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Clamping Split Rail Node to GND using Transistor(s)
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Ian.M:

--- Quote from: Zero999 on August 02, 2019, 09:46:53 am ---Why not use an analogue switch IC?

DG468 single channel
https://www.vishay.com/docs/74413/dg467.pdf

DG445 four channel
https://docs-emea.rs-online.com/webdocs/1300/0900766b813009f9.pdf

--- End quote ---
An excellent suggestion, which would work well with a smaller integration capacitor, however Mechatrommer's got very large integration capacitor and the analog switches you suggested are only good for max 30mA, (100mA 1ms pulse), so for the DG468 you'd need a 1K
series discharge resistor to keep it safe, which with a 235uF integration capacitor (2x 470uF back to back) and +/-15V rails, gives a worst case time to discharge of well over one second.  Use a DG445, and it can do 30mA per channel - parallel them all each with its own series 1K resistor and it will handle 120mA, which lets you get the reset time down to around 0.28 seconds (to 1% of initial voltage).

That's still pretty poor compared to my back-to-back MOSFETs, as the BSH114 MOSFETs I selected can handle 3.4A peak for long enough to be useful, and around 0.75A continuous (derated to 50 deg C), which lets you reset the integrator an order of magnitude faster.

The H11F1M PhotoFET optocoupler Mechatrommer briefly considered is also unsuitable for resetting such a large capacitor - its nominally good for 100mA abs. max. but its output characteristic (fig. 2) shows it will be unlikely to pass  more than a few mA. A TLP175A PhotoMOS relay would be more suitable as that can actually deliver its 100mA rated load current.


--- Quote from: Mechatrommer on August 02, 2019, 12:32:10 pm ---... thinking about it, before i send the pcb, i will implement analog switch as well on the pcb along with the triac opto solution, so i can switch between them to see which works better. analog switch has size and component count advantage, so if triac opto does not show significant improvement, maybe analog switch is the solution in the next revision. this can be part of my learning process.

--- End quote ---
I think your OptoTRIAC idea is going to disappoint you.  The OPAMP you are using to cancel its On state voltage drop wont be able to supply enough current for a fast reset, and as Zero999 just pointed out, a MOC3041 has a zero crossing detector so you cant even trigger it with a significant steady DC bias voltage across it. (I disagree with Zero999: worst case, full windup railing the output plus continued input at the opposite rail you could have over 25V across the integrator cap, 30V if the OPAMPs have rail-to-rail outputs.)

A MOC301xM or MOC302xM series OptoTRIAC, without a zero crossing detector would work as a coarse integrator reset. I wouldn't recommend trying to cancel its on state voltage with an open-loop OPAMP as you are likely to run into problems with failure to commutate due to excess dV/dt, unless you limit the OPAMP slew rate + you'll still need a beefy push/pull booster stage after the OPAMP.  A better idea would be to pair it in parallel with one of Zero999's suggested analog switches -  Fire the OptoTRIAC first to get it 95% of the way to reset and to handle the high current, then use the analog switch to discharge the last few volts. 
Ian.M:
An alternative idea that may be worth looking at for a *really* beefy integrator reset would be a small isolated DC-DC converter, switched on its input side, providing gate drive to a pair of back to back N-MOSFETs (with respect to their commoned sources).  It will need a discharge resistor as well and will be rather slow compared to a PhotoMOS relay, but may be cost-competitive compared to a PhotoMOS with a high peak current rating.    As the DC-DC converter only runs during the reset pulse, its EMI shouldn't be an issue for the rest of the circuit.
Zero999:
I agree  the 235µF capacitor charged to 30V would no doubt zap the analogue switch. The solution is to make the integration capacitor much smaller. There's no point in over-engineering the reset circuitry, when using a much smaller capacitor and higher value timing resistor would be much easier.

Unfortunately, the DG468 doesn't specify how much of a capacitive discharge it can withstand. Do you think 2.2µF would be OK? I think a 1k discharge resistor is a bit too conservative. It can withstand 100mA peak and will still be able to take more than 30mA, at a low duty.
Mechatrommer:

--- Quote from: Ian.M on August 02, 2019, 01:09:49 pm ---That's still pretty poor compared to my back-to-back MOSFETs

--- End quote ---
still figuring out your circuit. i think i understand the shorting current will shot through body diode of one of the mosfet to the other capacitor's pin, correct? and the rest is i think as you said, source biasing etc. unfortunately i have no more room space to put all the components, let the opto and analog switch go first (i stacked them one on top the other because only one will be populated at a time, and i dont have much space other than about the DIP8 footprint size. i will learn their behaviour, i'll try to understand how they work (learn the hard way) if they indeed dissapointing, i will make add-on pcb to populate your circuit in the next spin. i think if i have 2x2cm board size will be enough. the pcb is ready to go maybe tomorrow or the next monday.

otoh, buying DG468 switch will not hurt as i always wanted this type of part for another project. the MOC3041 bought because i need to repair my stuff using it, and also extra part to keep handy for other project, its not so expensive. i think i already have all the parts to build your circuit, i have 6A nmosfet AO3400 for that, zeners bjt etc, but the space to populate them is currently unavailable.


--- Quote from: Zero999 on August 02, 2019, 02:51:07 pm ---The solution is to make the integration capacitor much smaller. There's no point in over-engineering the reset circuitry, when using a much smaller capacitor and higher value timing resistor would be much easier.
--- End quote ---
the capacitance value will be decided during PID tuning, different motor will use different integrator value, i cannot choose whatever value. the values shown are only copied from the circuit i follow in the above link. if the system i will be driving requires less capacitance and smaller power rails, then maybe analog switch will be just fine, but bigger integrator system maybe need Ian's circuit. so it depends, i cannot give the answer right now..
Zero999:

--- Quote from: Mechatrommer on August 02, 2019, 04:32:27 pm ---
--- Quote from: Zero999 on August 02, 2019, 02:51:07 pm ---The solution is to make the integration capacitor much smaller. There's no point in over-engineering the reset circuitry, when using a much smaller capacitor and higher value timing resistor would be much easier.
--- End quote ---
the capacitance value will be decided during PID tuning, different motor will use different integrator value, i cannot choose whatever value. the values shown are only copied from the circuit i follow in the above link. if the system i will be driving requires less capacitance and smaller power rails, then maybe analog switch will be just fine, but bigger integrator system maybe need Ian's circuit. so it depends, i cannot give the answer right now..

--- End quote ---
Oh, I do understand that, but the integration time constant is proportional to the RC time constant, so if you change 470µF for 47µF and the resistor from 100k to 1M, then it should work the same.

How about using a small relay? There are plenty of tiny relays available which should fit the bill.
https://omronfs.omron.com/en_US/ecb/products/pdf/en-g6k.pdf
https://docs-emea.rs-online.com/webdocs/1398/0900766b81398222.pdf
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