Problems caused by capacitors with solutions

[A.Anand Srinivasan]

like inductive kickback of a inductor which results in voltage spike due to the property of the inductor of trying to keep the current constant
is there any problems due to the property of capacitor to try and keep the voltage constant....

if so can someone please provide me with the possible problems and solutions due to a capacitor????

 

[the_edge]

Re: capacitive kickback

I think it is analog to inductance... i=C*du/dt...

 

[A.Anand Srinivasan]

Re: capacitive kickback

i know it is analogous to inductor.... everyone knows that inductor and capacitor are dual and hence would be analogous in some way or the other.... what i want to know is whether there is any problems due to this behaviour of capacitor....

 

[pinoy]

capacitive kickback

I think only inductor has kickback voltage... Not applied to capacitors.

 

[invent]

capacitive kickback

I agree with pinoy.
Infact, the capacitor can be used to absorb the voltage spike.
If the voltage across the capacitor is removed, it will slowly discharge. This will not cause a voltage spike.

 

[A.Anand Srinivasan]

Re: capacitive kickback

i know that it wont cause a voltage spike.... what i'm asking is that would it create a current spike or some other problem....

 

[kevpat]

Re: capacitive kickback

Capacitors can cause current spikes during charging.
If you have a large amount of capacitance on the input of your PCB then several amps of current can flow for a few milliseconds when you first switch on the device until the capaitors are charged. This can cause problems with over current protection on your power supply. One of the most common problems with this is on USB powered devices where the device must maintain less than 500mA current drain, hence why you get a warning sometimes when you plug USB devices into your PC/laptop.
When power is switched off you won't get a high current discharge analogous to an inductor kick, due to the RC time constant of a capacitor discharge, you are more likely to see the capacitor take a very long time to discharge as it doesn't have a path to easily discharge to ground. Hope this helps.

 

[tshankar501]

capacitive kickback

so is there a solution to avoid this current spike. in inductor we use an diode to stop the voltage spike.....here could you think some other device.....since we cannot have a diode here....

 

[A.Anand Srinivasan]

Re: capacitive kickback

i referred a few things about this... the current is called surge current and generally they are avoided using current limiters(current diode)....

 

[exeiji]

Re: capacitive kickback

in power supply, inproperly driven MOSFETs sometimes have small shoot through. Not observable on main power supply because surge current is supplied by decoupling cap but MOSFET will be hot. If you monitor the current via current probe by inserting a wire between cap and mosfet, you can see it on scope. Not much can be done here except to improve the drive signals.

Anand: what are those current limiter (current diodes) you mentioned? Care to share a few part number for me to dig up their datasheets? Thanks!!

 

[A.Anand Srinivasan]

Re: capacitive kickback

current diodes are nothing but JFETs with their source and Gate shorted... it fixes their current to a particular value.....