Yesterday, Mike (Crackle) produced the batch of capacitors he changed to cure his recent microwave oven problem
and demonstrated the bad ESR.
It became apparent that, although everybody knows that ESR is BAD for you, it is not obvious what is is or even what the initials stand for - although the fact that is measured in ohms is a good pointer! (Actually, it is E
It doesn't help that, if you Google ESR, all you seem to turn up are very complicated articles riddled with complex formulae!
Another thing is that, to anybody who has dabbled in electronics for a long time - and particularly anybody who has come back to it after a long break - it is a relatively new thing that was never heard of years ago.
That doesn't mean that ESR didn't exist but that it was something that nobody needed to know anything about - unless, of course, you were a capacitor manufacturer or a designer of particularly demanding equipment.
There is no such thing as the perfect capacitor. A practical capacitor will have all sorts of unwanted 'additions' - pure resistance and inductance, for example. The effect of all of these is lumped together to give the ESR, in other words, the equivalent - I prefer effective - total series resistance between the capacitor proper and the outside world. ESR is not restricted to, but is of particular importance to us in connection with, electrolytics. Of course, the current flowing into and out of the capacitor creates a voltage drop across the ESR, which creates heat ...Although the term wasn't in common use, the effects of ESR are prominent in the marking of one important component in every vintage piece of mains operated equipment. Just take a look at the smoothing can - assuming it is the usual multiple type - and note the red paint dot on the reservoir section. There will be one, even if both sections are the same value, as they commonly were. It was imperative that this section was connected to the HT rectifier because of its superior ripple current rating - often marked on the can.
The reservoir cap was the section wound on the outside of the others, so that the can could be used to help dissipate heat to the atmosphere but, of course, it was the ESR that determined how much heat was produced!
As the capacitor aged, its capacitance could drop or its ESR could increase - or both. The effect on the radio or TV would be much the same in any event, which would result in replacement. Everybody assumed
the capacitor value had dropped, but it didn't matter anyway - nobody had an ESR meter and, in truth, nobody needed one!
So what changed?
The answer to that is the introduction of the Switch Mode Power Supply. ESR increases with frequency - note the inductive component I mentioned earlier on. Our old reservoir capacitor was quite happy with its low 50 or 100Hz charge/discharge cycle but increase that a thousandfold and everything changes!
Add an ever increasing demand for miniaturisation - thus more heat inside and around ever smaller capacitors and ESR becomes a very critical factor.
In Mike's case, the ESR increased so much and destroyed the capacitor's efficiency so much that, although his oven failed, the failure mechanism was self limiting. Indeed, it is what probably happens with most caps or they'd be exploding by the million every day!
So, what's new about ESR? Nothing! What is
new is its increasing importance do anybody who want to dable with the more modern forms of electrickery ...
I've tried to keep this simple and, in all honesty, made it up as I went along but I hope it explains what this curious ESR thing is. If anybody wants to challenge the fine detail, might I respectfully point them in the direction of Google and all those complex equations ...?