Eload again, and maybe a kit...

[Harvs]

I’m not sure if anyone’s still interested in electronic load projects anymore, one could suggest it’s kind of been done to death by now.  However, I’ve had some good success of late in the physical design side of fitting electronics into enclosures, so it’s brought me back to finishing off the eload project I started probably close to a year ago.  Albeit with a totally different outlook on the form factor.

In fact, I recon this could be a good candidate for a kit.

So what I’m doing is to package the components so they can make use of cheap COTS IT hardware.

First off, I’m using standard COTS CPU coolers as have a few others on the forum.  I’ve designed a PCB that will mount two TO247 case MOSFETs in the same way as a cpu does.  Basically it’s nothing more than physical mounting block that routes all connections to a convenient location where it can be connected via pluggable headers or terminal blocks.  First the two MOSFETs are tacked in place using CA glue, soldered, then everything is bolted on around them.

I’m using a Cooler Master Elite 120 case (albeit these parts would fit it most cases.) http://www.coolermaster.com.au/product.php?product_id=6777

So here’s some snaps prototyping, this case will hold two of these load cells:

http://i1085.photobucket.com/albums/j432/harvie256/2013-04-24150311_zpsdf5b5c49.jpg[/img]



And the board that’s ready to head off for manufacture.


The control boards are being designed to fit into 3.5” drive bays.  It’ll be a single fully isolated control board per load cell, allowing the system to be directly scaled depending on the total power, current or channel requirements.

I’ve been using the Thermaltake contact 21 cooler with two fans fitted and arctic silver thermal compound.  A quick survey of the local computer store showed this to be a very high bang for buck price point, at just $25ea for a 140W rated cooler.  My design goal was 150W per load cell, I’ve load tested this at 175W for an hour, with the maximum device case temp getting to just under 30 degrees C above ambient.  So very manageable as long as 300+W of heat can be extracted from the case (yet to be seen, and I’m open to any good ideas here.)  I've also tried it with a water cooler, but I don't think the advantage anywhere near justifies the price and hassle.

The control boards will be pretty much what I’ve built before but repackaged to fit the drive bay form factor, and the software re-jigged to fit.  They’ll consist of an isolated supply, Cortex M0 (STM32), isolated asyc link, 16bit DAC, 16bit ADC with 0.05% reference, auto-ranging voltage, etc. With a powerful uC and its fast on-chip ADC, constant resistance, voltage and power can be implemented digitally with a relatively high bandwidth.

I’m also putting together a separate board with basically two isolated auto-ranging DVMs. Again just using the circuitry I've already prototyped, just repackaging.  As pretty much whenever I’m loading a supply I’m always also log multiple voltages around the circuit, which is a bit of a PITA at the moment.

Front panels will be in the form of laser cut acrylic panels with banana jacks etc that would fit into the standard 5.25" drive spaces.

The current idea is then to bring this all back to a central board with USB link (and/or Ethernet etc.)  That final board is pretty low on the priority list, since there really won’t be a lot to it.  Maybe even a linux SBC with plenty of serials.

So from a kit perspective, it would be pretty straight forward to supply the boards assembled and tested.  Then the builder could locally source all the relevant IT hardware to make a complete system.

Any thoughts?

[Alana]

This is first electronic load that is complete device ready to be manufactured rather than just a working PCB.  And this is what is cool about it - that someone is trying.
Do i see that your metal case is mini atx/itx computer case? Nice thinking.

[Harvs]

This is first electronic load that is complete device ready to be manufactured rather than just a working PCB.  And this is what is cool about it - that someone is trying.
Do i see that your metal case is mini atx/itx computer case? Nice thinking.

Yep it's a mini it case, $59aud. I had one for a htpc, and thought it would make a very nice eload box.

But, I was actually thinking you could really grab any case that fit your requirements. A dumpster dive case would work just as well.

Btw, it's not read yet. It'll probably take a pcb spin or two to get all the form factor spot on. But from an electronic perspective it's pretty much all working on the bench.

