Is there a way that quite slowly discharge a capacitor. So i want to get a 40V capacitor, charge it the hook it to a button and short it (Not a good idea.)
So as you press the button it will short the cap and the wire will heat up. This is possible with a 9V battery but a cap can hold more voltage. Basically all I need is a simple diagram that shows a good way to slowly discharge a capacitor.

Thanks

 

What you need is a resistor in series with the button to limit the current.

 

What you need is a resistor in series with the button to limit the current.

A resistor is the standard solution - but a NTC thermistor will discharge the capacitor quicker while still protecting the pushbutton.

 

What's the capacitance of the capacitor?

 

The capacitor: Either a 40v 330uF or a 40v 15000uF capacitor.

 

Put a resistor in series with your button (switch) and connect that across the two terminals of the cap.

The 15000uF cap will discharge to 63% of initial voltage in about 15 seconds.

The 330uF cap will discharge 63% of initial voltage in 33 seconds with a 100k resistor in series with your button (Switch)


Cap+ ---------- switch ------- 100k resistor ------- Cap - post

 

Basically all I need is a simple diagram that shows a good way to slowly discharge a capacitor.

How slowly?

ak

 

Well as I said later in the post I am going to short it to heat up a wire. So I don't know how slowly but i would hope not to fast. But couldn't I use a Multi-Meter to see how fast it discharges?

 

Thanks guys!

 

Well as I said later in the post I am going to short it to heat up a wire. So I don't know how slowly but i would hope not to fast. But couldn't I use a Multi-Meter to see how fast it discharges?

In that case you need enough to heat the wire. How much current do need through the wire?

 

Well as I said later in the post I am going to short it to heat up a wire. So I don't know how slowly but i would hope not to fast. But couldn't I use a Multi-Meter to see how fast it discharges?

what size wire,
how long is the wire,
is the wire straight or coiled,
how tightly coiled,
what is the target temperature,

The milliamp level of discharge is not going to heat any wire.

The energy in Joules = 1/2CV(2)

In your case, the work (joules) = 0.5 x 0.015F x 1600 for a 40-volt system

That is 12 joules
The discharge time will be about 3x what I quoted above to get 95% of the 12 joules (about 11.4 joules).

Spreading 12 joules out over 33 seconds gives you a heater of .36 watts (average). Since the capacitor is discharging, the power will initially be higher and drop off over time to about 0.25 watts.

Also, the capacitor will absorb some of that power (equivalent series resistance - ESR) and the resistor will take some of that power (unless you consider that to be the "wire").

 

What size: You mean thickness?
Wire length: 4cm
Straight or coiled: Depends on what will be better.
How tight: Depends on the one above.
Target temperature: Hot enough to light paper/dry leaves when the wire is against it.

 

I personally wouldn't know how much current to put through but it should be enough to make a very very think piece of copper wire glow a yellow colour when the button is pressed. It works with a 9v battery (The PP3's) so it should work with one of those two caps.

 

I personally wouldn't know how much current to put through but it should be enough to make a very very think piece of copper wire glow a yellow colour when the button is pressed. It works with a 9v battery (The PP3's) so it should work with one of those two caps.

a capacitor will have much, much, much less energy than the 9V battery. The capacitor will discharge before you have any heat - especially if you have a thick wire to heat up. It will not get yellow-hot.

 

Hate to disappoint you. A capacitor will not store enough energy to do what you are planning.

 

Well as I said later in the post I am going to short it to heat up a wire. So I don't know how slowly but i would hope not to fast. But couldn't I use a Multi-Meter to see how fast it discharges?

You will get far more energy out of a battery than a capacitor of equal physical size.. Discharging a capacitor "slower" means less energy to heat up a wire.
Not doing the math, you would need capacitors in the hundred to thousands of Farads (not micro-Farads) to compete with lithium batteries or alkaline D cells.
The charge of a battery would typically be measured in Amp-Hours where even a 1 Farad capacitor might be measured in Amp-Minutes.

 

Hate to disappoint you. A capacitor will not store enough energy to do what you are planning.

Well, the one's you have might not........................

 

Well, the one's you have might not........................

What type of capacitor do you have? 100,000 uF has a capacity of Amp-Seconds. Caps measured in Farads amounts to Amp-minutes. Batteries are Amp-hours.
You can extend discharge time by increasing resistance which means much lower heat.
Which laws of physics do you think you can change?
You have already tried it and it doesn't meet your expectations, does it? What part do you not believe?

 

What type of capacitor do you have? 100,000 uF has a capacity of Amp-Seconds. Caps measured in Farads amounts to Amp-minutes. Batteries are Amp-hours.
You can extend discharge time by increasing resistance which means much lower heat.
Which laws of physics do you think you can change?

The gap between a thunder cloud and earth is basically a capacitor.

They have to make the lightning conductor straps on the sides of buildings fairly sturdy, otherwise they'd scatter in millions of molten metal globules when they get struck - sometimes they do melt.

There were plans to use supercapacitors in cars for energy recovery during braking, they could heat a wire for a short time - probably longer if you control the discharge rate with a hefty buck regulator.

Did you think there was some "set in stone" maximum size a capacitor could be?

 

use a high watt resistor