Summary:
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.
MANUFACTURER'S EXAMPLE:
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.
ADDED:
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.
MECHANISM:
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.
Yes, you can do that.
To be on the safe side of things I suggest an addition:
Put two 50KOhm to 100kOhm resistors in parallel to the capacitors. These resistors make sure that:
- The voltage level at the junction between the capacitors is close to 1/2 of the total voltage.
With ideal capacitors the junction would be at 1/2 of the total voltage. The world isn't ideal though and you will get capacitors that have tolerances in capacitance and the internal series resistance. When you charge them they will in practice get a voltage that is somewhere else but not exactly at 1/2 of the total voltage.
I suggest you try this out at home using low voltage (12V or so) and two different 100µF capacitors. You may be surprised how far off the voltage after a charge cycle is.
Adding the resistor voltage divider gives the capacitors a voltage reference.
- The capacitors discharge over time making your device a bit less dangerous.
Capacitors of that high capacitance and voltage can easily kill you if you touch them. They also hold voltage for quite a long time. Worse: Even after discharging, the capacitors may re-charge on their own due to an effect called dielectric absorption.
Having a resistor in parallel to the capacitors keeps that somewhat in check.
Last word of warning repeated:
The charge stored in the capacitors can easily kill people. If you don't yet know what you're dealing with please carefully read safety rules from the DIY tube-amp community. They deal with with that stuff each day.
Edit: Since the OP asked why such a circuit can kill people, even if it's powered just by a 1.5V battery:
Your disposable flash charger is a circuit that transforms the 1.5V up to some much higher voltage at a much lower current. This current is used to charge up the capacitor. Charging takes a while because the charging current is low, but once the capacitor is charged the energy can be let free instantly and currents of multiple amperes are possible.
Now what happens if you put your fingers across the leads of a 330µF capacitor loaded with 300V?
First thing is, that current starts to flow through your skin. The skin resistance is somewhere between 1KOhm and 100KOhm.
Lets say it's a hot summer day and your skin resistance is on the lower side of things. 10KOhm let's say. You'll get a shock, but this will likely not kill you because the current is not high enough yet. 300V at 10KOhm gives 30mA.
However, something else will likely happen: You get burn marks at the point where the current enters your body. And this is critical: Suddenly the high skin resistance is partly gone and your flesh is in direct contact with the voltage. The resistance will drop down to 1000 to 500 Ohm now.
Part of the energy stored in the capacitor will be consumed now and the voltage dropped down a bit. Let's say it's down to 280V now and your body resistance is at 500 Ohm. How much current will flow? 560mA. OUTCH!
There are different sources how much current is required to kill. It also depends on a lot of factors and differs from person to person. A number that I've picked up on the Internet was 300mA for DC currents.
The capacitor will now rapidly discharge through your body and the current will be down in the safer region after half a second or so.
Will that kill you? The answer is: Maybe. You only got one single discharge cycle, not a prolonged exposure to the current. This is good. If the current passes your heart (easy to do: Just make contact with both hands) the chances are quite high that your heart gets out of rhythm. Have bad luck and you'll drop dead. If you touched the capacitor with a single finger the chances are much lower.
That is by the way the reason why you're often advised to put one hand into your pockets if you're poking around in anything with high voltages. It prevents having current flow through the heart.
So best case it will hurt a lot. If worse comes to worse you'll drop dead from from a 1.5V battery.
Best Answer
Why not? The process is completely linear. There's no "threshold effect". I can use a capacitor rated at 50V to couple a signal that might be only a few µV in amplitude.
Also, it isn't the electric field that causes the electrons to move. It's the displacement of electrons (caused by an external source of voltage) that creates the field in the dielectric.
Suppose you have two capacitors of the same value, but one has 100× the dielectric thickness (and therefore 100× the area) of the other. If you charge them to the same voltage, they have the same charge — the same number of electrons have been shifted from one side to the other. Sure, the E field is 100× less intense in the one with the thicker dielectric, but there's also 10,000× as much (thickness × area) of it, so the total energy stored is also the same.