[kizzap]

This is first electronic load that is complete device ready to be manufactured rather than just a working PCB.  And this is what is cool about it - that someone is trying.
Do i see that your metal case is mini atx/itx computer case? Nice thinking.

http://www.bkprecision.com/products/dc-electronic-loads.html Not the first. Won't be the last.

What I do like is the fact that it has been designed to fit a PC heatsink. Having that option is AWESOME. I can already see it now....someone watercooling one to get real good power outputs out of it.

-kizzap

Edit:

What would be cool (no pun intended) is is the control boards would stack under the mosfet boards, or even better, if the control H/W is on the other side of the PCB, with the mosfets isolated off the board in some fashion. DO IT

Also designing around a PC case is a silly idea. Most computer cases can be quite bulky, and that is just a waste. With the design you have, you could almost just use a couple of sheets of perspex or sheet metal or something and make it simple.

[Kevin.D]

Nice one
I have just done one based on 2  old amd athlon cpu heatsinks and fans which you can can get for a few $ ,it does 150 Watt and could be expanded upto 240 with 2 more mosfets , it's a full anolgue control one with multiple ranges and a constant resistance mode,no digital involved as yet unless I decide to add a uC managed display.
It's got some nice features you wont find anywhere else .
I may as well join the crowd and post my design,it was just a personal project ,it's not designed to be a kit or anything like yours is. I will write up some extra circuit notes  for it and post it when I get time .

[Harvs]

I can already see it now....someone watercooling one to get real good power outputs out of it.

Water cooling is expensive, and MOSFETs are cheap.  If you want to get rid of more power, then just add more MOSFETs, op-amps and cheap air coolers.  Not only is it cheaper and easier, but you're keeping the MOSFETs miles away from their SOA boundary.  The way I'm designing this is my little mini ITX case will be 300-350W, but say you wanted a 600-700W load, then just add another 2 channels in a larger case.

It seems many of these projects follow a familiar track of pushing and pushing to dissipate more and more power.  So much so that they end up rigging up multiple power supply's to feed their load.  Honestly IMO, technically it's not hard, nor most of the time useful, to get rid of a lot of power.  On the other hand, PC automation, multiple independent channels, and accuracy up to a few hundred watts is seriously useful on a regular basis.

What would be cool (no pun intended) is is the control boards would stack under the mosfet boards, or even better, if the control H/W is on the other side of the PCB, with the mosfets isolated off the board in some fashion. DO IT

So you can't really put much on the area that the heatsink is clamped by due to the nature of the heatsink attachment (the plate underneath I'm talking about.)  So one could extend the board out making a longer board, that would no longer fit in my mini ITX case though.  As this isn't a simple all analog design, it actually needs quite a bit of board area to fit all the control on.  At first guess, there isn't going to be much room left over on a 10x10 2-layer board.

I have certainly toyed with the idea of stacking the control board under the heatsink.  In fact about a year ago that was my original idea, then mount them on top of a 1U 19" rack case so the coolers were showing off to the world.  Then I came to my senses and realized having live chunks of metal in the open was a stupid idea.  But as it is in the photos above, it already stands about 160mm tall, and only clears the top of the case by about 15-20mm.  Adding the control board underneath will add an extra 17mm and makes it collide with the support braces.  Seriously adding the control boards underneath would be very nice and make the case simpler, but it just makes it too high for PC cases.

Also designing around a PC case is a silly idea. Most computer cases can be quite bulky, and that is just a waste.

I'm going to have to strongly disagree there.  Mini ITX cases are an awesome size and form factor.  If the one I've got is too tall for you (it's no wider than any other piece of lab gear I've got), there are others that are about half the height, but the trade off is you're going to have to use the short heatsinks.  Also, for the money there is nothing else on the market that comes close in 1 offs, that's available at a locally, has very decent internal mounting options and can look very professional. Last but defiantly not least, I can make up the boards/faces and mounting hardware and anyone in the world can go out to a local shop and buy a case that it will pretty much bolt straight up to.

and make it simple

And that I believe is exactly what I have done.

