
Here is shown how smooth it tears away after drying.
Alas, after drying, the surface just refuse to be 100% flat! I gave up.
Another day of drilling and boring, soldering, finally done.
The 2 taller connectors are for battery source connection. They were from 4mm plug and the longest I can find:
And here's the final version:
|Oct 18, 2012, 12:49 AM|
Hi there, I'm back from the weekend!
Did a little here and a little there during weekends.. And the connector's pins arrived this afternoon.
Ok let's rock with pictures!
So the PCB is done, lets hook up the motors with the 4mm bullets soldered.
Here's the view of how the connection is done:
Then its time to bolt it down with damper. This version is stiffer..
Plug in all the ESC and weigh it to get a feel..
|Oct 18, 2012, 12:50 AM|
So I should clarify this in my blog too..
I have foot print of 100x100mm to play with. Much larger than any in the market. After some drawing, it was extended to 114x114mm.
And this copper sheet is already thicker than 1oz or even 2oz copper found in most PCB.
Definition of 1oz of copper: Means it is rolled out to 1 square foot, which yields 1.4mils or 0.03556mm thickness.
I just measured the copper sheet in my hand, 0.15mm. That's about 5.9mils.
Coming from a Mil-Spec definition, as a guide to Mil Std 275E - Trace Current Carrying Capacity of Copper, http://www.armisteadtechnologies.com/trace.shtml
every 1/2oz thick of copper PCB, under 0.2" wide track width, can carry 7.5A.
My average track width is 5mm minimum, ie. 0.19685", close to 0.2" definition.
I have 5.9mils, so that makes it 63A if it travels for one long foot! Since it is shorter, it can take in much more.
Hope this help in theoretical planning at least. No, I dun intend to put it through any bench test to see how high it can take.. I dun have a 2nd piece yet..
|Oct 18, 2012, 12:51 AM|
I relocated the battery to be at the side.. Have not completed the paltform yet but I can start taking out one CF plate and lower the FC..
Look how flat it became compare to the earlier 'Pagoda Tower'! Yucks!
The FC.. Still have not figure out (ok I haven't read the manual entirely..) the pin to plug into and out to the receiver..
I have insulated the connectors..
I left one +ve socket unwrapped so that I can put on the Dean's connectors later and tap on that +ve to power the rest.
In actual fact, I can hook up battery packs on all 4 sides of the square baseplate to parallel endurance. Just have to make sure the packs are almost same voltages.
So that's it for the work on the PDB.
|Oct 18, 2012, 12:58 AM|
About how much weight saved?
The Pin Connector from Astro is 1.1gm. Socket is 1.4gm. Total is 2.5gm.
I will use pin all around with the ESC so that is 1.1gm x 5 = 5.5gm.
Multiply by 4 ESCs and that is 22gm
Wire for 90mm length with socket bullet is 3gm. Multiply that with 5 cables that is 15gm! Multiply by 4 ESCs, that is 60gm
So ESC with pin and socket as a pair is still lighter by 38gm.
The variable now is the PCB. Can it be 38gm? Yes definitely.
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