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. [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 ;-) -- Greetings Bertho (disclaimers are disclaimed) ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
