Question about capacitance multiplier output voltage - Page 1
Forum section: electronics / projects

author: laejf
author_rank: Regular Contributor
timestamp: May 29, 2021, 10:17:47 pm
content: I made a capacitance multiplier as the one in Dave's video below to remove some power supply ripple in a 24V AC-DC adapter for my audio amplifier. I used an IRF520n MOSFET with a 10k resistor and a 10uF capacitor on the gate. When connected to a 24V supply the ripple disappears so the circuit works, but when I plug it in to my amplifier I only get 3V out of it. How do I make sure I get the 24V I need?Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2021, 10:23:21 pm
content: If I remember rightly, Dave recommended a Darlington BJT in his video, not a MOSFET.I would expect it to drop a fair voltage, with a MOSFET, depending on the current, but not that much. The maximum gate-source voltage rating of the IRF520 is only 20V. I think it's possible it exceeded that, during the start-up transient, causing it to fail. Try the circuit again with a Darlington pair.

author: laejf
author_rank: Regular Contributor
timestamp: May 30, 2021, 10:01:45 am
content: Quote from: Zero999 on May 29, 2021, 10:23:21 pmHe mentions a number of different transistors and says that a mosfet can be used in the case of needing large current such as an audio amplifier. I'll see if I can test it with a Darlington without burning it. Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2021, 06:49:56 pm
content: Quote from: laejf on May 30, 2021, 10:01:45 amI admit, it's been awhile since I saw the video and didn't have time to watch it again. Another thing you could try is a 10V zener diode between the gate and source, to clamp any voltage spikes.

author: exe
author_rank: Unknown
timestamp: May 30, 2021, 09:15:23 pm
content: Getting 3V sounds weird to me. I'd measure voltages in the circuit. Like, gate voltage with respect to ground, voltage across mosfet, and voltage between gate and source.Also, I'm not sure ripple should completely disappear. I think it only attenuates it. I didn't do measurements on mosfet cap multipliers, but with with 2n3904 I only got some modest attenuation, like by a factor of 20-40x. Probably, darlington will work better at modest frequencies.I'd also prefer darlington because, imho, it should give lower voltage dropout. With mosfet the dropout voltage will be more than Vgs(th), which is typically 2-3V. Darlington would give 1.3-1.5V dropout (my estimation).PS do you have an oscilloscope?

author: laejf
author_rank: Regular Contributor
timestamp: May 31, 2021, 06:51:03 pm
content: Quote from: exe on May 30, 2021, 09:15:23 pmWhen I said the ripple disappeared I didn't mean completely, but it was attenuated to about +- 5mV.The thing I don't really get is how this circuit could act as a supply. The gate-source voltage will depend on what load I connect so I would never get 24V out of it if the source is not grounded. The same should apply with a Darlington, right?

author: mikerj
author_rank: Super Contributor
timestamp: May 31, 2021, 06:59:48 pm
content: Quote from: laejf on May 31, 2021, 06:51:03 pmWere you expecting this circuit to provide a regulated voltage? If so I don't know where you could have got this idea from, certainly Dave didn't claim it. It's a ripple reduction circuit, if you want regulation then you'd need to follow it up with a regulator.Getting 3v does mean something is wrong with your circuit, but even when working the output voltage will be dependant on load as you say.

author: exe
author_rank: Unknown
timestamp: May 31, 2021, 07:17:53 pm
content: Quote from: laejf on May 31, 2021, 06:51:03 pmNot sure what "the source is not grounded" means, but the rest seems correct: the output voltage is always lower than the input.

author: laejf
author_rank: Regular Contributor
timestamp: May 31, 2021, 07:42:41 pm
content: Quote from: mikerj on May 31, 2021, 06:59:48 pmI don't, I just thought it would work better than this since Dave said it would be no worries. The circuit works, I get about 21V with a 10k load resistance, but with my amplifier I get 0.57 gate-source voltage. I guess with a too high load impedance it doesn't even get a chance to get going.

author: exe
author_rank: Unknown
timestamp: May 31, 2021, 08:10:17 pm
content: I'm not an expert, but what you say doesn't sound right to me. A photo of your setup and more voltages (referenced to ground) would help.

author: laejf
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:18:09 am
content: Quote from: exe on May 31, 2021, 08:10:17 pmI'll try to get that when I got some more time. But if a very high impedance (my amplifier) is connected to the source, why would the transistor conduct at all? It's almost an open circuit.

author: exe
author_rank: Unknown
timestamp: June 03, 2021, 08:38:21 am
content: Quote from: laejf on June 03, 2021, 06:18:09 amBy conduction you mean current? Well, I guess regulation is affected by load impedance, but I don't see how output voltage should be affected by this. Anyway, I suggest try simulating circuit in a circuit simulator to see how it operate.PS have you seen this article?https://sound-au.com/project15.htm. Btw, the auther mentions that mosfet cap multipliers have poor regulation. So, darlington might be a better option.

author: laejf
author_rank: Regular Contributor
timestamp: June 03, 2021, 04:52:17 pm
content: I finally found the problem which was a poorly crimped wireThanks for your support though!

author: exe
author_rank: Unknown
timestamp: June 03, 2021, 06:34:43 pm
content: Huh, me too have problems with wiring all the time. Even if I solder them.