I am trying to replace a bad 820 µF, 6.3 V capacitor on my PC's motherboard. Right now I don't have a 820 µF capacitor in my home and also don't have enough time to buy a new capacitor. I have two 470 µF capacitors rated at 16 volts. The damaged capacitor is located near a RAM socket of the motherboard. Can I use a combination of two 470 µF capacitors that is almost 940 µF? Is it safe to use higher value capacitors in computer systems?
Electronic – Is it safe to use a higher value capacitor
Electronic – Can you make a non-polar electrolytic capacitor out of two regular electrolytic capacitors
Yes "polarised" aluminum "wet electrolytic" capacitors can legitimately be connected "back-to-back" (ie in series with opposing polarities) to form a non-polar capacitor.
C1 + C2 are always equal in capacitance and voltage rating
Ceffective = = C1/2 = C2/2
Veffective = vrating of C1 & C2.
See "Mechanism" at end for how this (probably) works.
It is universally assumed that the two capacitors have identical capacitance when this is done.
The resulting capacitor with half the capacitance of each individual capacitor.
eg if two x 10 uF capacitors are placed in series the resulting capacitance will be 5 uF.
I conclude that the resulting capacitor will have the same voltage rating as the individual capacitors. (I may be wrong).
I have seen this method used on many occasions over many years and, more importanttly have seen the method described in application notes from a number of capacitor manufacturers. See at end for one such reference.
Understanding how the individual capacitors become correctly charged requires either faith in the capacitor manufacturers statements ("act as if they had been bypassed by diodes" or additional complexity BUT understanding how the arrangement works once initiated is easier.
Imagine two back-to-back caps with Cl fully charged and Cr fully discharged.
If a current is now passed though the series arrangement such that Cl then discharges to zero charge then the reversed polarity of Cr will cause it to be charged to full voltage. Attempts to apply additional current and to further discharge Cl so it assumes incorrect polarity would lead to Cr being charge above its rated voltage. ie it could be attempted BUT would be outside spec for both devices.
Given the above, the specific questions can be answered:
What are some reasons to connect capacitors in series?
Can create a bipolar cap from 2 x polar caps.
OR can double rated voltage as long as care is taken to balance voltage distribution. Paralleld resistors are sometimes used to help achieve balance.
"turns out that what might LOOK like two ordinary electrolytics are not, in fact, two ordinary electrolytics."
This can be done with oridinary electrolytics.
"No, do not do this. It will act as a capacitor also, but once you pass a few volts it will blow out the insulator."
Works OK if ratings are not exceeded.
'Kind of like "you can't make a BJT from two diodes"'
Reason for comparison is noted but is not a valid one. Each half capacitor is still subject to same rules and demands as when standing alone.
"it is a process that a tinkerer cannot do"
Tinkerer can - entirely legitimate.
So is a non-polar (NP) electrolytic cap electrically identical to two electrolytic caps in reverse series, or not?
It coild be but the manufacturers usually make a manufacturing change so that there are two Anode foils BUT the result is the same.
Does it not survive the same voltages?
Voltage rating is that of a single cap.
What happens to the reverse-biased cap when a large voltage is placed across the combination?
Under normal operation there is NO reverse biased cap. Each cap handles a full cycle of AC whole effectively seeing half a cycle. See my explanation above.
Are there practical limitations other than physical size?
No obvious limitation that i can think of.
Does it matter which polarity is on the outside?
No. Draw a picture of what each cap sees in isolation without reference to what is "outside it. Now change their order in the circuit. What they see is identical.
I don't see what the difference is, but a lot of people seem to think there is one.
You are correct. Functionally from a "black box" point of view they are the same.
In this document Application Guide, Aluminum Electrolytic Capacitors bY Cornell Dubilier, a competent and respected capacitor manufacturer it says (on age 2.183 & 2.184)
If two, same-value, aluminum electrolytic capacitors are connected in series, back-to-back with the positive terminals or the negative terminals connected, the resulting single capacitor is a non-polar capacitor with half the capacitance.
The two capacitors rectify the applied voltage and act as if they had been bypassed by diodes.
When voltage is applied, the correct-polarity capacitor gets the full voltage.
In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolytic capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case.
Of relevance to understanding the overall action is this comment from page 2.183.
While it may appear that the capacitance is between the two foils, actually the capacitance is between the anode foil and the electrolyte.
The positive plate is the anode foil;
the dielectric is the insulating aluminum oxide on the anode foil;
the true negative plate is the conductive, liquid electrolyte, and the cathode foil merely connects to the electrolyte.
This construction delivers colossal capacitance because etching the foils can increase surface area more than 100 times and the aluminum-oxide dielectric is less than a micrometer thick. Thus the resulting capacitor has very large plate area and the plates are awfully close together.
I intuitively feel as Olin does that it should be necessary to provide a means of maintaining correct polarity. In practice it seems that the capacitors do a good job of accommodating the startup "boundary condition". Cornell Dubiliers "acts like a diode" needs better understanding.
