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Finishing up the Enclosure for the Power Supply

The final enclosure with everything mounted

The final enclosure with everything mounted

Way back there I did all the sheet metal bending to form an enclosure box for my power supply.  (Post “Brains not tools… Making an Enclosure for Electronics“).  I did not like the power supply board open because it has 120 V on it in some areas and I do not like being turned into a “crispy critter” just because I happen to reach across an area.  I also wanted to add fuse protection and a switch so I could turn it off without having to unplug it.   The work I am showing tonight would have to be done even if I had purchased one of those Bud Boxes.  (It would look better than the bends I did, but I would not have learned as much.)

The first problem was mounting the actual printed circuit board in the enclosure.   The circuit board had three holes already drilled in it for the purpose.  Two were used to mount the transformer on the board and the third was at the opposite end.  It was simple to use the board itself as a template to start the holes and then I drilled each hole to the size needed.  (#6-32 bolt size if I remember right.)  The bottom of the board could not rest directly on the bottom of the enclosure without shorting everything out.  Sometimes a thin piece of plastic prevents this from happening, but in this case I needed something that would firmly hold the transformer without flexing the board.    I needed some sort of standoff.

There are several types of standoffs available.  There are metal and plastic standoffs and ones with internal threads as well as ones without threads where the bolt simply goes through the standoff and the board and a nut is either placed on top of the board or on the bottom of the enclosure.  The standoffs come in various diameters.   The problem is like everything when you do a one-of type of project is being able to purchase a reasonable quantity at a reasonable price.  I lucked out on this project.  The holes had enough clearance around them that I was able to feed the bolt through the bottom of the enclosure and use a not to hold the bolt in place as wall as act like a standoff.   There were no circuit traces in the area of of the holes.

Initial planning - determining the size of the components.

Initial planning – determining the size of the components.

After checking the fit of the board on these screws acting like studs in the bottom of the box it was time to move on to mounting the the other components.  The board was removed so metal filings would not get on it while I did the other work.   The first step was to determine the size and shape for the various holes.  I took accurate measurement of the components as well as I pulled data sheets for each of them off the web.

The power cord came with a grommet that required a rectangular slot.   I decided this would be in the back of the box.   The switch I had laying around also required a rectangular hole.  Since I already had this switch available, I decided to use it although a simple toggle switch could be installed with a round hole.  The terminal strip would require 3 holes for the terminals and 2 additional holes for mounting screws.  The LED light that I moved from the board required a simple drilled hole and I glued it to the enclosure on the inside of the enclosure.  The only thing necessary after drilling the holes is to deburr the back side of the holes.  This can be done with a file, but I used a small grinding wheel on a dremel to do this.  The final device, the fuse holder could be installed with a drilled hole, but I decided to do it the hard way.  (I will explain why very soon.)

The fuse hold and switch holes.

The fuse holder and switch holes.

To make rectangular holes and the slot there is a special tool called a nibbler.  The nibbler is a very small hand operated shear that takes a small rectangular bite out of the metal with each squeeze of the handle.  To use it, it is necessary to drill a 1/4″ hole to be able to insert the tool into the metal and then nibble away the rest of the hole in the sheet metal.   If you are careful about how far you insert the mouth of the tool for each nibble you can make precise holes.   I got a little sloppy on the hole for the switch but all of these devices have a lip on the front to cover the edge of the hole.   It was necessary to make the center of the top of the hole a little larger to allow a flexible plastic catch to slide in.  Once the switch is inserted into the hole this catch snaps out and holds the switch securely in the hole.

I ended up cutting a slot for the three terminal strip connections.  I was concerned about clearance to the side of each terminal to the enclosure wall and was afraid of a short. It was easier to cut the slot than it was to continue to drill bigger and bigger holes.

Filing the Fuse Holder hole.

Filing the Fuse Holder hole.

The biggest hassle of the whole project was making the hole for the fuse holder.   The fuse holder could have been installed in drilled hole.  The problem with doing it that way is the holder has to be mounted very very tight to keep it from twisting when the cap is screwed on.   I have never seen one mounted that tightly and the twisting of the fuse holder can pull wires loose and create problems forever,   A better solution is to make a hole with one side flat so the holder cannot turn.   There are special punches designed to do this, but again I did not feel like purchasing something for a one-of project.  I drilled a much smaller hole and then filed the hole big enough to fit the holder.   This required many tries and determining where the binding point was and then filing some more and trying again.   However, making the hole was a one time issue.  Living with the problem is a forever issue so I chose to do it the hard way.

Checking Fit of the fuse holder in the hole.

Checking Fit of the fuse holder in the hole.

After all the holes were drilled and all the metal filings were shaken out, I mounted the PC board and the rest of the components.   The wires between the components were soldered and then the whole thing was tested without the cover on.   One thing I would do differently is I would make the flat side of the fuse holder either at the top or bottom of the hole and not to the side like I did.   This location put one of the solder connections at the bottom of the holder and it was hard to do the work with the wire underneath the holder.

Everything soldered and ready for the first test.

Everything soldered and ready for the first test.

Before plugging the power supply into the wall outlet, I used an ohmmeter and checked each of the power prongs to the enclosure to make sure there was no short to the enclosure. All I need is to do all this work and see it go up in smoke, or even worse fry myself by touching the enclosure. There was one minor glitch, the fuse holder was slightly defective and the fuse would not fit. The cylinder inside the cap was bent, but I was able to take a pair of long nose pliers and fix the problem. While I had the cover off and had the unit operating, I adjusted both sides of the power supply to exactly 15 Volts. Notice in the picture how the power cord grommet fit into the slot on the back of the enclosure.

Final after the cover was installed.

Final after the cover was installed.

After checking it out, I installed the cover and did a final check. I will probably remove the cover one more time and put some glue on the nuts of the screws to prevent those from vibrating loose and causing me problems in the future.

The nibbler I used was exactly like this one available from Amazon.

 

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Gary


 

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