# The collapse of a magnetic field

#### thew

Mar 18, 2016
11
Hi Guys, whats up?, I m trying to create a circuit very difficult about the possibilities of charge a capacitor using the collapse of a magnetic field of a inductor where the inductor is previously charged obviously from a external source of power such as a battery as is showed in the circuit I attached, the real difficulty here is the switch 1 must be turned on when the switch 2 is off, and the switch 2 must be turned on instantly when the switch 1 is off, the Simultaneity must be perfect, it must be instantly at the same time in order to take advantage entirely the collapse of the magnetic field to charge the capacitor, I drew in the circuit switches to understand what I want, but I think the best way will be to use other electronic component, but I don't know what electronics components are able to do something like that, I hope somebody can help me

#### Attachments

• Untitled-1.jpg
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#### Bluejets

Oct 5, 2014
6,920
What happens if you bridge out switch 2?

#### thew

Mar 18, 2016
11
What happens if you bridge out switch 2?
If there is current directly from the battery to the capacitor it means that I m not charging the capacitor using just the collapse of the magnetic field of the inductor

Last edited:

#### duke37

Jan 9, 2011
5,364
This is the circuit of a boost convertor for increasing the input voltage to what is required. They are used in many situations and there is much information on the net.

Normally a diode is placed across switch2 so that the current can pass instantly. If the voltage drop of the diode is too large, then the switch can be closed the odd microsecond later (use a fet) to pass most of the energy.

Moderator
Nov 17, 2011
13,719

#### thew

Mar 18, 2016
11
I was watching the circuit of a boost convertor over internet but is not exactly the same, because, in the boost convertor circuits the capacitor is charged directly from the power source, and I want the capacitor CHARGED ONLY from the inductor. Also I was looking the possibility to use transistors but I m not sure if it could be the solution.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
The way you have your circuit drawn the capacitor will charge from the battery and when the switch is opened the voltage spike will destroy the diode and discharge the capacitor (and maybe ring for a while).

Keep switch 2 closed and change the polarity of your battery and it will work the way you want it to.

#### thew

Mar 18, 2016
11
The way you have your circuit drawn the capacitor will charge from the battery and when the switch is opened the voltage spike will destroy the diode and discharge the capacitor (and maybe ring for a while).

Keep switch 2 closed and change the polarity of your battery and it will work the way you want it to.
I was researching and I thing what I want is a
Buck-Boost circuit

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#### thew

Mar 18, 2016
11
But then, a inductor it is different from a capacitor, right? when you charge a capacitor and after you discharge it is as a spring, its reverses the current, but a inductor is not as a spring, when a inductor is being charged the magnetic field is trying to oppose the current and for that reason when the magnetic field collapses goes in the same direction?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
No, the current flows the same way through the inductor, it doesn't reverse.

Edit: or maybe yes. Whatever... the current doesn't reverse.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
I was researching and I thing what I want is a
Buck-Boost circuit

Yep, that looks to be what I was suggesting ;-)

#### BobK

Jan 5, 2010
7,682
Why are you rejecting the boost mode converter? The fact that current comes from the battery and the inductor just increases the voltage available to charge the capacitor. And it does not have the possibly annoying property of inverting the voltage.

Bob

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,719
I was researching and I thing what I want is a
Buck-Boost circuit
It would be so much easier to help you if you stated what you want to achieve, not how. We can then direct you to possible methods. It is quite typical that several solutions exist for one and the same task. Which solution to chose depends on several factors, among them:
• price
• size
• power
• experience
• ...

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,880
Simultaneity must be perfect, it must be instantly at the same time
There is no such thing as simultaneity. Read about it in the Special Theory of Relativity by Albert Einstein. There is no such thing as instantly either. For a beginner's introduction, written pretty much in layman's terms, read this paper.

OTOH, if all you want to do is create a magnetic field, and then collapse it to recover part of the energy stored in the field, all you need is a fast switch and a diode, as @Harald Kapp directed you to in his post #5. Here again is his link, just in case you missed it.

It appears from your posts that this is a learning experience for you, rather than having a specific application in mind. There are several things you need to know about practical (real) switches. First, two switches will never change states simultaneously. You can get close, within a few femtoseconds with the best technology, but never simultaneously, for reasons I won't go into. Second, the same goes for instantly. Any switch requires a finite period of time to change from a closed circuit to an open circuit and vice versa. The transition never occurs instantly, although with the best technology it can occur very rapidly.

May 25, 2011
25
The original requirement that the capacitor is ONLY charged by the discharge of the inductor, is odd or synthetic, and sounds like more of an assignment or thinking challenge. The "simultaneous" would only apply to the 2 switch version - so the original post seems to mix both Initial Requirements - and some of THEW's expected solution...

Here is a great reference for the field...

#### AnalogKid

Jun 10, 2015
2,892
I have versions of that chart dating back to the 80s. I think the first was from Harris. Thanks for a PDF version.

ak

Mar 2, 2012
34
I always wondered if you could use something like " http://www.linear.com/product/LT1158 " - where the load=inductor, source of lower mosfet=cap. Q: Is gate drive to lower mosfet high enough to follow source as cap charges up? Also, STMicroelectronics makes a slew of nice mosfet drivers - look at some of those?

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