I think the following describes how the system works.
As I described above, once one capacitor is fully charged at one extreme of the AC waveform and the other fully discharged then the system will operate correctly, with charge being passed into the outside "plate" of one cap, across from inside plate of that cap to the other cap and "out the other end". ie a body of charge transfers to and from between the two capacitors and allows net charge flow to and from through the dual cap. No problem so far.
A correctly biased capacitor has very low leakage.
A reverse biased capacitor has higher leakage and possibly much higher.
At startup one cap is reverse biased on each half cycle and leakage current flows.
The charge flow is such as to drive the capacitors towards the properly balanced condition.
This is the "diode action" referred to - not formal rectification per say but leakage under incorrect operating bias.
After a number of cycles balance will be achieved. The "leakier" the cap is in the reverse direction the quicker balance will be achieved.
Any imperfections or inequalities will be compensated for by this self adjusting mechanism. Very neat.
It is likely but not certain that the capacitor as a tantalum electrolytic of about 10 uF to 33 uF capacitance.
Most of the following assumes this was the case.
THIS MAY NOT BE TRUE.
It may have been a Dilithium energy source or a transzorb or something else interesting and/or strange, but probably not.
If faulty, then removing it, regardless of what it was, may improve things. Removing it if not dead is less wise. See below re testing to see if this is a tantalum cap that has suffered the infamous tantalum eat-spike-and-die meltdown.
IF it is/was it may be hard short circuited. The location suggests it was a filter capacitor. It is not uncommon to use tantalum capacitors for this purpose. A voltage spike can persuade tantalum capacitors to break down and let their internal metal and produce a hard short circuit across their terminals.
If this is/was a tantalum cap then just removing it my allow the circuit to function again for test purposes with no cap in place, although it would then be wise to add a new cap.
It MAY have been an eg 0.1 uF ceramic.
More likely tantalum as above 10 uF to 33 uF, 16V rated.
Use of a solid Aluminum cap if same rating MUCH preferred - they do not self immolate.
Using a leaded aluminum electrolytic of similar raing in this location would work OK enough if original was tantalum. Short leads. Observe polarity. bend cap parallel to board when soldered.
Try this test:
Measure across capacitor with a meter set to low volts range.
If no voltage, proceed.
If any voltage at all, short across capacitor with a wire for 10 seconds and then repeat above test.
Set meter to low ohms range.
Measure resistance across capacitor with both polarities.
IF this was a tantalum cap and IF it has suffered the famous and common hard short circuit failure mode then the above test should show consistently low resistance both ways. If it does it is a good but not certain indication that is a tantalum cap that has failed. Repeat this test afer the cap has been removed (see below). If the resistance has now changed and is higher one way the other or open circuit both ways you can be mear sure that this is a failed tantalum cap. Measuring across the removed component should give the same low resistance both ways result.
- Does the laptop still work?
If not, what are the symptoms?
If it works, dos the PCI socket work?
How did the fault occur?
If it was dead, does it work when the above component is removed?
There is a vast amount on web re soldering SMD components. Starting in a motherboard whch MAY be salvable is not ideal. I suggest you try the following on another dead PCB first.
This is aimed at removing the current capacitor. It is a sub-sub-sub set of smd soldering practice. That is too large a subject to deal well with here. The following sounds complex but is simple and easy in practice. Much harder to describe than to do.
Tools / material:
Some liquid flux suited to smd soldering use IF AVAILABLE. Not essential here. But IS essential for general smd use.
Fine tweezers that you are comfortable with. I prefer super sharp needlenose tweezers - others like blunter versions.
Good light. magnifier of sort that suits that lets you view component comfortably. I use headset with flipout screens and swivel out small round magnifier for extra detailed viewing. Makes you look like a Vogon starship officer.
Some use magnifying glass on stand etc.
Soldering iron, finest sensible tip available, well tinned, temperature controlled.
Sounds horrendous but really just a usual fine soldering setup plus tweezers.
Tin each pad of bad component with iron and solder till new solder nicely evident.
Place tweezer tips alongside component and then heat one pad vigorously while applying lifting pressure with tweezers. You can try to swap iron to and fro from end to end but heat transfer is usually good enough and fast enough that whole component heats. Dont do it quite this way if you were going to re-use the component :-).
Do not apply heat that way for more than 2 or 3 seconds - that should be enough. Continual "cooking" of board at a point may cause damage elsewhere. Unlikely but possible.
The above should allow removal OK. If not try laying iron alongside component and heating both pads at once while providing mechanical pressure with iron tip and with tweezers. Once you have moved it at all it should be easy to do as above.
Most likely the two 470 µF capacitors in parallel will be fine. At these values, they are almost certainly power supply caps. About the only drawback of more capacitance is higher inrush current when the supply comes up, but the 15% extra capacitance is very very unlikely to cause any problem whatsoever.