Thanks Russell, for the helpful feedback. There isn't really a duty cycle on this particular circuit - It's battery operated, so you turn it on once and then run it for several hours. I did finally track down the issue, and it wasn't an issue with the above circuit, it was an issue with the motor driver board. I've got 2 traces side by side going all the way across the (7" wide) board and then down a ribbon cable to a control box and none of it was grounded (since the power board above is a ground-on board) voltage was leaking from the 24v constant-on side over to the switching side before I ever touched the pushbutton. So the lack of a ground tie coupled with leakage seems to be the root cause= .. You guys definitely helped by letting me know that while the details of my circuit weren't perfect the basic design was ok. Thank you! DougM On Mon, Jul 1, 2019 at 2:30 AM RussellMc wrote: > On Mon, 1 Jul 2019 at 03:31, doug metzler wrote: > > > > > Correct, fat finger. The actual part number is IRLR3636PbF. > > > > That's the one that I provided a datasheet link to so we are both talking > about the same part. > > Because of my "it's been my understanding" statement - I've tried to swit= ch > > 24V with logic level fets in the past and it hasn't worked as I expecte= d. > > My solution was to use a transistor to raise the gate voltage. I'm > sure > > Russell is correct that I'm not reading the datasheet carefully enough. > > > > Q1 - What is the switching repetition rate? > Q2 - How long on and how long off? > Q3 - Is there any reason apart from device dissipation to require rapid > turn on and off? > Q4 - What are worst case ambient conditions? (eg not in Erebus crater or > ... ?)(As happens ...) > > *Dante II volcano robot * > > Q5 - Is the load inductive or has it an inductive component - or might it > have? > > > I feel that there is something 'generally wrong' here. > It partly depends on what "it hasn't worked' means. > > At the sort of currents and voltages involved I'd expect a very basic gat= e > drive circuit would suffice, unless there is an exceptional requirement f= or > rapid turn on or off. > eg at 24V and 16A peak (which may or may not occur at turn on or off) if > you dissipated all of the energy in the FET (which you won't) during > switching transition you;d get 24 x 16 =3D~ 400 Watts or 0.4 mJ/microseco= nd > off transition time. > Look at datasheet page 5 figs 13 14 15. > Allowed "avalanche energy" is far in excess of what you should need with= a > simple driver. > If you did need higher current drive then more current (but no more > voltage) can be obtained with two jellybean transistors acting as dual > emitter followers. > More on that if required. > > > > rUSSELL > > > > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .