Equivalent Series Resistor (ESR) Value in Electrolytic Capacitors
Equivalent series resistor (ESR) in electrolytic capacitor is an interesting topic. Electrolytic capacitors can be found in many circuits in a circuit board but in today’s article I’m going to share with you about the electrolytic capacitors ESR found in the SMPS DC output line (filter circuit). I will not talk about ESR in details because there is already tons of information about ESR in the internet where you can refer to. In my previous article about different type of capacitors connected in parallel, I have explained about the function of this filter capacitor.
In order to test or to check the filter capacitor ESR, you need an ESR meter. In the market you can find many brands of ESR meters and comes with many different prices. One of the good features about ESR meter is that it can check the capacitor ESR while the capacitor is still in circuit (on board). Unless it is connected parallel with another filter capacitor or if the DC line is going to a low ohm component such as the heater circuit of CRT Monitor then you need to remove one of the leads and to test it.
If you found a capacitor with high ESR value or the capacitor already bulged, your next action would be to remove and replace with a known good capacitor and retest the equipment and hopefully the equipment will work again. This is a common procedure for troubleshooting and repairing electronic equipment. I myself also follow such procedures but what you need to pay attention is the brand of capacitors you are replacing.
From my repair experiences, I had encountered problem where the repaired set was returned for warranty claim because the capacitor that I have replaced became bulged after just couple of months only! A repeated call by customer means a lost time to you because you need to settle problem without getting any extra pay. If it happens few times to the same customer, you will be labeled as a non-professional repairer.
So, in order to solve the issue you really need a replacement capacitor with a good brand and are able to withstand higher temperature (105 Celcius). It is always a good idea to check the capacitor datasheet if you need more info about that particular brand of capacitor. For your information, there are certain conditions that can significantly affect the performance of a capacitor include high current, high frequency, and extreme temperatures but only frequency affects ESR.
In the above paragraph, I did not mention about using a super low ESR value capacitor. Basically low ESR capacitors mean it has:
- Less power losses across that capacitor
- Lower resistance thus lower heat generated from ripples (less internal heating problems means higher lifetime)
For your information, lowest capacitor ESR is not always a best choice to certain circuits such as the regulation feedback loop, Low Dropout Regulator (LDO) type of circuit, boost regulator circuit and etc. If a capacitor that has a bit lower ESR value than original is fine, but going ultra-low on a circuit may cause oscillation problem (instability).
Some says it is okay to replace with a very low ESR value capacitor but to some they will replace with the same type and brand of capacitor only. All electronics circuit design is eventually a compromise between theoretical perfection and practical real world working. This means if you have replaced the capacitor with a too low ESR value and the device have shown weird symptom (possible due to oscillation) then try replace with another capacitor (or original capacitor) that have higher value of ESR and do a retest. If all works fine then you should know what capacitor ESR value you should use.
Reliability of capacitor
Usually for branded capacitors, they will have longer service life and rarely bulge as compare to the non-branded one. When I searched for the price of a branded and a non-branded filter capacitor in my country local online website, this is what I have found:
Non-branded 1000uf 25 volt 105 Celsius- USD 0.15 each
Branded (Nichicon) 1000uf 25 volt 105 Celsius- USD 0.80 each
The above price is just a general guide and varies from country to country. I’m showing you how significant the price different between both kind of capacitors. There is no doubt that I will opt for the branded one to avoid repeated calls.
Repair tip- You can use a higher working voltage of capacitor for replacement. Example- for a 1000uf 16 volt, you can use 1000uf 25 volt as replacement in the filter output circuit. There are repair techs that used 2200uf 16v or 25 volt for replacement if the space is permitted.
Why there is more than one filter capacitor (Figure 2) connected parallel in the same DC line?
If you look at figure 1 below, you could see only one filter capacitor in the DC line (2200uf 16 volt):
Connecting more than one filter capacitor is actually to drastically reduce the total ESR value and to increase the total capacitance thus storing more current. If there is only one filter capacitor in the DC line, that particular capacitor would be working under stress but again it depends on the design. Now you have understood why certain power supplies have more than one filter capacitors in the DC line.
Conclusion- You always has the choice of what brand of electrolytic capacitor you wanted to use. For your information my choice would be always the branded one so that I can avoid problem after the repair have been done. If the replaced branded capacitor has caused weird problems in the circuit then I will use a different capacitor ESR value and retest the set. Hope you have enjoyed the article and if you have any question related to this topic you can post it in the comment section below.
This article is brought to you by Jestine Yong. He is from Kuala Lumpur Malaysia and he loved electronics repair and blogging about electronics repair information. He is the author of the famous Basic Electronics Repair and SMPS Repair ebook . He is also a trainer and conduct electronics repair courses at Noahtech Electronics Training Center.
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You may also interested in his previous repair article on the Functions of RC Snubber Circuit in SMPS