On Wednesday 06 January 2016 17:05:27 Bertho Stultiens wrote: > On 01/06/2016 10:13 PM, Gene Heskett wrote: > [snip] > > >> This setup is probably also cheaper than using SSRs. > > > > Perhaps, but your parts count is many times what I have in mind. > > But then again, the components are small and cheap. The setup you > described would probably use more space. > For the resistor, yes, its a 200 watter. > > [snip] > > > So if after a time, it seems your circuit is getting slower, that is > > probably the failure mode. A slower rate of failure at 130 volts > > might be expected, but I have doubts its completely hideable by the > > passage of time. > > [snip] > > That is where you are wrong afaik. > > The failure mode you describe is attributed to high dU/dt applications > (aka fast switching). In that case, you have many problems with the > output capacitance, which can cause the mosfet to burn the isolation > and the channel(s) (hexfet is just a particular physical construction > of a mosfet). > > The elegant part of using a mosfet as a bleeder in current limited > mode is the explicit lack of high dU/dt, it is actually very slow. As > long as you don't let it heat up too much it will hold forever. > > The circuit cannot get "slower" because the speed is not dependent on > low Rdson, but on constant (low) current, which regulates Rds in the > linear region. The mosfet I used in the diagram is just (a cheap) one > that I have on the shelf. You can use one with higher Rdson without > problem, as long as it has a high enough Vds max. > > > There are few things that you need to ensure with mosfets (regardless > physical construction): > - Never ever exceed Vgs max. This is typically +/- 20V (or lower) and > you need to have a protection zener mounted on the mosfet if you have > a noisy environment or inductive leads up to the gate. Modern small > mosfets have integrated TVS protection built into the package. > Otherwise you would just mount an 18V zener over the gate-source > leads. - Never exceed the Vds max. This is often a cause of trouble in > noisy lines, where high dU/dt noise exceeds the limit and causes > failure. In this specific application it is easily mitigated using a > small ceramic cap over drain-source to short the noise. > > > Thank you Bertho, you made me think... > > That is always good, that thinking thing ;-)
Which got me trolling on fleabay & found this: <http://www.ebay.com/itm/1-set-LY4NJ-12V-DC-Power-Relay-HH64P-4PDT-14-Pin-10A-250VAC-With-Socket-Base-/120897376163?hash=item1c260b8fa3:g:3~kAAMXQgPhRkf79> That gives me a 4pdt, and I can use 2 of them to move that 51 ohm resistor from in series with the AC input when its enegized, and to across the caps when its de-energized. One set to tally back to the hal file to run a status led. Maximum closing current would be about 2.6 amps RMS for coming on or 2.6 peak and fading in DC, which those 10A contacts should handle, and since it would already be closed, it should be able to handle the full 10 amps to the motor when the motor needs it. The third set of contacts would be placed in series with the SSR crowbaring the resistor so it can only be crowbared when active. So I just ordered 3. Never can tell where I might need more 4pdt's. :) One of these first days I need to order me some more 14 gauge wire in assorted colors. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
