help , what does the C8 do?  - Page 1
Forum section: electronics / beginners

author: Touch3
author_rank: Newbie
timestamp: November 25, 2023, 10:06:50 am
content: This is a comparator circuit as shown. The negative terminal is ISEN taken from the current detection resistor of the brushless motor, and the positive terminal is the set level. What I wonder is what the function of C8 is? Who can help me look at it? Thank you.

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2023, 10:15:04 am
content: C8 is a filter, to remove high frequency noise from the input, and to average the current, so the turn on spike as the capacitance of the wiring and mosfets will not falsely trigger the comparator, as it slows the rise of the current sense voltage, allowing for the motor inductance to be the dominant source of voltage, so the driver can switch off the mosfet switch when the current in the coil has exceeded the required current cut off point.If it is not there the charge injection of turning on the mosfet Q2,4,6 will otherwise appear as an extra current flow in the sense resistor, and thus with a moderately high current in the coil turn off the driver early, before the current is correct to provide the required torque in the motor.

author: Touch3
author_rank: Newbie
timestamp: November 25, 2023, 10:45:38 am
content: Are you saying C8 is a filter? So the filter loop of interference signal is C8->C14->GND?

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2023, 11:14:58 am
content: C8 is removing noise from the comparator input, as the desired trigger voltage is between the 2 comparator inputs, the filtering is there, so that it slows down the response of the input, only R4 and C8 are the filter, C18 is going to allow the reference to rise up with the edge of the impulse, so the comparator will still not trigger on this edge, but only after a RC time constant when the voltage finally settles down to steady state, and thus the comparator input will have inverting input above non inverting input. Roughly 2ms, so enough time for transients in thew wiring and the motor to have settled, and the current to have ramped up through motor inductance to the desired current.

author: Touch3
author_rank: Newbie
timestamp: November 25, 2023, 11:40:46 am
content: C8 is connected between the two ends of the op amp input. Does it have any other function besides filtering?Can I change C8 like this? One end of C8 is connected to the negative terminal of the amplifier, and the other end is grounded.

author: Konkedout
author_rank: Frequent Contributor
timestamp: November 26, 2023, 06:01:39 am
content: I think that might work, but layout (component placement) is an under-appreciated consideration. Ignoring the exact bandwidth differences and assuming no layout issues, yeah that is OK. But f there is a lot of current flowing through ground, or if there is a lot of wiring distance that is not shown and the environment is noisy it might not work as well. C8 should be a small capacitor that is up close to the comparator input.I find it interesting that one capacitor is shown as 220 nF while the other one is shown as 0.1 uF. Two different conventions of nomenclature. I personally prefer nF rather than decimal uF. But neither is as bad as uuF which was common once upon a time.

author: dietert1
author_rank: Super Contributor
timestamp: November 26, 2023, 07:46:33 am
content: It may be possible that C8 = 220 pF (not nF) was meant initially. With the 10K resistor R4 the filter time constant would be 2.2 usec, certainly enough to suppress mosfet turn-on and turn-off spikes.Power mosfets often have some nF of input capacitance, times 10R gate resistor gives 50 nsec or so.Regards, Dieter

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2023, 03:25:15 pm
content: Quote from: Touch3 on November 25, 2023, 11:40:46 amC8 also delays the signal to the comparator.QuoteThat will almost always still work, however placing C8 between the inputs has the advantage of not turning common mode noise into differential noise. If C8 is grounded, then common mode noise in the ground will affect the two inputs differently and could possibly trip the comparator.But this is unlikely and either connection should work fine.