On Wed, Oct 2, 2013, at 01:58 PM, andy pugh wrote: > On 2 October 2013 18:28, Jon Elson <el...@pico-systems.com> wrote:
> > > or use a FET to control the dump resistor. > > I would like to do this, but I am not sure how to wire a FET to > discharge the cap when AC power is removed (Whereas an NC relay does > this easily) One way to use a FET (or IGBT) for this: Put a 12 to 15V zener diode from gate to source. Put a resistor from gate to drain. The resistor should be chosen to deliver 10-20mA. For 300V, that means 15-30K. It will dissipate 3-6 watts, so size it conservatively. Connect a low power relay such that when power is on it shorts the gate to the source, turning the FET off. When the power goes off, the relay opens, and the 10mA current charges the gate until the zener clamps it. That turns the FET on. Once the bus discharges below 12V the gate voltage will droop, but at that point you don't care. Note that just like the relay, the FET can fail in the ON position, applying steady state power to your bleeder resistor. In one project I worked on, we used the water heater elements and mounted a small bi-metal snap-disk thermostat to the element. If the element overheats because of continuous power, the thermostat opens and kills main power by turning off the main contactor. -- John Kasunich jmkasun...@fastmail.fm ------------------------------------------------------------------------------ October Webinars: Code for Performance Free Intel webinars can help you accelerate application performance. Explore tips for MPI, OpenMP, advanced profiling, and more. Get the most from the latest Intel processors and coprocessors. See abstracts and register > http://pubads.g.doubleclick.net/gampad/clk?id=60134791&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
