We have now a new over-voltage protection circuit design for the mass production Mesh-Potatos. We were not satisfied with the previous version. Ideally the over-voltage protection circuit has a snap-on characteristic that triggers the fuse and interrupts the supply voltage without a grey zone. The new circuit triggers at 43 Volts and acts as a powerful crowbar circuit. I have connected the prototype of this circuit to a Mesh-Potato and went through the robustness tests according to our test plan. You can find the test plan and the schematic in the svn respository. A little video at Vimeo.com is documenting some of the tests – thanks to Katrin Lang, who acted as editor and camera operator this time.
Mesh-Potato smoke testing with 230 Volt AC from Elektra Berlin on Vimeo.
The list of tests included a reverse DC voltage test from a unfused 36 Volt source (consisting of three powerful 12 V lead acid batteries in series), excessive DC voltage tests and finally a really scary test involving 230 Volt AC (330 Volt peak) from mains. Please don’t try this at home. Even if the Mesh-Potato survives, there is a 50% chance that you might have mains potential on ground of all components connected to the MP. Also don’t try this with your alpha or beta series Mesh-Potatos, because the previous over-voltage protection circuit is not up to that challenge.
Technically inexperienced people can easily make mistakes, so our idea was to design the Mesh-Potato as robust as possible. In 2005 I was helping to set up a large scale WLAN network in the Sylhet area in Bangladesh. The network consisted of high towers (up to 100 feet tall) and strong directional antennas, interconnecting towns and a school with wireless long shots (up to 32km). One of the trainees damaged a important wireless relay on a tower by taking the open ends of a 12 Volt cable and plugging it straight into the mains socket. Of course the equipment (a Mesh-Cube from 4G Systems) subsequently looked like a lightning strike had hit it, which was actually what I supposed first. However there had been no thunderstorm in the night before. It took me a while to find the reason. The trainee either hadn’t realized what he had done, or he didn’t want to admit it. He watched me trying to find the problem without saying anything. It is common practice in Bangladesh to plug cables into sockets without plugs. The quality of sockets and plugs is miserable, so loose contacts are the rule, not the exception. Now a important relay was down and it was hard to get a replacement. The problem wasn’t so much the financial loss. Shipping and particularly customs can take weeks in Bangladesh. So during the first Villagetelco workshop I suggested to design the MP as robust as possible.