How bad electrolytic cap behaves in a circuit

[Askic]

Hello guys,

I hope there are people here who can explain me what bad or blown electrolytic cap change in the circuit. How its capacity is changed?

Recently I had a problem with my Samsung 940N monitor. When computer is started for the first 1 or 2 minutes the picture is slightly flickering, later is OK.
The problem is solved by replacing three bumped caps 470 uF, 25V on power supply board.

I also found two slightly bumped caps on my motherboard, but the it is working fine, so for now I didn't change them. In this case, there is absolutely no indication that something is wrong or not functioning properly because of bad caps. The caps are 1000 uF, 16V.

So, in two cases I had bumped caps and in one case I had indication something is wrong (startup flicker) in other case everything seems OK.

[PeteS in CA]

Re: How bad electrolytic cap behaves in a circuit

Well, the bad capacitor doesn't do what it is supposed to do, or it does something it should not.

If it fails short circuit things may blow up or a fuse may blow or a power supply's over-current protection may be tripped.

If a capacitor's capacitance drops it can't hold the charge it is supposed to store.

If a capacitor's equivalent series resistance goes up, it may not be able to supply the energy it is supposed to be storing. Or it may not reduce the ripple and noise in a circuit.

The monitor problem you described sounds like the capacitors were not supplying needed energy until they warmed up, causing the flickering.

The domed capacitors in your motherboard are probably not filtering VRM output noise adequately, and you may see erratic performance, errors, etc.. I believe that if those capacitors are not replaced and deteriorate badly enough, the increased ripple amplitude could damage components, but I'll defer to others here on BCF with greater experience regarding that possibility.

[televizora]

Re: How bad electrolytic cap behaves in a circuit

Capacitors are elements used to store energy or because of their freq characteristics - as part of a filter.
Here are some examples of capacitor use
1. Capacitor used as a part of RC generator
2. Capacitor as a filter, buffer
3. Capacitor as a ballast
Ex 1 - the circuit that relies on the RC generator will not work correctly. Ex - PSU.
Ex 2 - the capacitor on the exit of the PSU with high ESR will cause low voltage on exit.
Ex 3 - complete malfunction of the circuit
Ex 4 - PFC of the PSU

Ex1 details - cheap PSU-s use IC-s to drive the MOSFET of the switching PSU, that don't have internal oscilators.
Use external RC to provide base freq
Ex2 details - most PSUs have capacitors on their output. To smooth the voltage after rectification. If ESR too high, then they become like resistors connected to the output. Then you have lower than specified voltage on the output of the PSU.
Ex3 details - for example cheap AC led lightbulbs. Capacitors have certain reactive resistance for certain freq.
Some of the cheap lighbulbs use them instead of dedicated drivers, on place of what usually will be resistor.
This cannot be used for DC. Capacitor for DC = infinite resistance.
Ex4 details - malfunction of the PSU, that relies on CAP to boost the voltage.
---
There are rare cases, when they just blow up or one of the legs is torn or corroded. Replace ASAP.
These are some uses in digital electronics and PSU-s.
Analog uses include filters - low pass, high pass or combined and again - oscillator circuits.
For ex - NE555 relies on external components to tune up the oscilation.
Read about time constant. Caps can make the circuits around them to fail as well.
Not properly working PSU, wrong freq - heat, shorting of MOSFETS and so on.
Usually the PSU IC-s are smart enough to turn off the PSU if not operating in the specified parameter range.
But don't count on this, because of many cheepies, where the component number is extremely reduced.
Caps are also used in buck voltage converters.

One more ex - you have Switching mode PSU that relies on RC generator to generate impulses, so the MOSFET is driven.
The MOSFET-s have a different parameters. Constant load and impulse load. For ex, you have MOSFET that can tolerate
duty cycle of 20Khz. At one moment, because of failed cap, the frequency becomes 5Khz or total bust.
If there isn't a smart IC the mosfet will overheat, blow, short, take half of the live side of the PSU and possibly damage the transformer.
The short will also take the input rectifier with it, as well as the fuse. Full cascade failure.
What could happen depends on where where the cap is and what it's function is.. From almost nothing to total failure.
The bright side is that the caps don't usually go out of spec or blow up suddenly. Usually you will have much time and prior notice
that something is not okay. And time to fix it. If you ignore it, you know what awaits you.

Also - one topic to read fro our forum - https://www.badcaps.net/forum/showthread.php?t=211

[Askic]

Re: How bad electrolytic cap behaves in a circuit

Excellent explanation televizora!

Thank you very much.