# Charging capacitor in parallel and discharging in series

I wanted to use multiple capacitors to step up the voltage in a circuit. A little bit of google searching told me that it is called a Charge Pump. I figured out the charging each capacitor individually in parallel and then discharging them in series should result in the same capacitance and same voltage. Since charging capacitor in parallel will allow each capacitor to charge upto its rated capacitance(ideally!) and then discharging in series will add their voltages to give me Higher voltage without having to bargain with capacitance. Also I found there is a circuit called Marx Generator which uses the same principal,

Q1) Is my understanding correct? (which I think is)

Now If my understanding happens to be correct, I take four 50 volt capacitors and put them in series after charging them in parallel at say 36 volts. Then the output voltage from the capacitors should be approximately $$36v*4=144 v$$ .

Q2) Will the overall voltage damage the capacitors as it exceeds their individual ratings? (which I don't think will happen, but obviously need some experts help)

Now if I connect this output to two 200 volt capacitors in parallel and then put them in series. The resultant voltage should be 288 volts. Same as above followed and in theory I should end up with 288 volts approximately.

Q3) I think I have the circuit figured out but it will use a lot of switches to change the capacitor from parallel to series, Do you guys have a better way to do so?

• Be aware that the capacitance of four caps in series is 1/4 of the value of each cap, but that is basically correct. Marx and Cockroft-Walton both use this basic idea, the latter using diode switches.
– user16324
Commented Jun 15, 2016 at 15:13
• @BrianDrummond I know that there capacitance reduces when in series $$Q=c*v$$ But I am planning on charging them in parallel. Thanks for suggest Cockroft-Walton, I never heard about him, Is he the guy that used the Z-diode? Just guessing I am gonna search about them right now though. Commented Jun 15, 2016 at 15:50
• I saw a circuit that did this out in the wilde while fixing some equiptement, now Im trying to think of the name of it.
– j0h
Commented Apr 14, 2019 at 14:59
• Paralleling the capacitors give you extra capacitance, and putting them in series gives you less capacitance. If you have (say) 3 50uF capacitors then in parallel they are 150uF and in series they are 16.667uF. Commented May 17, 2019 at 5:07

Now if I connect this output to two 200 volt capacitors in parallel and then put them in series. The resultant voltage should be 288 volts. Same as above followed and in theory I should end up with 288 volts approximately.

No of course not. Sure you made a voltage of 144 volts but as soon as you apply that to an extra capacitor that voltage is going to fall - in other words the charge is going to be shared by the series bank and the added extra capacitors. Charged capacitors are not infinite sources of charge - their voltage drops when you remove charge to "charge up" more capacitors.

Q = CV i.e. charge = capacitance x voltage.

• Ohkay, So I can scavenge only 300 volt at maximum from 6 capacitors right? if i give each 50 volts right? Commented Jun 15, 2016 at 15:43
• It all depends. Are you charging all 6 in parallel? Commented Jun 15, 2016 at 17:03
• Yes, all 6 in parallel and then add them in series. Commented Jun 15, 2016 at 17:09
• Then yes you can but please ensure the voltage ratings for each capacitor exceed 60V by a good margin and they are all nominally the same value. If they are not the same value you might find that smaller caps exceed the voltage rating when you connect up a heavy load. Commented Jun 15, 2016 at 17:11

There are some un-addressed issues and incomplete information in some of the responses I have read here. Essentially, you are correct and it WILL work. But there are things to keep in mind.

1. DO NOT ASSUME that total capacitance will be 1/4 of single capacitance. ALWAYS use the formula to calculate it to be certain. 1/Ct = 1/C1 + 1/C2 + 1/C3...1/Cn
2. You can charge the capacitors as a parallel bank as long as you do not exceed the working (breakdown) voltage of any of the caps.
3. You will not exceed the WV of the caps when discharging in series for two reasons. !.) The voltage across any individual cap is the voltage it was charged to, NOT the total voltage of the series circuit; and 2.) In a series circuit, the WV of the caps (treated as a single cap, mind you) is NOT the WV of an individual cap, but the TOTAL WV of the caps added together. This is because when you put caps in series, you are effectively adding the distance between the plates of all the individual caps, and that distance (along with dielectric strength) is what determines the WV

NOW...to address another issue brought up somewhere in this thread - that of switching the caps from charging to discharging. I am currently teaching one of my sons about electronics, and we are currently discussing this exact issue. Mechanical switching CAN be done, although using simple switches you run the risk of accidentally discharging one or more caps while switching. If your knowledge of electronics is sparse, I would suggest using a bank of DPDT relays, all operated by a common manual switch to energize them. This will ensure the fastest switchover possible using manual means. I am working on a transistor based circuit that will sense when the capacitors have achieved full charge and then activate more transistors to switch the total voltage over to discharge into the holding cap. But I haven't done that yet. Still working on the drawings.

That's my story and I'm sticking to it.

I would say that you are right from a theoretical standpoint if I understand your meaning. You can place charged capacitors in series and the voltage will be additive, as long as there is no load or if the capacitors were ideal and perfectly balanced.

However, in real life, capacitors in general and electrolytic capacitors in particular behave badly when placed in series. If you have all of your capacitors in series fully charged, and shorted the output (top capacitor) to ground, mismatches in the capacitor values will result in voltages with respect to ground between individual electrolytic capacitors, and some will be reverse biased (and conduct). A general rule: don't try to use capacitors in series or inductors in parallel.

• But say I only use a 9v power supply to charge up 6 capacitors, the overall voltage should be around 54 volts. Now the capacitors I plan on using have 60 volt limit to keep atleast 10 volt headroom. Shouldn't they be able to handle a short like a charm? Commented Jun 15, 2016 at 16:25

Swapping caps charged in parallel, to a series setup, increases the voltage, but decreases the capacitance.

n caps of value C in parallel have capacitance nC, and charged to voltage V, total energy stored 1/2nCV^2.

Rearranging the charged caps to series, the effective total capacitance is now C/n, whilst the voltage is nV. The energy is therefore 1/2C/n(nV)^2. Simplifying gives the same stored energy of 1/2nCV^2.

Voltage is increased, but energy is conserved because the "available" charge is reduced (it can only flow from the "external" plates of the stacked caps).

That isn't to say charge pumps aren't useful, just that you can't break the laws of physics!

As other note, Marx uses spark gaps for discharge and resistors for charging, whilst CW use AC charging with diodes for switching