Author Topic: Using a diode to make a capacitor drain in 1 direction?  (Read 1273 times)

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Offline MWisBestTopic starter

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Using a diode to make a capacitor drain in 1 direction?
« on: November 06, 2017, 06:10:22 pm »
Just want to make sure this idea will actually work before I try it.



The idea here is to have a capacitor that stabilizes voltage on the output without feeding back into the input. The application is an automobile, specifically the headlights. I have an audio amplifier that draws sudden surges of current, which causes a momentary voltage drop while the alternator regulator adjusts for the load (approximately 0.2V for 50-100ms). This is slightly visible with the headlights. I'd like a capacitor to "buffer" that so it's not noticeable, but if the capacitor drains back into the rest of the car this will be fairly useless without an enormous capacitor because the headlights are just a fraction of the overall power usage. Will a diode work here?
 

Offline jm_araujo

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Re: Using a diode to make a capacitor drain in 1 direction?
« Reply #1 on: November 06, 2017, 06:16:29 pm »
In theory, yes.
In practice, the headlights are power hogs and you will need a beefy diode and a huge capacitor.
 

Offline MWisBestTopic starter

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Re: Using a diode to make a capacitor drain in 1 direction?
« Reply #2 on: November 06, 2017, 06:54:13 pm »
In theory, yes.
In practice, the headlights are power hogs and you will need a beefy diode and a huge capacitor.

Yes of course, it will take some beefy components. They use somewhere around 5A of current each. Each headlight will get its own diode+cap(s). Heatsinked TO-220 package diode and somewhere between 4,000 and 12,000 uF of caps should do the trick I think.
 

Offline Psi

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Re: Using a diode to make a capacitor drain in 1 direction?
« Reply #3 on: November 06, 2017, 06:59:54 pm »
From memory headlights are around 5A so i would spec a 20A continuous diode each.

The cap has to be quite big, maybe supercap size. Thats not too much of a an issue. You can get them easy enough. Cost a bit through.
The issue is when the system first powers up and the caps are empty it's going to draw huge current (30A+) and may burn out the diodes. So maybe add a series resistor that limits inrush current under the diodes pulse current.

Your may get a better result just putting big caps at the amp though. I dunno
« Last Edit: November 06, 2017, 07:05:13 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline MWisBestTopic starter

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Re: Using a diode to make a capacitor drain in 1 direction?
« Reply #4 on: November 06, 2017, 07:49:08 pm »
From memory headlights are around 5A so i would spec a 20A continuous diode each.

The cap has to be quite big, maybe supercap size. Thats not too much of a an issue. You can get them easy enough. Cost a bit through.
The issue is when the system first powers up and the caps are empty it's going to draw huge current (30A+) and may burn out the diodes. So maybe add a series resistor that limits inrush current under the diodes pulse current.

That's a good point about empty caps. I was already planning on having the relay after the capacitor rather than before it, so that there's no current surge every time the headlights turn on, but when the battery is first hooked up to this it could definitely be an issue. Perhaps I can add a switch for the resistor, so it can just be disconnected easily once the capacitor is charged.

Your may get a better result just putting big caps at the amp though. I dunno

That's what I'm trying to avoid. Those things scare me, anything off-the-shelf for that is very sketchy and stupid expensive.
 


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