# Changing rise time of circuit?

Discussion in 'General Electronics Chat' started by wes, Aug 20, 2010.

1. ### wes Thread Starter Active Member

Aug 24, 2007
242
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Hello I have been thinking and is it possible to create a variable rise time and fall time for a Coil. I know that by increasing the frequency, the risetime and fall time can be increased, also depending on type of wave. But I want to know is it possible to create a square wave which has a short rise time of say 1 ns and a pulse width of maybe 5 ns and a fall time of about 1 ns and it would only creat this signal about 10 times a second.

So is it possible? If so, does anyone know of maybe a pulse generator or signal generator capable of this.

2. ### JDT Well-Known Member

Feb 12, 2009
658
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By "coil" you mean a "transformer". Even a single coil, an inductor, is in a way a transformer in which the primary and secondary are the same winding. It uses "self induction". The induced voltage across the terminals of the coil depends on a CHANGING current in the coil (an ideal coil has no resistance so that is the only way you can have voltage across the coil). The magnitude of the induced voltage depends on the rate of change (how fast it is changing) of current. To get a an induced voltage with a fast rise-time you need a current that goes from not changing (steady) to a high rate-of-change - very quickly. So there is a second thing that has to change very fast. If you see what I mean. I'm trying to stay away from mathematics here (I know most people switch off when they see a formula - including me!).

So, yes it is possible - but only if you feed in the right input.

A classic example is the ignition coil in a car where the primary current goes from constant (with 12V across the coil) to nothing (when the contact points open). This sudden high rate of change causes the high voltage for the spark. But in fact, if we used this system as above the change in rate-of-change would be so high that the energy would flash across the contact points before it got to the spark plug.

So a capacitor is added (the condenser) that slows down the change in rate-of-change to control the rise-time of the high voltage. Basically to give the contact points time to open. See?

3. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
ok so since the current would be changing so fast, this would cause a huge back-emf that would oppose the change and cause the current to take time to build to steady state.

you said
"To get a an induced voltage with a fast rise-time you need a current that goes from not changing (steady) to a high rate-of-change - very quickly. So there is a second thing that has to change very fast. If you see what I mean"

I am not sure what you mean. doesn't the induced voltage oppose any change? or is it that you want it to a have a fast rise time as well so it will dissapate faster and so coil current get's to steady state much quicker?

I just want to know is it possible to get a coil (electromagnet) with a rise time for the current on the order of 1 ns. also what math would I need to calculate if it is possible.

4. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
also i am talking current of like 1 amp or more. I have heard of pulsed emps with coil current of like 1k amps or something like that with rise time's in the nanoseconds, ( I think, lol)

5. ### JDT Well-Known Member

Feb 12, 2009
658
87
Correct!

So you need a high driving voltage.

One way to generate a high current pulse with a fast rise-time is to charge a capacitor to a high voltage and then discharge it through your solenoid. The solenoid and the wiring (and the capacitor) need to have a very low self-inductance to get a fast rise-time. Nanoseconds though, probably difficult.

For an example of someone who has taken this to the limit see:-

6. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Ok yeah I figured that much from what I read. I found a formula for finding the rise time on a coil.

it's rise time in u/s = (uh / Voltage) * peak current and it give the time at which the current reaches 1 amp

Is this formula correct?

and according to that formula, I would need 1 million volts to get a 1 mh coil to 1 amp in 1 n/s.

first is it even possible to put a million volt's through a wire.
I know a taser can put out a million volts and that voltage runs through the a wire to the electrodes. also where could I find more info about this.

2. say the coil used 14 awg wire which is capable of more then 1 amp of current. would it be possible to run a million volts and 1 amp or more through it but only for like 1 n/s and then you shut it off. So basically a pulsed coil

also is there a good book that has all the different formula's for calculating thing's like rise time and magnetic field strength etc....

thanks for the help,
Wes

7. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
also would using a superconducting coil allow the current to reach the same level as a normal coil in a shorter amount of time? it's seem's it would but not sure

8. ### tom66 Senior Member

May 9, 2009
2,613
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A million volts...

More than fatal.

A taser does not put out one million volts. It puts out around 50kV open circuit and 5kV when an unfortunate person becomes the conductor. The only place you'll get close to 1MV is utility power lines. Some of those can run up to 750kV. But don't go messing with them or you'll end up as the "conductor".

One important point is isolation. The arcing voltage for air is somewhere from 10kV/cm to 30kV/cm. Assuming best case, 30kV/cm, you would need at least 35cm between any two conductors.

9. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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Ahhh phooey.

I had a million volts running "through" me at age 13! And i'm FINE! (well...)

The Maryland Science Center used to have a vanDeGraff generator that produced 1,000,000v. They would let kids come up, and me being a longer haired youth, was snatched up so the audience could see my hair stand on end.

Thing was, you had to touch the thing from before it powered up, until after the final discharge to GND at the end of the demonstration.

I just now realized that the first 5 letters of demonstration spells demon. eriee...

So, it is quite possible that the man giving the demonstration was an evil sob who enjoyed shocking 12 year olds with a million volts.

hmmm.

10. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Ok so maybe taser's dont produce 1 million volt's. 1, why does my stun gun say 2 million volt's on it, then?

2. then is thier any way to put 1 million volt's through a wire, say I use some ultra insulator and have the coil's spaced 1 inch apart so it does not cause a breakdown of the insulator. is there any insulator capaple of this?

Dec 5, 2009
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12. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
but wouldn't I need some special insulation to avoid a short circuit between the coil's of a inductor