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Whenever i build chargers/power supplies for myself, I always add reverse current protection to them in the form of a diode reversed, parallel to the charge lines, with a fuse in line with the positive line.. that way if a diode shorts, or if the charger is hooked up backwards, then a fuse pops quick. Another good bit of cheap insurance is a mov and NE lamp parallel to the ac line input, and a fuse between the plug and the mov/NE lamp. Depending on the mov, you can get clamping at 150-400v.. it's not a super fast device.. but it helps.. and will shunt enough current to open a primary side fuse fast enough for non computerized equipment... the natural inductance of the transformer primary should keep the current lagging for a few micor/milliseconds. though the voltage will come up ( that's where the mov/NE buld comeinto play ). The NE bulb won't shunt much current before failure.. but will act a tad faster than the MOV... thus using the two together gives you decent and cheap protection. You can also expans to other 'modes' of protection and ne/buld-mov the neutral to ground line, as well as hot to neutral, and hot to ground. This way you can also help prevent damage in areas where there are site wireing faults.. and/or insuficient grounds.. etc. As an added bonus, the NE lamps give you an idea of the site wireing, and of the 'health of the charge/power supply. An 'out' bulb across the HN & HG lines indicates primary fuse failure or no power.. so you know to check the system. Also, a 'lit' NE bulb on the NG lines lets you know there is significant voltage betwixt neutral and ground, and that points to a site wireing fault.. etc.. All these components, including a tip package or to packages regulator ( in the case of a power supply.. not a charger ) only add 2-3$ to a circuit cost, but add lots of protection. For the person with a tad bit of electrical experience, a zenir shunt diode can be added to the rectified portion of the secondary side to help grab 'over voltage' spikes that make it thru to the secondary, in case the filtering on the primary side fails to shut down the circuit fast enough and lets a brief amount of high primary current incuce a brief over volage situation on the secondary side. Soundguy
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