I finally got around to trying this using a 200V supply and one of these <http://www.mouser.com/ds/2/268/supertex_dn2530-522899.pdf>. I tried it first for a 2mA load current. I had to determine the Vgs experimentally. The datasheet looks like it should be about -2.5V. I got about -1.9V. I gave it a 5K load. The mosfet heats up quite a lot while operating, so the current across the load gradually climbs, though it wasn't too bad - the end result was about 2.05mA
I then tried it for a 4mA load current. The mosfet got really hot, and the current climbed up to about 4.5mA before I disconnected it. In short, I won't be using this method to limit current. BTW I tried two different pieces of the same mosfet - the Vgs was the same for both. I ordered them at the same time so they could well be from the same batch. So far, if I wanted to use a constant current source, I would go with the very first option. On Friday, April 7, 2017 at 5:34:49 PM UTC-4, Paul Andrews wrote: > > Here is a link > <http://www.infineon.com/dgdl/Infineon-Application_Note_Applications_for_Depletion_MOSFETs-AN-v01_00-EN.pdf?fileId=5546d4624cb7f111014cd63d1a197d94> > > to a paper about using a depletion mode MOSFET for the same purpose. It > gives this equation for calculating the resistor value, which I assume > would be the same for all depletion mode MOSFETS (!?): > > > <https://lh3.googleusercontent.com/-BIZNnjZU_6Y/WOgFjAlbUJI/AAAAAAAAAi4/6FaT3NlNgJ0hFwLOnQChLGzvyur6iVq-QCLcB/s1600/Capture.JPG> > > VGS(th) is the gate threshold voltage of the MOSFET, IDSS is the on > current at VGS = 0 V and ID is the required current. > > > There are plenty of these around with suitable voltage characteristics. > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/b1fe91c3-2cff-490e-a49c-b7bfec8b4a5d%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
