John Larkin <jlarkin@highland_atwork_technology.com> wrote:> On Fri, 11 Aug 2023 21:22:10 +0100, piglet <erichpwagner@hotmail.com> > wrote: > >> On 11/08/2023 20:38, John Larkin wrote: >>> On Fri, 11 Aug 2023 19:19:14 +0100, piglet <erichpwagner@hotmail.com> >>> wrote: >>> >>>> On 11/08/2023 16:00, John Larkin wrote: >>>>> We have a mosfet closed-loop programmable current sink that works fine >>>>> most of the time but behaves badly at startup from zero current or at >>>>> very low currents. The fix is to drool in a bit of keepalive current >>>>> so that the loop never goes open and rails low. >>>>> >>>>> https://www.dropbox.com/scl/fi/3ql0ew6a2elh562wbsx96/Keepalive.jpg?rlkey=fujknponcgtf31mi3rf072x5e&dl=0 >>>>> >>>>> The 400 volt MMBD5004 would be better for high-voltage versions. >>>>> >>>> Does the sink device have to be a FET - a BJT or Darlington might be >>>> better in startup? >>>> >>>> piglet >>> >>> The fet is sure nice. It goes to milliohms and is easy to drive from >>> an opamp. >>> >>> The thing about startup, with any transistor, is to not let the servo >>> opamp see zero-zero inputs and wind up to some rail. In fact we want >>> the fet to be biased slightly ON and be ready to go. Audio amps have >>> the same issue, wanting to keep the output transistors always on a >>> bit. >>> >> >> Yes, I am hoping with a BJT the base current should guarantee the opamp >> is always servoing and not railed. >> >> piglet > > If both the opamp inputs are zero volts, and it has any offset, it > will rail high or low. The keepalive, and a tiny DAC offset, makes the > opamp regulate a small constant mosfet current, and that's nice. > > This is a textbook opamp+mosfet controlled current sink, which can go > bananas at zero current. > > Right, a BJT and some minimum demand offset will keep the loop closed > even with nothing available on the collector. > > A gate-source resistor on the mosfet might have a similar effect. Kill > its infinite beta! > > >The FET avoids the latchup state where the BJT saturates and the loop regulates off the (greatly increased ) base current. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
keepalive
Started by ●August 11, 2023
Reply by ●August 11, 20232023-08-11
Reply by ●August 11, 20232023-08-11
On Fri, 11 Aug 2023 13:49:22 -0700 (PDT), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Friday, August 11, 2023 at 12:12:18?PM UTC-4, John Larkin wrote: >> On Fri, 11 Aug 2023 08:51:43 -0700 (PDT), Fred Bloggs >> <bloggs.fred...@gmail.com> wrote: >> >On Friday, August 11, 2023 at 11:01:02?AM UTC-4, John Larkin wrote: >> >> We have a mosfet closed-loop programmable current sink that works fine >> >> most of the time but behaves badly at startup from zero current or at >> >> very low currents. The fix is to drool in a bit of keepalive current >> >> so that the loop never goes open and rails low. >> >> >> >> https://www.dropbox.com/scl/fi/3ql0ew6a2elh562wbsx96/Keepalive.jpg?rlkey=fujknponcgtf31mi3rf072x5e&dl=0 >> >> >> >> The 400 volt MMBD5004 would be better for high-voltage versions. >> > >> >Isn't 0.12V on that drain rather scanty? >> The 10 ohm resistor can always be tweaked, but this works in >> simulation. The opamp that closes the loop has a 3 uV max offset and >> the fet goes to milli-ohms. My project engineer can make it work; I >> just donated the idea. >> >> Of course dual diodes are usually separate chips and don't exactly >> match. But they do have exponential i-v curves and some ohms, not >> brick walls. >> >> If we specify a minimum input voltage, 2.5 maybe, the keepalive shuts >> itself off when the voltage is applied so contributes no error. > >Is this a current source or variable resistor?Current sink or resistor. It will be an 8-channel isolated dummy load module. It's easy to have both programmable resistance and constant-current modes, as most electronic load boxes do. Some even have constant-voltage, essentially zener diode, mode, but I don't think we'll do that. They do want open and short states. In resistance mode, our customer wants to apply +28 volts to one side and a PWM mosfet to ground. Or connect us to a full-bridge bipolar switcher, like a torque motor or stepper driver. The dynamic issues are horrendous. The best way to simulate resistors is with resistors, but that had problems too.
Reply by ●August 12, 20232023-08-12
On a sunny day (Fri, 11 Aug 2023 08:00:42 -0700) it happened John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote in <vuicdi5u8vfimu5cgkh9mvl0fkih5sfprr@4ax.com>:>We have a mosfet closed-loop programmable current sink that works fine >most of the time but behaves badly at startup from zero current or at >very low currents. The fix is to drool in a bit of keepalive current >so that the loop never goes open and rails low. > >https://www.dropbox.com/scl/fi/3ql0ew6a2elh562wbsx96/Keepalive.jpg?rlkey=fujknponcgtf31mi3rf072x5e&dl=0 > >The 400 volt MMBD5004 would be better for high-voltage versions.This has among other things, my MOSFET heater, driven by filtered PWM from a PIC micro https://panteltje.nl/panteltje/tri_pic/tritium_decay_experiment_black_box_circuit_diagram_IMG_3883.GIF The feedback is via the temperature sensor using the Vbe change in a BUX86 mounted on the same hotplate as the MOSFET Worked OK for many years, switched of now. Tritium decay experiment https://panteltje.nl/panteltje/tri_pic/ control loop is in software.
