Electronic constant current DC load

[jwrelectro]

Nice job arekm!  I have done several pcb's using a laser printer and photo positive boards.  Your outlines and lettering on the top side look good and I have never done that process.  What did you use to create the artwork for the traces?  John

[arekm]

The PCB was designed in Sprint Layout software (http://www.abacom-online.de/uk/html/sprint-layout.html) by someone else. I've done the rest. Copper layer and silkscreen layer were printed on a laser printer  on separate pages.

The whole thermotransfer idea is to transfer toner (laser printer) printed on slick paper to laminate, then do etching (toner is immune to sodium persulfate B327). The same method is used for silkscreen - just transfer toner and silkscreen done.

Some video (sorry, polish subtitles but you will get the idea) - http://www.youtube.com/watch?v=vgHFet8ci-I
Note: I'm using unmodified lamination device (Lervia KH4410) instead of iron (clothes iron) used on this video.

Etching done in home made device http://www.youtube.com/watch?v=Ui0G9cptFcs

The whole process is like 30-45 minutes per pcb (not counting drilling).

Drilling is easier after etching - small etched points in pads make drilling easier.

Edit: final product (well, case is missing) http://imageshack.us/photo/my-images/6/img8288wn.jpg/

[jwrelectro]

Thanks for the links arekm.  John

[jwrelectro]

The picture below is the first pass at the el-load pcb.  The resolution is not very good because it is a screen capture of the print preview of the artwork.  This picture is also just the traces of the board with no lettering.  The board size is a little less than 5 x 3 inches.  I was happy that so far there are no jumpers in the layout. This board contains all the controller circuitry and the dual voltage power supply.  Many of the pads around the edges are for the wiring connections of the off-board components.  I tried to bring those connections out to the appropriate edge of the board to reduce the wiring lengths.  I had thought of designing the board so that the rotary switch was solder directly to the board but I think the board would either require  being double sided or have many jumpers to accomplish the task.  I also didn't like having the AC 120 V on the board.  John

[MJLorton]

Again...Arekm and John...great posts. It certainly inspires me to give the PCB manufacture process a bash.

Cheers,
Martin.

[Kiriakos GR]

Count me in,  about getting one PCB too.  ;-)   

[jwrelectro]

I completed a second version of the artwork using KiCAD and from that artwork made the pcb for  the el-load.  Looking at the pictures below, in the first one you can see the exposure lamp, artwork, and photo-sensitive printed circuit board (pcb).   The pcb is exposed to the light for 6 minutes using the timer show in the right side of the unit.   The next picture shows the board placed in a glass tray with a photo developer and then rocked back and forth for approximately 3 minutes to remove the exposed resist.  The next picture shows the developed board ready for etching.  The next picture shows the ferric chloride solution used to remove the exposed copper from the board.  The final picture shows the etched board before the resist is removed, sheared to size and drilled.  This system is a positive photo-fabrication process and the total time from start to finish is under 20 minutes.  More to come....   John

[Kiriakos GR]

The above is what I like to avoid, and why my vote was in favor of a mass-order PCB solution.
The older boys haves much less patience to do this things.

[jwrelectro]

I understand where you are coming from.  I am one of the older boys too.  The reason I like this process occasionally is to really test out the design and if it fails I can have another version in an hour or two.

[Kiriakos GR]

I was making PCB at the age of 18, and actually with just a waterproof pen, that makes it a totally hand made process.
But now days if you invest in a laser printer everything becomes more automated.
The point is that I do not design experimental circuits.
Now days I prefer to buy a ready Kit and to just assembly it.     

[jwrelectro]

Please refer to the pictures below of the el-load control board.  In the first picture you can see the back of the board and the board is shown trimmed, drilled, and tin plated.  The second and third pictures show the front and side of the control board populated with components.  There are still a lot of pads that will allow external wiring connections later.  The board is larger than required but I like the extra room on a first prototype and also the size helps keep the external wiring distributed across the front panel of the case.  The three TO-220 cases are the two voltage regulators and the one crowbar E-MOSFET.  There is no high current connections on this board and none of the TO-220 devices require heatsinks.  All the resistors are 1% metal film, 1/4 watt.  All the capacitors are over-rated for working voltages and you can see power supply decoupling capacitors distributed across the board.  More to come...  John

[jwrelectro]

Please refer to the pictures below on my el-load.  In the first picture I have set the basic parts into the case to determine proper mounting locations.  In the upper left corner is a 12.6-V CT, 2-A shielded power transformer which is way over rated but I am think of future additions.  In the lower left corner is the control pcb on metal stand-offs. In the right side area is the heatsink tunnel or load module.  In the second picture is a preliminary front panel layout.  I am trying to make sure that the panel components do not interfere with the other chassis components, (need to think in 3D).  I had hoped to use a larger input air filter, (shown on right side) but settled on this smaller one and hope it doesn't impact the air flow in too negative of a way.  More to come...  John

[iloveelectronics]

Looking very neat!!

[SeanB]

Should not be a problem, limiting the airflow a lot. Still you will find the case will let in air via slots. You might want to turn the fan around and put the filter on the back of the case, letting the air out via the sides of the case and sucking the air past the heatsink after it is filtered  by the filter. The fan will be more effective then with laminar flow into it.

[MJLorton]

Brilliant work John. Thanks for the great posts.