Question about capacitance multiplier output voltage

[laejf]

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.

[Zero999]

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.

[laejf]

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.

He 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.

[Zero999]

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.

He 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.

I 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.

[exe]

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?

[laejf]

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?

When 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?

[mikerj]

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?

Were 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.

[exe]

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?

Not sure what "the source is not grounded" means, but the rest seems correct: the output voltage is always lower than the input.

[laejf]

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?

Were 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.

I 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.

[exe]

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.

[laejf]

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.

I'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.

[exe]

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.

I'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.

By 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.

[laejf]

I finally found the problem which was a poorly crimped wire Thanks for your support though!

[exe]

Huh, me too have problems with wiring all the time. Even if I solder them.