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.
-John ================ > 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.
