Capacitors in series for voltage - Page 1
Forum section: electronics / beginners

author: Poe
author_rank: Frequent Contributor
timestamp: September 09, 2014, 02:19:34 pm
content: Has anyone used capacitors in series for the increased voltage limit? e.g. Two 100V 10uF caps to create a 5uF 200V cap?I have only seen that technique used in products plagued with numerous design issues so I have hesitated to consider it.Are there problems with this method?

author: PepeK
author_rank: Regular Contributor
timestamp: September 09, 2014, 02:35:04 pm
content: Voltage divider having ratio 1:1 (100k - 1 M ohm resistors) is highly recommended. Otherwise different parameters like capacitance or parasitic resistance can cause unbalanced voltages across capacitors and one of them can be overloaded. Do you want to use them as a filter after a diode rectifier ?

author: Poe
author_rank: Frequent Contributor
timestamp: September 09, 2014, 02:58:24 pm
content: Quote from: PepeK on September 09, 2014, 02:35:04 pmI have seen them used after a rectifier, but rarely with those resistors. The example I'm looking at now has two capacitors rated at 250V (500V total) with an expected 110VAC USA supply.It just got me pondering potential problems. The balance resistors makes sense. They probably become more important as the operating voltage gets closer to the total voltage limit.

author: German_EE
author_rank: Super Contributor
timestamp: September 09, 2014, 03:08:38 pm
content: This is a standard technique in tube power amplifiers where you might see an HT supply of two or three thousand volts. Electrolytic capacitors are placed in series across the supply along with a chain of 100K resistors which have two functions a) equalizing the voltage across the capacitors and b) providing a discharge path after the power is removed, good for safety.http://wb0nni.dakotamade.com/doubler1.jpg

author: DanielS
author_rank: Frequent Contributor
timestamp: September 09, 2014, 03:15:09 pm
content: Quote from: Poe on September 09, 2014, 02:58:24 pmIn the voltage doubler line input configuration, there is no need for equalization resistors since the "positive" capacitor gets charged by the positive half-wave and the "negative" capacitor gets charged by the negative alternance. Imbalance in that setup (in terms of one capacitor ending up bearing the whole rail-to-rail supply voltage) is not possible since each cap gets charged by its respective half-wave rectifier with neutral line connected in-between - that's what the 115V switch does: connect neutral to mid-point.

author: Poe
author_rank: Frequent Contributor
timestamp: September 09, 2014, 03:57:46 pm
content: Quote from: DanielS on September 09, 2014, 03:15:09 pmI'm specifically talking about circuits like the one I described earlier where the node between the two capacitors is not connected to anything else. The two caps were selected and placed in series solely for the purpose of increasing the caps voltage limit.I have one in front of me that is simply two capacitors in series after a half wave rectifier. No transformer so they get the full 110Vac. No filtering such as a common mode choke, etc.Are there problems with this technique?

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2014, 04:24:35 pm
content: Quote from: Poe on September 09, 2014, 02:19:34 pmProvided that you have two identical capacitors, there are no problems. Of course in practice youwon'thave two identical capacitors, and the voltage willnotbe equally shared!The capacitor that has the lower capacitance will end up with a larger voltage across it.The capacitor that has the lower leakage current will end up with a larger voltage across it.This cannot be ignored with ultracapacitors, so read the manufacturer's info to understand the causes and ameliorations.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 09, 2014, 04:51:38 pm
content: Quote from: Poe on September 09, 2014, 03:57:46 pmIf you mean in a classic line voltage doubler like what I attached with identical caps for both sides, yes, there is absolutely no problem with it since both caps get equally topped up on their respective alternance. The main reason those setups should still have some form of bleeder resistor in them is for safety reasons: some safety regulations require that high-voltage caps be drained within some given time from mains voltage being removed.

author: Poe
author_rank: Frequent Contributor
timestamp: September 09, 2014, 05:11:01 pm
content: The first circuit illustrates what I'm referring to.Most I have seen did not have the bleed resistors though.If one capacitor has a close-enough voltage rating by itself, I'm guessing that using two in series (as they did with the circuit in my hands) would mostly eliminate any need for balance resistors?

author: KJDS
author_rank: Super Contributor
timestamp: September 09, 2014, 05:21:15 pm
content: It's quite possible that the leakage of one cap to be 10x the leakage of another, especially after they have aged a little, with one closer to a heat source than the other. so I'd always use the balancing resistors.

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2014, 05:22:22 pm
content: Quote from: DanielS on September 09, 2014, 04:51:38 pmThat's two half-wave rectifiers, which is not the subject of the thread.I'll note that in your schematic, removing the AC voltage will cause the smaller capacitor to become reverse charged. Solution: use a separate bleed resistor for each capacitor.

author: Seekonk
author_rank: Super Contributor
timestamp: September 09, 2014, 09:13:28 pm
content: Besides electrolytics, there are other common uses for series capacitors. X2 rated line capacitors are actually two capacitors in series, there is an intermediate metalized film in between. If you used a common 1000V, the actual AC rating may be only 100V. That is because when they are wound, microscopic air bubbles are formed between the layers. With AC a tiny corona forms that eventually degrades the capacitor. I manufactured thousands of commercial products with three film capacitors in series acting as a voltage drop that operated at 480V AC without failure

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2014, 10:14:19 pm
content: With some capacitors you can get away without the balancing resistors because increased leakage as the voltage rises will keep them roughly balanced. In most cases though you want the balance resistors.

