What does 2 capacitors in series do?

[wsixsmith]

In the attached circuit could someone explain what the 2 diodes in series would be accomplishing? Also If I was using 120 Volts instead of 220 volts would allow me to use a 200 volt capacitor instead of a 400 Volt capacitor?

 

[banjo]

The two diodes are redundant. In normal operation, they only cause an additional small voltage drop. A series connection can increase the peak inverse voltage. However, I believe these diodes are already rated to 1KV. Probably, the designer added the second diode as a protection against capacitor failure due to one diode shorting. Any one diode can short without blowing up a capacitor.
Voltage rating of capacitor should be greater than 1.25 times peak voltage. (The 1.25 is a safety margin.) Peak voltage is rms voltage * 2^0.5 . In the original circuit, 1.25*220*1.414 = 389. A 400V capacitor is the next larger size. In your case, using a 200V capacitor would be decreasing the safety margin. A 250V cap would be better, but 200V would work.

 

[wsixsmith]

Thank you very much for the explanation. When you mentioned that 2 diodes in series can increase the inverse voltage is there an easy way to tell by how much it would be increased? Is it as simple as adding the value of the 2 diodes together or is there a formula to be used? And you were bang on, those diodes are 1 KV reverse voltage.

 

[banjo]

Yes, in general the effect is additive. Therefore, the two diode string would have an effective peak inverse voltage of 2KV. At voltages above the peak inverse voltage, the diode can start to act like a zener diode. (Zener diodes are specially made to have a low and tightly controlled peak inverse voltage so this property can be used for voltage regulation.)
Recitifer diodes are met to allow current only in one direction and are designed to have as high a peak inverse voltage as practical.
The rule of thumb is to double the circuit's peak voltage and then pick a diode with an inverse voltage at least that high or higher. For the 220V system, the peak is again 220*2^0.5 or 311 volts. Doubling that would give 622V. So 1KV should be fine.

 

[wsixsmith]

I actually meant to say diodes in the topic but since I said capacitors. Does anyone have a formula or explanation for what 2 capacitors in series would do?

Thanks,

 

[banjo]

Capacitors in series reduce the total capacitance by the formula:

1/Ctotal = 1/C1 + 1/C2 + 1/C3 +.....

You may recognize this as the same formula for resistors in parallel.

Therefore, connecting capacitors in series reduces their total capacitance. Why would you do this? Because it increases the working voltage that can be applied. The DC voltage across capacitors is distributed with the inverse of their capacitance value. Therefore with two capacitors in series, the smaller capacitor drops the larger voltage. The reason for this is that they have the same charge level and since Q=CV, then the smaller capacitor has the larger voltage. Since Q's are equal.
C1V1 = C2V2. And Vtotal = V1 + V2 . From these equations you can solve for the voltage across either capacitor.

If the two capacitors are equal in value, then the calculation is easy. The total effective capacitance is 1/2 the capacitance of the individual capacitor and the working voltage is twice the rating of the individual capacitor.
Connecting capacitors in series is most often seen in high voltage circuits.

 

[servocontroller]

by connecting two capacitors in series you can get half voltage at the central joint subjected there capacitances are same

 

[Snoopy_94]

This is the theory.
In practice it is also necessary to take in account of the parasitic resitor in parallel on each capacitor. Very important with DC voltage, because these resistors governs the distribution of the total voltage.