Electronics > Beginners
Clamping Split Rail Node to GND using Transistor(s)
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Mechatrommer:

--- Quote from: Zero999 on August 02, 2019, 04:47:07 pm ---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.

--- End quote ---
oh ok, maybe thats possible. when pcb completed, i will try biggest R in order to reduce C as much as possible.


--- Quote from: Zero999 on August 02, 2019, 04:47:07 pm ---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

--- End quote ---
yes as stated in OP, mechanical relay can be used but only very low switching frequency, maybe 1Hz and below will be workable. i have mini relays in stock omron G5V, if i need it i can make bodged connection to pcb later. thanks for the suggestion..
Ian.M:

--- Quote from: Mechatrommer on August 02, 2019, 04:32:27 pm ---
--- 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.
--- End quote ---
Not quite.  If the MOSFET on the more positive side has a lower threshold, the negative side MOSFET's body diode may conduct briefly, but as its gate voltage rises, its channel very soon becomes fully enhanced, and conducts in reverse, bypassing the body diode.  If  the thresholds are the other way round, the negative side MOSFET channel becomes enhanced before its body diode conducts.   Therefore the total voltage drop is only limited by 2*I*Rds_on  so it can nearly fully reset the integrator.  Once the pulse ends so there is no current from the gate drive circuit, the C2+ 2*Cgs, R3 time constant gives extra time with no bias current causing offset voltages to complete resetting the integrator.  It should be noted that ideally the integrator input (to the left of R1) should be at 0V during the reset pulse to minimise the residual offset.

--- Quote from: Mechatrommer on August 02, 2019, 04:32:27 pm --- 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.
....
 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.

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Knock it up on protoboard and patch-wire it in for testing.  N.B. if your logic ground is at -15V, you can omit Q1 (short C-E).  If using 3.3V logic, reduce R4 to 4K7 (5K6 if no Q1).
The Zener should be chosen to be around 3/4 of the MOSFETs' Vgs_max. 


--- Quote from: Mechatrommer on August 02, 2019, 04:32:27 pm ---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.

--- 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.
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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..

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I think you are premature going to a PCB layout.   More development on solderless breadboard or protoboard would have been helpful.    e.g. 22uF non-polarised electrolytics are readily available so if you follow Zero999's suggestion of trading C for R to maintain the same time constant, you could potentially save quite a bit of board space.  However increasing R makes the integrator proportionately more sensitive to leakage current, so some testing at elevated temperature would be advisable.

A broader overview of the whole problem would be helpful, as it smells like you have an X-Y problem, and unless you have a *HARD* constraint of no MCUs,  software PID, with the MCU PWMing an H-bridge would almost certainly be easier to tune, more compact, and have far lower dissipation.  It also removes the need for an external windup detector and integrator reset pulse generator.
Zero999:

--- Quote from: Mechatrommer on August 02, 2019, 06:11:50 pm ---
--- Quote from: Zero999 on August 02, 2019, 04:47:07 pm ---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.

--- End quote ---
oh ok, maybe thats possible. when pcb completed, i will try biggest R in order to reduce C as much as possible.


--- Quote from: Zero999 on August 02, 2019, 04:47:07 pm ---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

--- End quote ---
yes as stated in OP, mechanical relay can be used but only very low switching frequency, maybe 1Hz and below will be workable. i have mini relays in stock omron G5V, if i need it i can make bodged connection to pcb later. thanks for the suggestion..

--- End quote ---
I'd even try smaller capacitors and higher value resistors, say 10M and a 2.2μF but as Ian says, there will come a point when leakage becomes significant.

How fast does this need to reset? Sorry, if you've said already. I've quickly flicked through the thread, but don't have time to read it in great detail. Small relays are relatively fast. The ones I linked to all switch on in under 3ms.

I agree with Ian, that going to a PCB at this stage is jumping the gun.
Mechatrommer:

--- Quote from: Ian.M on August 02, 2019, 08:19:07 pm ---I think you are premature going to a PCB layout....
A broader overview of the whole problem would be helpful, as it smells like you have an X-Y problem, and unless you have a *HARD* constraint of no MCUs,  software PID, with the MCU PWMing an H-bridge would almost certainly be easier to tune, more compact, and have far lower dissipation.  It also removes the need for an external windup detector and integrator reset pulse generator.

--- End quote ---
i'm well aware of soft-pid have done that but not have enough time yet to tune correctly. this time i want to try hard-pid using opamps, mostly for educational purpose, the pcb consists many other small designs, so this pid circuit is only to fill up the free space on the $5 10x10cm pcb. since its cheap, i dont want to mess with protoboard and flying wires anymore, sometime took much longer than on proper pcb, and i usually left the incomplete proto/bread board circuit to the side while doing other stuffs, when i come back few weeks later, i dont remember whats that protoboard is doing, i have to unsolder again to recover the parts/board etc. pcb have labelling that reminds me what it is. anyway, i've sent the pcb to seeedstudio an hour ago, then i made last time modification but i cannot change the file anymore dough, but nevermind just a small change i can cut traces later :-\


--- Quote from: Zero999 on August 02, 2019, 08:53:11 pm ---How fast does this need to reset?
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well as fast as it can be, 10Hz? 1KHz? 100KHz? i dont know but faster is better. i'll see how high the limit of that small pcb module can go, if i need higher performance, i can do rev2 later. as i said, just for educational only to get a picture what application can use this circuit.

btw, i also put jumpers on the pcb to entirely disable I or D or both term, so i tried to make this circuit to be highly configurable to find sweet spot for various systems that i'm going to test.. the board is like a small module with IO and power pins. see attached to get the better picture.
Ian.M:
As the aim is self-education, what you probably should have done is design simpler more general purpose analog computing building blocks (e.g. summers, differential amplifiers, integrators, differentiators, multipliers, lin=>log and log=> lin, etc.) to use up the spare PCB space.  Add pin headers and pop them in a solderless breadboard to build your PID controller. When you have something workable, if you want to take it further then's the time to do a final PCB layout.

However that will have to wait for another time.   Do follow up with the PID controller results + details of the motor, its mechanical load, and whatever you are using for a power stage to drive it.
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