Hello , Is there a good variable resister you could reccomend 0 to 1K so it will not be much larger then 0805 dimention and most importantly so it wont have any parasitic capasitance in it because it goes into opamp circuit and I dont want the parasitic capacitance to cause stability issues.
Thanks.

 

Hello , Is there a good variable resister you could reccomend 0 to 1K so it will not be much larger then 0805 dimention and most importantly so it wont have any parasitic capasitance in it because it goes into opamp circuit and I dont want the parasitic capacitance to cause stability issues.
Thanks.

How big is the screwdriver you want to adjust it with?

 

Hello , I think I'll try this one.How can I know that it doesnt have parasitic capacitance?
https://www.digikey.com/en/products/detail/bourns-inc/TC42X-2-102E/9677741

 

Hello , I think I'll try this one.How can I know that it doesnt have parasitic capacitance?
https://www.digikey.com/en/products/detail/bourns-inc/TC42X-2-102E/9677741

Yes, that one looks like you could use the same screwdriver you use for removing hubcaps and opening cans of paint.

 

Hello, is there a good variable resister you could recommend 0 to 1K so it will not be much larger then 0805 dimension and most importantly so it won't have any parasitic capacitance in it because it goes into opamp circuit and I don't want the parasitic capacitance to cause stability issues.
Thanks.

Everything has parasitic capacitance. I don't see how you can avoid it. It might be instructive for you to measure it.

 

Hello Papabravo, so you say that TC42X-2-102E will not cause som extra capacitance on my feedback then regular 0805?
I am afraid of getting the opamp unstable.

 

Hello Papabravo, so you say that TC42X-2-102E will not cause som extra capacitance on my feedback then regular 0805?
I am afraid of getting the opamp unstable.

Actually, I believe it is the opposite. Extra capacitance in the feedback loop reduces the bandwidth and increases stability. Unless you are talking about positive feedback.

 

Actually, I believe it is the opposite. Extra capacitance in the feedback loop reduces the bandwidth and increases stability. Unless you are talking about positive feedback.

Did you assume that the preset would be placed in the feedback loop between output and inverting input? He didn't say it would. It could be between inverting input and ground, in which case it would have the opposite effect. Also, a preset would have rotary track, which will introduce inductance.

The only thing that is for certain is that extra capacitance will introduce extra phase shift. The amount of gain still remaining when that phase shift reaches 180° determines whether or not it will oscillate.
The TS doesn't specify the op-amp, but nearly all op-amps these days have a dominant pole capacitor which introduces 90° of phase shift, a capacitor will introduce another 90°, and there will be enough parasitic capacitance somewhere to push it beyond 180° at some frequency.

 

Yes, I made that unwarranted assumption.

 

Hello, I am using the opamp is AD8034 as shown below.
I want to place TC42X-2-102E in R4 so I wont need to resolder my 0805 regular resistor over and over.
The big questiois does placing TC42X-2-102E instead of 0805 will pose instability danger?
Thanks.
https://www.analog.com/media/en/technical-documentation/data-sheets/ad8033_8034.pdf

 

Yes, I made that unwarranted assumption.

. . but you guessed correctly:

Hello, I am using the opamp is AD8034 as shown below.

 

. . but you guessed correctly:

so should I use regular 0805 and not risk with TC42X-2-102E

 

Hello, I am using the opamp is AD8034 as shown below.
I want to place TC42X-2-102E in R4 so I wont need to resolder my 0805 regular resistor over and over.
The big questiois does placing TC42X-2-102E instead of 0805 will pose instability danger?
Thanks.
https://www.analog.com/media/en/technical-documentation/data-sheets/ad8033_8034.pdf

View attachment 351836

I doubt it will make the circuit into a bigger mess than it is already. Why the gain of 2500 below 3Hz? With 1mV DC offset that will give you 2.5V offset at the output. Why the increase in gain above 20MHz when the op-amp hasn't any gain above 20MHz? You might get a little variation in gain at the higher frequencies, but it won't be much compared the effects of the op-amp on the lower third of the circuit.

 

Hello Ian, yes you are correct my basic goal is to make the "P" part of the PIDcurrent stronger at most 70mA
but it seams the I component is the dominant and P not getting much influence.
How whould you set the gain in this situation.
Ltspice file are attached.

 

hi yef,
Where is the Closed loop feedback that a regular PID controller requires in order to function.?
E

 

Hello Eric, When I built this circuit I basickly looked the the inputs and output of each stage .
But now I see that the load current of the total system absolutely ibeys the integrator.
I look at it in open loop.Why the proportional has no effect on the load current shape?
Photos and Ltspice files are attached.

 

I don't see anywhere that it measures the current.

 

Hello Ian, So rry for the missunderstanding .
yesthe final goal for the PID is the current on L1 insuctor.
Why the "P" is so weak in the simulation files I posted at post 16?
https://forum.allaboutcircuits.com/...-replace-1k-0805-resistor.207224/post-1989880

 

update:
maybe these little steps show the effect of P?
the larger the step the bigger influence P has on the output?
Thanks.

 

hi yef,
R4 and R5 give a Gain of only 1.
E