You are picking very unimportant nits. If there were a small noise spike from the opamp, it'd goose the pendulum a tiny amount. That would be corrected on the next swing.
You are turning a trip to the corner store into an Apollo Moon Mission. BTW, since the =drive does not to be bipolar, one of the NPN and PNP transistors can be deleted. They never turn on. So you are left with two opamsa, =each with a simple emitter follower. -John ============== > J. Forster wrote: >> OK. You know better. >> >> BTW, op-amp noise is essentially irrelevant in this application, and the >> C's across the FB resistors limit slew rates so there is no significant >> dI/dt to cause voltage spikes. >> >> > Noise is never irrelevant. > You havent shown that its insignificant either. > > In the real world such dv/dt assumptions with inductive loads lead to > fried parts. > For example if the circuit oscillates at high frequency because the > compensation isnt correct/effective or the feedback wire becomes > detached or the power supply goes down suddently due to a crowbar event > then high dv/dt at the opamp/buffer output is possible. > > >> -John >> >> ================ >> >> > > Bruce >> >>> Your naive stabilisation scheme wont work, try simulating it. >>> 741's are somewhat noisier than necessary. >>> Omitting the diodes with an inductive load almost inevitably leads to >>> >> transistor or opamp destruction. >> >>> Bruce >>> >>> J. Forster wrote: >>> >>>> IMO, far too complicated. >>>> >>>> I'd use a series pair of u741s each with a complementary emitter >>>> follower. >>>> 2 u741s, 2x 2N2102, 2x 2N4036, 5 resistors. Maybe 2x .01 caos to >>>> stabilize >>>> the thing >>>> --------------------------------------------- >>>> |\| |---|c >>>> DAC --o--| \ | |\ 2N2102 >>>> | | / --o-o |------CCCCC >>>> R |/ | | |/ 2N4036 >>>> | | | |---|c >>>> | |-------------------------------------------- >>>> | | >>>> |--------o-------------------------to input of mirror image >>>> >>>> Best, >>>> >>>> -J >>>> >>>> ========= >>>> >>>> >>>> >>>> >>>> >>>>> The attached circuit schematic illustrates the Howland current source >>>>> >> plus inverting amplifier drive technique. >> >>>>> It also illustrates a method of frequency compensation (series RC >>>>> >> connected across the coil). >> >>>>> Of course one can either use discrete buffers or high current opamps. >>>>> >> However for improved accuracy using a difference amplifier with built >> in >> >>>>> pretrimmed resistors for the Howland current source may be >>>>> preferable, >>>>> >> in which case a discrete buffer stage or equivalent may be required. >> >>>>> Bruce >>>>> >>>>> J. Forster wrote: >>>>> >>>>> >>>>>> There are cheap, split supply audio amp ICs that'd work, or you >>>>>> could >>>>>> >> use >> >>>>>> a u741 with a complementary-symmetry output buffer of discrete >>>>>> >> transistors. >> >>>>>> Crossover distortion would be essentially irrelevant, keeping the >>>>>> parts >>>>>> count very low. >>>>>> >>>>>> -John >>>>>> >>>>>> ============ >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>>> The 60mA load current would be problematic for most common opamps >>>>>>> >> without an output buffer stage. >> >>>>>>> High voltage opamps are relatively rare. >>>>>>> >>>>>>> Bruce >>>>>>> >>>>>>> >>>>>>> J. Forster wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>> Since it's inside a closed loop, the design is uncritical. >>>>>>>> >>>>>>>> One option is a high voltage Op-Amp with +/- 25 to 30 VDC >>>>>>>> supplies. >>>>>>>> >> You >> >>>>>>>> would set the OA gain to about 10, so 2.5 V in would yield 25 V >>>>>>>> >> out. and >> >>>>>>>> sum in a negative offset voltage so that +2.5 from the DAC yields >>>>>>>> 0.0 >>>>>>>> V >>>>>>>> out. I'd use something like a 100 K FB resistor and a 10K from the >>>>>>>> >> DAC, >> >>>>>>>> assuming it's a voltage output DAC. A 1 M to the -25 V supply >>>>>>>> would >>>>>>>> >> provide the 2.5 V offset. >> >>>>>>>> Another option would be to use two series opamps with the first >>>>>>>> set up >>>>>>>> as >>>>>>>> above, and the second as a unity gain inverter with input >>>>>>>> connected to >>>>>>>> the >>>>>>>> output of the first. The coil would connect between the two OA >>>>>>>> >> outputs. >> >>>>>>>> As >>>>>>>> one output swings high, the other mirrors that and goes low (just >>>>>>>> >> as in >> >>>>>>>> an >>>>>>>> H bridge). Stability might be an issue, but this has the advantage >>>>>>>> of >>>>>>>> only >>>>>>>> needing a +/- 15 supplies. >>>>>>>> >>>>>>>> FWIW, >>>>>>>> >>>>>>>> -John >>>>>>>> >>>>>>>> ============= >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> Hi all, >>>>>>>>> >>>>>>>>> I have a Seimens master clock with a Reiffler pendulum. A lovely >>>>>>>>> >> piece >> >>>>>>>>> of work that used to provide time services in the 40s. >>>>>>>>> >>>>>>>>> Being a master clock it has contacts that open and close on each >>>>>>>>> >> pendulum swing and so I can monitor it's accuracy quite easily >> using >> >>>>>>>>> gps and my 5370B. >>>>>>>>> >>>>>>>>> I've adjusted it as best I can and the best I can get is about 50 >>>>>>>>> ms >>>>>>>>> over 24 hours. However that was a one off. Temp and air pressure >>>>>>>>> >> cause >> >>>>>>>>> variations of up to 300 ms and it changes direction too. >>>>>>>>> Basically >>>>>>>>> >> it's hard to keep accurate. >> >>>>>>>>> It also has a coil mounted near the pendulum and a fixed magnet >>>>>>>>> on >>>>>>>>> >> the >> >>>>>>>>> pendulum bar and this coil connects to a box down below with a >>>>>>>>> meter >>>>>>>>> and a knob. They are labelled in sec/day. The electronics in the >>>>>>>>> box >>>>>>>>> are not clear (being quite old) but by measuring the current in >>>>>>>>> >> the coil it quite simply increases the current one way to slow the >> clock >> >>>>>>>>> and the other way to speed it up. (I'll admit the physics of this >>>>>>>>> >> doesn't make sense to me - but it works!) >> >>>>>>>>> It's about 25v in the coil and goes up to 60mA max. Even at >>>>>>>>> levels of >>>>>>>>> 2mA has an effect. >>>>>>>>> >>>>>>>>> Using this control it's quite easy to manually bring the clock >>>>>>>>> >> back to >> >>>>>>>>> the right time if it's say half a second fast. >>>>>>>>> >>>>>>>>> What I want to do is control the current in the coil with a micro >>>>>>>>> >> controller which I have attached to a rubidium oscillator. Getting >> the >> >>>>>>>>> pps from the pendulum clock in and comparing to actual time is >>>>>>>>> easy, >>>>>>>>> but I need a way to control the current through the coil so it >>>>>>>>> can >>>>>>>>> >> dynamically adjust the clock. >> >>>>>>>>> I need the current to go from say -10 to +10 mA (at 25v) and this >>>>>>>>> >> needs to be controlled via a micro controller output (which goes >> from >> >>>>>>>>> 0 to 5 with 2.5 being the 0mA point). >>>>>>>>> >>>>>>>>> I can either use the D/A in the controller (or PWM an output I >>>>>>>>> >> suppose). >> >>>>>>>>> I'd appreciate some thoughts on circuits to do this. Software >>>>>>>>> side is >>>>>>>>> not a problem. >>>>>>>>> >>>>>>>>> Jim Palfreyman >>>>>>>>> >>>>>>>>> _______________________________________________ >>>>>>>>> time-nuts mailing list -- [email protected] >>>>>>>>> To unsubscribe, go to >>>>>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>>>>> and follow the instructions there. >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>> _______________________________________________ >>>>>>>> time-nuts mailing list -- [email protected] >>>>>>>> To unsubscribe, go to >>>>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>>>> and follow the instructions there. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>>> >>>>>> >>>>> >>>>> >>>> >>>> >>>> >>> >>> >>> >> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