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2014, 11:31:21 pm
content: Quote from: David Hess on September 09, 2014, 10:14:19 pmIs that part of the capacitor's specification - or could the manufacturer "improve" it without notice?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2014, 12:05:36 am
content: Quote from: tggzzz on September 09, 2014, 11:31:21 pmIt's characteristic of electrolytic capacitors, but it changes with time as each one reforms to withstand the higher voltage. You'd be comparing the reform rate of two caps, which seems dubious to me.Tim

author: ovnr
author_rank: Frequent Contributor
timestamp: September 10, 2014, 01:07:20 am
content: Nothing wrong with it - I recently finished a project which had six caps in series (2.7v supercaps - I needed 12v, and some extra headroom is always nice). Just be sure to add a resistor divider in parallel, as has been mentioned - if you have a reasonably constant load and input voltage, look in the capacitor datasheet for the leakage current, then configure your divider to present around 3 times as much leakage.(For example, say you have a leakage current of 100 µA, total voltage of 36v, and three caps in series, in which case you'd probably pick 47k resistors, with one resistor in parallel with each cap.)I had no problems with my stack, and capacitor voltages were all within +- 0.2v, even when using the cheapest supercaps I found.

author: tggzzz
author_rank: Super Contributor
timestamp: September 10, 2014, 07:59:25 am
content: Quote from: ovnr on September 10, 2014, 01:07:20 amIf that technique is recommended by the manufacturer, then it would be perfectly acceptable.Does the manufacturer guarantee the leakage current over time and temperature?

author: ovnr
author_rank: Frequent Contributor
timestamp: September 10, 2014, 11:28:35 am
content: Quote from: tggzzz on September 10, 2014, 07:59:25 amPassive balancing in this way is the generally accepted way to do it for smaller supercaps, yes. Larger supercaps will usually be actively balanced, but it's kind of a pain.In my case, no, there's no aging/temp spec on the leakage, but I've got enough headroom (2v out of 2.7v rated) that I don't really care. Since I'm using these for backup power (graceful shutdown in the event of power failure) I'm a bit reluctant to increase the balancing current a lot, but if you don't care much about parasitic power drain you can easily go for 10x - or even 100x - the default leakage.

author: mrkev
author_rank: Regular Contributor
timestamp: September 10, 2014, 07:08:50 pm
content: Just btw, you are talking about electrolytic caps. They have genuenly higher leakage than fe foil caps. And as was mentioned before, those currents can vary by an order. Usually, you won't get leakage mentioned in datasheet, but it's part of ESR or tan delta which is often given (usually in terms of worst case).This method is often used in really high voltage stuff like f.e. Tesla generator as it's often the only managable solution.

author: tggzzz
author_rank: Super Contributor
timestamp: September 10, 2014, 08:18:01 pm
content: Quote from: mrkev on September 10, 2014, 07:08:50 pmThinking of high voltage capacitors, how does the "solutions" for LT electrolytics translate to HT capacitors where "soakage" can be a significant issue.(Soakage: discharge a capacitor with a resistor until the voltage is very low, then remove the resistor. Come back later and find there is asignificantvoltage across the previously discharged capacitor. Ask anybody that has dealt with CRT HT PSUs, or high powered transmitters).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2014, 11:35:10 pm
content: Solutions..?It's worth noting that Tesla coils are AC only, so there's no opportunity for leakage to cause imbalance in the strings. It's okay to run film caps that way. At DC, even film caps will drift, and probably worse than electrolytics (the ultimate balancing force being physical breakdown -- which isn't fatal for self-healing types, but it only gets worse over time).You could run electrolytics in series that way too, except they're never good at AC, so it's a moot point.Tim

author: G4ZWI
author_rank: Newbie
timestamp: September 11, 2014, 06:19:38 am
content: ......And, just bear in mind, (If i ain't teaching Granny to suck eggs!! ) if you are 'stacking' electrolytics for a high voltage supply, (RF linear, or such), then the cap cans will be floating above ground, so must be WELL insulated!!! Also, ensure the bleeder resistors have a good wattage safety margin,..... the ones in my 4CX250B push-pull 144Mhz linear started to drift up after a couple of years, or so. If the resistance alters, then you risk the problems mentioned above.Cheers, Fred G4ZWI

author: tggzzz
author_rank: Super Contributor
timestamp: September 11, 2014, 08:10:57 am
content: Quote from: G4ZWI on September 11, 2014, 06:19:38 amAs I tell my daughter, "95% of the time you will be teaching your father to suck eggs", and "listen to whatisn'tbeing said".

author: anotherlin
author_rank: Frequent Contributor
timestamp: September 11, 2014, 01:44:12 pm
content: Quote from: Poe on September 09, 2014, 02:19:34 pmThis is very common in tube guitar amps, which require high voltage.Parallel the two series caps with two same values resistors for balancing purposes, so each cap has 100V across it.