Why can a battery charge the capacitor?

[kendall]

Hello there, guys!

I'm perplexed as to why the capacitor is

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I'd never considered it until I recently read it in a book. I honestly have no idea what's going on.

If anyone has any idea why this happens, please let me know.

Thanks!

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[zjj_wwa]

The mains voltage is rarely "pure" AC. 
Most of the time, it is AC with a small amount of DC.  
For example:  230V AC + 0,1 V DC. 
The 100 milivolts is pure DC, or as we call it a DC offset. 
You can thank your female neighbour who dries her hair with a hair dryer running on half power (via diode), or other such "consumers" who create an uneven load for the positive half-sine as compared to the load on the negative half-sine. 

If the primary winding of your toroidal transformer (the one sitting inside your power amplifier or other audio component) is exposed to this 0,5V DC,  the effects are very very unfortunate. 

DC does not understand "Induction" or "Henry's". 
DC does understand real Ohms.  As in Ohms of the wire used to wind your primary. 
My wild ass guess is that your 500VA primary has a wire resistance of say 5 Ohms. 

That means that the DC CURRENT  (i.e. MAGNETIZING current) flowing through your primary is something like I = U / R = 0,5 V / 5 Ohms = 0,1 A. 

Does not seem like a real big value ... does it? 

Whell - guess what.  You guessed WRONG. 

 

 

This value needs to be MULTIPLIED by the amount of turns of the primary winding. That would typically be 2300 turns. 

2300 turns * 0,1 A = 230 Amper-Turns.   Now, THAT is a really BIG value. Enough to upset the workings of your transformer, pushing it partially into magnetic saturation. Meaning into a region of the magnetic hysteresis, where it is far from optimal operational conditions. Providing that it will still work at all. 

So, now you have a brief knowledge of the "Problem".   But what about the "Solution" ? 

The solution is to basically use a bi-polar, high-capacity electrolytic capacitor (pair), which will charge up to the full amount of 0,5V DC and block it from reaching your primary. That is basically the whole story. 

A capacitor does NOT allow DC current to pass.  But it DOES allow AC current to pass. 

In short, that is the solution.   The bi-polar capacitor MUST be protected by series connected diodes from right to left and from left to right.   Such diodes LIMIT the amount of the maximum possible voltage, to which such an electrolytic can charge up to. 

Basically, when you use two right-going and two left-going power diodes, the forward voltage will be limited to 2 * Vf = 1,4 Volts. 

So the capacitor could theoretically be for a very low voltage. But practically, you want to use electrolytic capacitors (anti-series connected pairs:  +-  -+) for a DC voltage of not less than 25V. 

For a big power amplifier, I recommend that you use (at least) Two pairs of 22 000 uF / 25V. 

Remember: PAIRS.  Anti-Series connected.  I.e. with their minuses together, and with their "+" to the external circuit.   Remember, a capacitor is a polarized device and can withstand DC voltages only in ONE direction.  But when put into an AC circuit, it needs to be converted into an AC device.  Hence the PAIRS. Each capacitor working in the opposite direction within the series connected pair. 

That is basically it. 

The simplest DC blocker consists of a pair of high-capacity, medium voltage electrolytics and four diodes. 

 

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[zjj_wwa]

>>>I'm perplexed as to why the capacitor is DC-blocking, but the battery (DC) may charge the capacitor.

Consider a capacitor as an equivalent to your fuel tank in your car.  You can "fill it up" to the maximum limit, but not more.  Once you fill it up, it is full, so no further filling it up can occur.  We then say that the capacitor is CHARGED. Up. To the limit.   Hence no further charging current can flow (because it is already FULL).  Hence, the device is BLOCKING any further flow of current. 

But if the tank is empty or almost empty - there is no problem with charging it up, providing that you do not "blow the top". 

Sorry for the simple language - no offense intended. 

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