When working with electronics, it is important to ground yourself and your setup. Without proper grounding, you may accidentally damage or destroy sensitive electronics by zapping it, discharging the charge you’ve built up just by walking around. Grounding keeps you, your setup, and whatever electronics you’re working on all at the same voltage, which means that an ESD (Electro Static Discharge) event does not occur.
As the name suggests, grounding means connecting something to earth ground, as in the dirt. Electrical connection to earth is achieved by power companies by driving long metal rods into the ground, and then providing wires that connect to that rod to buildings. The buildings, then, provide earth ground to people using three prong outlets.
All this to say that having a good, convenient access to earth ground is very useful. So how do you go about doing that? Well, the system I’ve been using gives me anxiety:
The cable shown above connects to a three prong outlet on one side, and has a metal stub that you can use to ground yourself (be sure to ground yourself through a large value resistor, typically 1 MΩ; more on that at the end of the post). Besides the fact that I feel like I’m going to get electrocuted using this thing, the cable’s thickness makes it cumbersome, and the fact that the part that plugs into the outlet only has a ground prong means the cable tends to move and rotate during use. I didn’t like this setup, so I looked into an upgrade.
The Qube shown above is what we use at my workplace. Unfortunately, each Qube costs about $20! This is a nice product that does what I want, but I’m not willing to pay that much, especially since it’s so simple; all this does is turn a ground prong into banana jacks. So I decided to make my own!
I made my grounding cube out of three pieces; one connects to the outlet (outlet plate), the other holds a couple of banana jacks (banana plate), and the third holds the other two pieces together. This multi-part approach makes the assembly very easy.
The fully assembled cube is shown above. The outlet face and the banana face are held together by a M3, 20 mm long, female-to-female, metal hex standoff. The banana face has an M3 screw going through one of its holes, and the outlet face has a M3, 20 mm long, male-to-female, metal hex standoff, which acts as the ground prong. The assembled and disassembled pictures are shown below.
As you can see from the disassembled picture, a o-ring is used to electrically connect the bananas jacks to earth ground.
Ground prongs, in typical cables, are about 5 mm in diameter. In my first design, I used an M5 screw instead of a hex standoff. However, the threads on the screw made it REALLY hard to push into and pull out of outlets, so I had to change the design. While sticking various metal things into outlets to test what would work well (carefully, of course; would not recommend), I found that M3 hex standoffs, with 4.5 mm width, work very well. The outlets I’ve used have a bit of resistance to the standoffs, but I actually like that; it means the outlet has a firm grip, which means a good connection, both mechanically and electrically.
The hexagonal shape of the standoff provides two advantages over a round shape. First, the counterbore I put into the outlet plate of the grounding cube prevents that standoff from rotating over time, which means the threading is less likely to be undone over time.
Second, the hexagon has a edge-to-edge distance of 4.5 mm, but a maximum vertex-to-vertex distance of over 5.7 mm. This means that the hex standoff, once inside of the outlet, cannot rotate very much. The two plastic prongs that go in the outlet prevent the grounding cube from rotating, but the hex standoff is also capable of resisting rotation as well, which means I won’t have to worry about the cube rotating while it’s in use, which is a great improvement from my old setup.
Two last things I’d like to point out.
- The grounding cube has two banana jacks that connect to earth ground. However, the screw below the banana jacks connects to earth ground as well. This means that if you have something that needs to be grounded, then you can connect to the screw instead of using the banana jack if that’s more convenient. This may be the case if the grounding mat you bought, for example, has a ground wire that ends with an o-ring.
- The grounding cube provides a dead short to earth ground; there is not built-in resistor. You should not use this cube directly to ground yourself; instead, you should use the cube to ground ESD mats and ESD straps. Mats and straps typically have 1 MΩ resistors built in to them. The reason you want high resistance between yourself and earth ground is to prevent large currents, since that may be dangerous. For example, if you’re tied directly to earth ground and touch a high voltage wire (above 100 volts with respect to ground), then you could get seriously injured since the wire will pump current to earth ground through you. The high resistor will significantly reduce the current, possibly saving your life.
All in all, I was very happy with this mini-project. It was a good excuse to use my new 3D printer, and I got to print something I’ll use regularly.
EE Diary 