[Harvs]

Nice one
I have just done one based on 2  old amd athlon cpu heatsinks and fans which you can can get for a few $ ,it does 150 Watt and could be expanded upto 240 with 2 more mosfets , it's a full anolgue control one with multiple ranges and a constant resistance mode,no digital involved as yet unless I decide to add a uC managed display.
It's got some nice features you wont find anywhere else .
I may as well join the crowd and post my design,it was just a personal project ,it's not designed to be a kit or anything like yours is. I will write up some extra circuit notes  for it and post it when I get time .

It's always good to see what people have done.  There are a couple of nice analog versions that have popped up on the forum, Blackdog's immediately comes to mind.

Just to clear up one point though, this wasn't designed to be a kit.  Simply I've got to this point and realised that pretty much every part is now going to be made via a standard process, so turning it into a kit would be a relatively simple job (i.e. there's no hand made items.)

[David_AVD]

Nice solution.   

[Harvs]

Put together a design for the 5.25" drive bay today.  The grey looking stuff will actually be black acrylic, but details just don't show up well in rendered black.

Any thoughts?

[peter.mitchell]

It would be really cool if you could have it so the board lined up with the mini itx mounting holes as one big board.
Have footprints for 2x channels of 2x mosfets, and don't populate them, then have the controller board(s) sit perpendicular to the board mounted on a 20~ pin 0.1" header, that way no custom screw holes/mounts in the case, and is pretty modular (put in another controller board/fets/heatsink as you wish)

[Harvs]

The single board idea is a good one.  I just did some mockups, it's going to be a big board, about 230x190mm, which makes it not so cheap (looking at about $30 ea in low quantities.)  But, if you fork out for a board that big, there's going to be enough room on the board even with the coolers to put the control circuitry on as well, so it saves in some other areas.

The down side is it needs to be much bigger than the 170x170mm maximum allowed in the standard to provide clearance between the coolers, so it's not necessarily going to fit in any mini ITX case.

Mechanically it'll be a challenging design to ensure nothing fouls including all the mounting holes.

[peter.mitchell]

not really, it shouldn't be too bad, mount the heatsinks so they are inline with the case if they geta  bit warm because they blow through each other, just lower the power dissipation, still better than single channel?

20x20cm board isn't *too* bad, and you could *just* fit everything on it imho, itead do 5(7 is you select open source it) 20x20cm 2 layer for $99, thats $20/$14/board and if you got in "bulk" i'm sure you could work something out cheaper

[Harvs]

They actually take up about a 100x100 foot print because of the bottom mounting plate.  Because I need to keep some separation between them, it starts to get messy trying to fit it on 200mm.  I think it's worth the extra $5 per board to get the extra width and do it properly.  How I've attached should work well as the case has large vents all the way down the sides.

BTW, itead's "Open Source And Get 2 More Additional Boards" means you get two random boards from other people, not you're own design.

[c4757p]

I like your design a lot!  I was doing a load kit myself, and was leaning towards a CPU cooler too. Sadly, my effort there was overtaken by a massive load of schoolwork and I have yet to finish. (I may throw together a cheesy bodged load for myself to use and just move on to something else, at this point.)

[Harvs]

So I got my drive bay unit back from the laser cutter.  A few things I'd change if I were to do it again:

- Not being so over zelous to correct for the laser kerf, which now requires a bit of filing to get it to fit together.
- Remove the laser etch text.  It's unreadable from across the bench, just using quality laser print labels will be better.
- Use 4.5mm acrylic for the front panel and dispense with the rear plastic mounting hardware for the banana jacks (3mm was the most I could use with that in place.)  3mm is bit flexible.

But overall I recon it's a pretty neat finish...

[skipjackrc4]

This is looking very nice!