Polymer vs. Electrolytic Capacitors?

[Zando_]

What's the difference between polymer and electrolytic capacitors? Is one better than the other? I'm going to be replacing some caps on an old AM3 ( I think) mobo, so I'd like them to last a decent while. The current ones are 6.3v 820uf. 

[Zando_]

Another question: will other parts of my mobo actually wear out? Or can I just replace (eventually) all the caps and have it keep chugging along?

[Mira Yurizaki]

3 hours ago, Zando Bob said:

What's the difference between polymer and electrolytic capacitors? Is one better than the other? I'm going to be replacing some caps on an old AM3 ( I think) mobo, so I'd like them to last a decent while. The current ones are 6.3v 820uf. 

The main difference is that electrolytic capacitors are "wet" and their material can evaporate or dry up over time. Polymer capacitors are "dry", so they have a longer service life. However, there's no "one is better than the other", both types have their electrical characteristics that one or the other can provide, among other things.

 

2 hours ago, Zando Bob said:

Another question: will other parts of my mobo actually wear out? Or can I just replace (eventually) all the caps and have it keep chugging along?

As long as you get capacitors of the same type, capacitance, and equal or higher voltage rating, you should be fine.

[Zando_]

1 hour ago, M.Yurizaki said:

The main difference is that electrolytic capacitors are "wet" and their material can evaporate or dry up over time. Polymer capacitors are "dry", so they have a longer service life. However, there's no "one is better than the other", both types have their electrical characteristics that one or the other can provide, among other things.

 

As long as you get capacitors of the same type, capacitance, and equal or higher voltage rating, you should be fine.

Awesome! So I should just go for electrolytic ones, since they're cheaper?

[Mira Yurizaki]

53 minutes ago, Zando Bob said:

Awesome! So I should just go for electrolytic ones, since they're cheaper?

Yes, and that the while I would like to believe it shouldn't matter, I'm going to assume the circuit was designed with electrolytic caps in mind.

[Zando_]

Just now, M.Yurizaki said:

Yes, and that the while I would like to believe it shouldn't matter, I'm going to assume the circuit was designed with electrolytic caps in mind.

The OEM ones I'm replacing are electrolytic. Though @mariushm said I could use either. 

[mariushm]

Electrolytic capacitors and polymer capacitors hold energy in them and then release the energy as needed. Think of them as a bucket with a nozzle at the bottom. You put this bucket of water at the water pump and depending on how fast you pump manually water will come into the bucket in bursts that will vary in water amount. Out the nozzle at the bottom however, the water will come out at a constant rate.

 

(in a very simplified way)

 

In order to hold energy in them, electrolytic capacitors use a solution called electrolyte spread between layers of metal and insulator material - there's a long strip of material ( insulator , electrolyte, metal) which is then rolled and squeezed in the round capacitor case.

If you exceed the voltage rating of the capacitor, or you heat them too much, the chemical solution inside - the electrolyte - can be damaged in some places and because of this gasses can form inside and that's what causes the bumps in the top .. those are stress relief for the capacitor, so they don't explode. As the electrolyte is damaged, the technical properties of the capacitor change often for the worse, but for a long time, the capacitor's technical properties will still be within some acceptable limits.

 

This electrolyte can be either a sort of liquid solution or it can be more like a paste ... the highest performance capacitors which technical properties close to polymer capacitors usually have a more "unstable" electrolyte, which can be damaged and produce those gases more easily compared to a paste or semi-liquid electrolyte.

In contrast, polymer capacitors don't have such an electolyte, they hold the energy in some polymer (plastic) material, so they are harder to damage and also have a longer life.

 

Capacitors have some characteristics, the most important being  (1) capacitance , which is usually specified as +/- 20% and (2) ESR / Impedance which is a sort of internal resistance of the capacitor,  how much the capacitor resists energy being taken out of it and (3) current ripple , how well the capacitor can handle bursts of energy going in it and out of it

 

ESR is a sort of bad thing for most circuits you see on motherboards, DC-DC converters, because it means that resistance will cause capacitors to warm up and some energy will be lost in them.. so overall the circuit will be less efficient. However, some circuits are designed to accept the fact that there will always be some ESR and use that property to their advantage... for such circuits, it would be bad to replace an electrolytic capacitor that has let's say 100 mOhm ESR value with a polymer capacitor that has 10 mOhm ESR value, because the circuit actually expects the ESR to be within let's say 80 .. 120 mOhm to function properly.

Nobody designs a circuit for exact values, because even in a batch of capacitors from the same series, the ESR value could vary a bit, and as the capacitors age in circuit or become damaged gradually by heat, their ESR will increase.

Anyway, the basic idea is that when you replace an electrolytic capacitor in a circuit, it's best to replace it with another capacitor that has an ESR value equal or slightly lower than the original capacitor.

 

This property of capacitors, ESR, is also dependant on the length and width of the strip of material in the capacitor and the quality (the chemical formula) of the electrolyte. The width of the strip often defines how high voltage will the capacitor support.

 

So knowing these, when a designer knows that it needs a capacitors with at most 50 mOhm ESR, this designed will often specify more capacitance than needed, or higher voltage rating, because this way he'll be sure a capacitor that has a higher diameter or is taller will be used, which in turn will have smaller ESR.

 

For example, in your case, the circuit may use a 820uF 6.3v capacitor but there's a high chance the circuit probably only needed 470uF and they chose 820uF in order to force the use of a taller and wider capacitor, which would have smaller ESR value compared to the original 470uF capacitor. 

But, a polymer capacitor would already have a much lower ESR value than even that fictional 470uF electrolytic capacitor so if you're sure the circuit would work fine even with less than 820 uF (something you'll learn from experience or by studying the circuit layout), it would be perfectly fine to install a polymer capacitor with less than 820uF in that place.

Of course, like I explained above, you have to keep in mind not to go too low with the ESR in case the circuit needs it.

 

If you're an amateur, it's best indeed to stick to safe things, like replacing with electrolytic of same capacitance and same voltage rating (or slightly higher) with ESR equal or slightly lower (you can look in datasheet of original capacitor at Impedance column)

 

 

 

 

 

[Zando_]

@mariushm Wow! You really know your stuff! I'll just stick with electrolytic caps with the same ratings. Do you have an inexpensive soldering iron you could recommend?

[mariushm]

Sorry, but all I would really be comfortable recommending would be soldering stations that have temperature sensor in the tip and allow you to adjust the temperature.

For simple things like soldering wires together or soldering some component on a prototyping board it would be possible to use a cheap 5$ soldering iron without temperature control.

Motherboards and computer components however are a different things.

Motherboards are made from multiple layers of fiber glass and copper which are glued together. Capacitors and components that are used to control power through the motherboard are often surrounded by big copper layers, so cheap soldering irons wouldn't be able to heat up the leads of a capacitor and keep them hot enough to be soldered or desoldered - all the copper in the motherboard will behave like a heatsink, taking away the heat of your cheap soldering iron.

 

A soldering station rated for at least 40 watts with temperature control is what i can recommend, cheapest would be all the Hakko 936 clones you can find on eBay, just search for "936 soldering station" .. here's an example (though i suspect it's not really 60w, it's more like 35-45w but it will be enough for a beginner ... and you'll have to buy one that works at 110v if you're in US, they're not universal voltage input)

 

Also don't be tempted to buy smaller stations like let's say this one - these have a knob to control temperature but they don't really control temperature and they don't have a temperature sensor in the tip, the knob just controls how much electricity goes in the heating element at the top which is not the same thing (it's worse, temperature sensor is really helpful)