Voltage doubler driving capacitor bank question - Page 1
Forum section: electronics / projects

author: CaptDon
author_rank: Super Contributor
timestamp: December 06, 2022, 02:43:00 pm
content: I have a voltage doubler consisting of a pair of 2200uf capacitors and I want to use the output to keep charge on a 10,000uf capacitor. I will be drawing a constant .25 amps out of the 10,000uf capacitor. As a reference let's assume the transformer is 9 v.a.c. and I need roughly 20 v.d.c. output. I can easily tolerate 1/2 volt of ripple on the 20 v.d.c. My question is when I draw the .25 amps will this thing act like a capacitive voltage divider and my output voltage sag like crazy? The operating frequency is 60 Hz and the circuit would be the classic half-wave voltage doubler. Can this be relatively accurately simulated in Spice? (I don't have access to Spice at home but probably can find a work station at my job that is 'Spice' equipped.Thanks!!

author: mawyatt
author_rank: Super Contributor
timestamp: December 06, 2022, 03:01:20 pm
content: You'll lose about 1/2V due to output Capacitor droop, then another few volts with the Doubler Capacitors and Transformer, plus another couple volts to the Diode drops. So likely you won't be able to create 20VDC with 1/2 ripple as intended.Spice should be able to simulate this very well, just get good model parameters for your components.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2022, 05:05:04 pm
content: Why does the output cap have to be so big..?Voltage doublers are more sensitive to source impedance and capacitor reactance. Whereas a full-wave rectifier is something like 2x sensitivity (a 10% regulation source gives more like 20% DC regulation), a doubler, well, about doubles that (so, a 10% regulation source might give 40% DC regulation). Add capacitor reactance/impedance to that as well, especially as you go up the chain in higher order multipliers (which are sometimes built with tapered stacks of capacitors for this very reason).My feeling is 0.25A won't be nearly enough to be a problem. Want to say I did a similar arrangement years ago, but with more like 5 or 10mF each, and got plenty of amperes out of it (10?) with surprisingly good regulation.Let me see here... my handy reference is:https://archive.org/details/RadioAmateurHandbook1976/page/n118/mode/1upScroll down to page 119.Note that "R" includes the transformer's impedance, just ballpark sum the line and leakage inductances (reactance at mains or low harmonics) if significant, with the resistances (secondary DCR, primary reflected DCR, source). As you can see, regulation is good above some point, and sloping off quickly below; from the curve you can calculate the effective DC output resistance, and take the ratio to the source impedance to get the sensitivity factor I started with.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: December 06, 2022, 05:26:47 pm
content: The voltage double does not act like a capacitove divider. So the capacitor ratio is not relevant. The pumping capacitor acts like an AC resistor. So the output voltage will sag soemwhat under load. The other series resistance (e.g. the transformer) can also reduce the putput voltage. With no load 9 V AC would have some 12.6 V peak voltage to start with and a small unloaded transformer will have an extra voltage overhead (e.g. 10-30 %). At low current the diode drop is also lower.250 mA for some 8 ms from a 2.2 mF capacitor gives about 1 V drop from the capacitor. The transformer may add a similar drop.One can get at least 2 Spice versions and front ends for free: LTspice and Tina. The simulation does work well, it only depends on how realistic the models are.

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2022, 06:36:48 pm
content: Your circuit is one of the variations that can be computed in PSUD2 freeware from Duncan Labs.Start with a voltage doubler and your capacitors, then add an R-C filter with your large capacitor and some appropriate resistance.https://www.duncanamps.com/psud2/(Note that you need to model the transformer in terms of no-load RMS voltage and effective secondary resistance, not the catalog values of voltage at maximum load current.)

author: magic
author_rank: Super Contributor
timestamp: December 06, 2022, 06:54:05 pm
content: Quote from: CaptDon on December 06, 2022, 02:43:00 pmWith this kind of requirements, just use two half-wave rectifiers to get unregulated ±12V rails.Converting this into regulated 20V is left as an exercise for the readerI recall making an attempt recently at analyzing DC output impedance of a voltage doubler. It's postedhere.Maybe you will make some sense of it.

author: CaptDon
author_rank: Super Contributor
timestamp: December 07, 2022, 06:36:28 pm
content: Thanks all!! I will try to download LTSpice/TINA and install it on my engineering laptop and then do the sims. I forgot about Duncan, I believe I have used that site for some calculations for a 2kvdc supply years ago.