I have a LCD monitor that is wavy and produces more ripple as the resolution goes up. I have just replaced 2 surface mount capacitors, their ESR ( using Dick Smith meter) readings are 88 and 97 respectively, so there must be bad . However, the monitor is still wavy after the repair.
There are couples of 16V 10uF that goes around 10-17, and according to the chart on the meter , good rating should be around 1.6
Question1: What ESR value is considered to be bad? How many times the bad one has to be larger than the good value on the meter's chart?
Question2: Is it possible that the ripple is caused by something other than the dying capacitors?
Depends on the Value of capacitance as you can tell from the meter chart.
chart?
The times depends of the type of cap, there are low ESR caps and low leakage caps. Towit the higher ESR the less ripple that cap can take out compared to a low ESR. of the same value of capacitance. The Lower the ESR the better.
Instead of trying to pass/fail a electrolytic based only on the chart it is wise to compare the readings with several new caps from your parts bin..... same value obviously. electricitym . .
When the ESR is too high, the circuit doesn't work :-) That is the criterion that really matters in the end.
Some circuits are more sensitive to the ESR of the capacitors than other circuits. An increase of 10 times means that it is worth replacing the cap, even though this is no absolute guarantee that it will fix your monitor. The ESR is sufficiently high that it *might* be the problem, and at least when you have replaced it, you will have ruled out that component and you can move on. Also if you don't replace it, then it might stop working soon anyway.
Although I've said on many occasions that the Dick Smith meter gives unambiguous readings, that statement has to be tempered with a degree of experience. Interpreting ESR readings is a mixture of art and science, and if you deal with the subject a lot, you will rapidly get a " feel " for bad caps. As others have said, a lot depends on capacitor type, and where it is located in the circuit, as to whether its going to cause trouble. Surface mount electros tend to have a higher ' natural ' reading than their corresponding-value conventional types, so it's often a good plan to do a comparison with a known-good similar type.
I use a Dick Smith virtually every day, and the chart on the front is, in general, a very good guide, but it should be used as just that, not a definitive good / not good indicator for all cap types. In normal circumstances, I would consider anything giving a reading of twice or more the *expected* value, to be at least on its way, if not truly u/s. Other indicators help with the suspicion of a bad cap. These are -
Its proximity to anything that gets hot
The ripple ( or other 'AC' ) circumstances under which it normally works, and how hot this causes it to normally run
To some extent, the normal voltage that's across it, compared to its rated voltage
That's an excellent explanation, mate! You wouldn't believe how many times I've been asked "What's a good and bad ESR reading?" as though it's a hard and fast value for a capacitor like its working voltage is. I'm going to make it into a text file and send it to people when they ask me that question. Thanks!!
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