Reply by ●August 12, 20232023-08-12
On Sat, 12 Aug 2023 05:40:32 GMT, Jan Panteltje <alien@comet.invalid> wrote:>On a sunny day (Fri, 11 Aug 2023 08:00:42 -0700) it happened John Larkin ><jlarkin@highlandSNIPMEtechnology.com> wrote in ><vuicdi5u8vfimu5cgkh9mvl0fkih5sfprr@4ax.com>: > >>We have a mosfet closed-loop programmable current sink that works fine >>most of the time but behaves badly at startup from zero current or at >>very low currents. The fix is to drool in a bit of keepalive current >>so that the loop never goes open and rails low. >> >>https://www.dropbox.com/scl/fi/3ql0ew6a2elh562wbsx96/Keepalive.jpg?rlkey=fujknponcgtf31mi3rf072x5e&dl=0 >> >>The 400 volt MMBD5004 would be better for high-voltage versions. > >This has among other things, my MOSFET heater, driven by filtered PWM from a PIC micro > https://panteltje.nl/panteltje/tri_pic/tritium_decay_experiment_black_box_circuit_diagram_IMG_3883.GIF >The feedback is via the temperature sensor using the Vbe change in a BUX86 mounted on the same hotplate as the MOSFET >Worked OK for many years, switched of now. > Tritium decay experiment > https://panteltje.nl/panteltje/tri_pic/ > control loop is in software. >Mosfets make great heaters and transistors make pretty good temperature sensors. One of my guys did an experiment yesterday. A drain-tab-up D2PAK mosfet is being sat upon by a G199 CPU cooler, with grease. A LabJack controls the gate of the fet to dissipate power. Once in a while it switches off the drain power supply and back-biases the fet substrate diode to quickly measure junction temperature. In this box, the giant optics oven is heated by mosfets. https://www.dropbox.com/scl/fi/d8zn6yca7bkun0jkj3riq/Man_Top_1.jpg?rlkey=352n236tashqclwpcn6dd2qf7&dl=0 The temp sensor is a thermistor Wheatstone bridge for high sensitivity at the target temperature, 30C.
Reply by ●August 12, 20232023-08-12
On a sunny day (Sat, 12 Aug 2023 07:39:21 -0700) it happened John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote in <cs5fdidecu7mkgtuopp0f8egg2mk8pk8pk@4ax.com>:>On Sat, 12 Aug 2023 05:40:32 GMT, Jan Panteltje <alien@comet.invalid> >wrote: > >>On a sunny day (Fri, 11 Aug 2023 08:00:42 -0700) it happened John Larkin >><jlarkin@highlandSNIPMEtechnology.com> wrote in >><vuicdi5u8vfimu5cgkh9mvl0fkih5sfprr@4ax.com>: >> >>>We have a mosfet closed-loop programmable current sink that works fine >>>most of the time but behaves badly at startup from zero current or at >>>very low currents. The fix is to drool in a bit of keepalive current >>>so that the loop never goes open and rails low. >>> >>>https://www.dropbox.com/scl/fi/3ql0ew6a2elh562wbsx96/Keepalive.jpg?rlkey=fujknponcgtf31mi3rf072x5e&dl=0 >>> >>>The 400 volt MMBD5004 would be better for high-voltage versions. >> >>This has among other things, my MOSFET heater, driven by filtered PWM from a PIC micro >> https://panteltje.nl/panteltje/tri_pic/tritium_decay_experiment_black_box_circuit_diagram_IMG_3883.GIF >>The feedback is via the temperature sensor using the Vbe change in a BUX86 mounted on the same hotplate as the MOSFET >>Worked OK for many years, switched of now. >> Tritium decay experiment >> https://panteltje.nl/panteltje/tri_pic/ >> control loop is in software. >> > >Mosfets make great heaters and transistors make pretty good >temperature sensors. > >One of my guys did an experiment yesterday. A drain-tab-up D2PAK >mosfet is being sat upon by a G199 CPU cooler, with grease. > >A LabJack controls the gate of the fet to dissipate power. Once in aI had to look up 'LabJack' https://labjack.com/pages/comparison seems some 24 bit ones are very expensive, 12 bits $260 I'd use a PIC and maybe a Raspberry Pi, relais?>while it switches off the drain power supply and back-biases the fet >substrate diode to quickly measure junction temperature.I never tried that, interesting, will measure some.>In this box, the giant optics oven is heated by mosfets. > >https://www.dropbox.com/scl/fi/d8zn6yca7bkun0jkj3riq/Man_Top_1.jpg?rlkey=352n236tashqclwpcn6dd2qf7&dl=0 > >The temp sensor is a thermistor Wheatstone bridge for high sensitivity >at the target temperature, 30C.Yes, that should work, I have used a simple forward biased diode in big industrial equipment as temperature sensor. But a LM35 gives you real temperature. Thermopcouples work OK too. This uses a LM35 for cold-side measurement: https://panteltje.nl/panteltje/pic/th_pic/ here measuring some cold stuff: panteltje.nl/pub/super_filter/cryocooler/minus_40C_90Vpp_60Hz_img_2617.jpg
