Capacitor question - Page 1
Forum section: electronics / beginners

author: Crambone
author_rank: Frequent Contributor
timestamp: November 11, 2020, 11:30:11 pm
content: I Need a 150pf 1200v mica cap. Can I use 2ea 75pf 1000v together? Will having 2ea 1000v give me 2000v?

author: BlackICE
author_rank: Regular Contributor
timestamp: November 12, 2020, 12:51:57 am
content: If you put two capacitors in parallel you will add their capacitance in the voltage rating will not change. If you put two identical capacitors in series the voltage rating will be added and the capacitance will be halved.

author: helius
author_rank: Super Contributor
timestamp: November 12, 2020, 04:45:57 am
content: In a series string, each capacitor receives a fraction of the voltage across the whole string. Each cap develops an equalchargeby KCL. Thevoltageon each cap will only be equal if 1) all really are exactly equal in capacitance; and 2) all have exactly equal leakage current. This is why in real series string assemblies, balancing resistors are used. The balancing resistors are chosen to exceed the greatest leakage current in any individual cap (forcing the leakage to be pretty equal in all).

author: drussell
author_rank: Super Contributor
timestamp: November 12, 2020, 08:03:48 am
content: Quote from: Crambone on November 11, 2020, 11:30:11 pmYou would need to use two 300 pF capacitors in series. When you put two identical capacitors in series, you get half the capacitance but double the voltage capability since each capacitor will see half of the charge. In this case, if you use 1000V capacitors, you should be totally fine at 1200V without any sort of balancing resistors.You calculate series capacitors the same way as calculating parallel resistors, so for two identical capacitors in series, you can use the shortcut that the value is simply half, for three in series it would be one third, etc.For two non-identical capacitors, the formula is: \$C(total)=\frac{C_1C_2}{C_1+C_2}\$For three or more, you use: \$C(total)=\frac{1}{\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}+\frac{1}{C_4}+.......}\$Edit:The voltage is shared according to their capacitance values. For example, if you had three capacitors in series, with values of 1µF, 2µF and 4µF and you put 1000 volts across the string of three, the voltages across the capacitors would be approximately 571V, 286V and 143V respectively.Since the actual charge will be distributed equally, the lowest value capacitor will have the highest voltage across it. In this example, the 1µF will be at about 571V, while that same number of electrons in the 4µF will only bring the voltage to about 143V, etc.

author: Simon
author_rank: Unknown
timestamp: November 12, 2020, 04:57:52 pm
content: Quote from: Crambone on November 11, 2020, 11:30:11 pmNO!You will get 2000V 37.5pF with the leakage current issues described above that will ultimately de-rate the combined voltage rating without the balancing resistors.

author: drussell
author_rank: Super Contributor
timestamp: November 12, 2020, 05:06:44 pm
content: Quote from: Simon on November 12, 2020, 04:57:52 pmIf the OP uses two 1000V mica capacitors in series on 1200V they will be fine without balancing resistors. Balancing resistors are needed when you're doing things like series strings of power supply reservoir capacitors, etc., but will be totally inappropriate for most applications where a tiny pF mica capacitor is called for!

author: Simon
author_rank: Unknown
timestamp: November 12, 2020, 05:08:52 pm
content: I would still not call it a 2000V combo, sure 1200V will be fine.

author: Crambone
author_rank: Frequent Contributor
timestamp: November 13, 2020, 04:21:51 am
content: Great info thank you everyone!