Electrolytics in a voltage doubler - Page 1
Forum section: electronics / beginners

author: electronwaster
author_rank: Regular Contributor
timestamp: March 05, 2013, 07:10:54 am
content: I've been trying to understand voltage multipliers, and how current flow into and out of capacitors, to make these circuits possible. I have seen diagrams of cascade type voltage doublers, like this one:.. but I'm having trouble understanding how the electrolytic/polarised cap is allowed to be in series with an AC source like that (the first cap, top left). Surely that cap is having voltage applied across it backwards for half the AC cycle? I thought electrolytics would go pop if you applied a reverse voltage to them?Clearly I've misunderstood something basic here, because the circuit is everywhere. Can anyone enlighten me please?Cheers,Electronwaster

author: AndyC_772
author_rank: Super Contributor
timestamp: March 05, 2013, 07:42:59 am
content: This is a question that you can answer very easily for yourself by simulating the circuit and probing the voltages at various points to see exactly what's going on. Grab yourself a copy of LTSpice, it's free at:http://www.linear.com/designtools/software/#LTspice

author: amyk
author_rank: Super Contributor
timestamp: March 05, 2013, 12:10:02 pm
content: Actually you will pop the cap if your AC input happens to be a very low frequency and you connect the circuit initially at just the right part of the cycle.

author: Gall
author_rank: Frequent Contributor
timestamp: March 05, 2013, 12:46:47 pm
content: This circuit may be used in two different ways - with AC input (i.e. from a transformer) or with pulsed DC input (imagine a 555 timer for the input voltage source). In the first case the capacitor is at AC, in the second one it isn't.

author: MikeK
author_rank: Super Contributor
timestamp: March 05, 2013, 04:00:27 pm
content: Like Andy said, it's very illustrative to see what happens in a simulator. You can even use simple simulators like falstad.com. Look at what the voltage is on the right side of the cap when the left side is at various points of the AC waveform (namely: zero, min peak, and max peak).

author: hlavac
author_rank: Frequent Contributor
timestamp: March 05, 2013, 05:53:18 pm
content: You can happily put AC current thru an electrolytic cap as long as voltage across it stays positive or zero. What must not happen is reverse charge (negative voltage).There is no reverse polarity charge in the cap here, so it is fine.First when the top input terminal is lower voltage than the bottom one the cap charges thru the lower diode.Then as the voltage reverses it is lifted up by the upper input terminal and its voltage is added to voltage on the upper input terminal and it discharges thru the upper diode into the storage capacitor on the right, where the double voltage appears (minus two diode drops of course).

author: electronwaster
author_rank: Regular Contributor
timestamp: March 05, 2013, 07:21:17 pm
content: Quote from: hlavac on March 05, 2013, 05:53:18 pmWhat about if the positive half of the ac cycle is the first thing the cap sees?

author: hlavac
author_rank: Frequent Contributor
timestamp: March 07, 2013, 12:17:49 pm
content: Nothing happens. Even if the cap was completely discharged and there would be higher voltage on the storage cap, the upper diode prevents any reverse current.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 07, 2013, 12:25:07 pm
content: You do need to be careful about the load though - this circuit is not suitable for applications like strobe/flash charging which can discharge C2 to less than +Vin(peak), as you have a DC path through D1 which will apply reverse polarity to C1 on the positive cycle.If powered on on the positive half cycle, C1 will potentially see a momentary reverse voltage as C2 charges, but this is unlikely to be an issue.

author: hlavac
author_rank: Frequent Contributor
timestamp: March 07, 2013, 02:30:25 pm
content: You can put a diode across C1, or directly from positive rail to C2 if you are worried