Thread: Circuit resonance

There are formulae available to calculate the inductance of a coil.

Answers:-
1. Yes, but D3 will severly dampen the reasonance.

2. No, the magnet will just add a "DC" bias to the field. However, the core may saturate on one half cycle.

3. I don't know. It depends upon whther neodymium is conductive. If so there will be eddy current losses which will dampen the oscillation.

4. Yes, but you need to ensure that the amplitude of the oscillation is within the limits of the FET

5. When the FET turns on, the voltage across the coil will start at 12 Volt and then decay expontentially to a value determined by the resistor and the coil resistance (provided that the pulse is long enough)

When the FET is turned off, D3 will clamp the back EMF at about 0.6 Volt; this will decay to zero if the off pulse is long enough.

I suggest you use circuit simulation software to simulate the circuit so you can see the waveforms.

John,
You're welcome.

It is a bit complicated, so it is difficult to fully express it in a verbal description. Hence a software simulation is better.

There are free simulation programmes available. The one I have is called
Switcher CAD III. I assume it is still available and still free.

Good morning John,
There are experts in this forum who can help you with Switcher CAD.

good luck,

What's the purpose of the 820R and 220R potential divider on the MOSET's gate?

Why does the 555 timer circuit have 100R in series with it?

You can look up LRC circuits on Wikipedia if you want to calculate everything but simulation software is probably the easiest option.

What's the purpose of your circuit?

You don't have to add a time stap to your posts, the forum software does it for you.

Hi again Hero999 and friends,

I have downloaded a time trial version of Multisim simulation software as advised but it blowing me over a bit. I can't seem to progress because the Footprints of certain devices are unfamiliar to me. To save you downloading I put a link to my Homepage showing part of my basic circuit counter schematic using a 741 and a 4017 decade counter, if anyone can help me I'd be obliged.

http://homepage.ntlworld.com/john.tilston/4017.html

Cheers for now

John

PS. In respect to the above request, and for other new users of Multisim, the program automatically allocates VCC and GROUND to component footprints to keep wiring to a minimum.
My decade counter is now working beautifully from the pulses of the LM741 opamp. You learn a little each day. Great !!!

Originally Posted by johntee

Hi everyone,

Sorry for the delay in answering, grandkids are very demanding !!

The circuit is actually off the internet with the INDUCTOR (L) coil added to the output stage. This is the actual text from the author:-

[I]"The output voltage from pin 3 of the second NE555 chip is reduced by the 220 ohm / 820 ohm resistor combination. The transistor acts as a current amplifier (actually it is a switch, not an amp), capable of providing several amps to the output electrodes.
The 1N4007 diode is included to protect the MOSFET should it be decided at a later date to introduce either a coil (“inductor”) or a transformer in the output coming from the MOSFET, as sudden switching off of a current through either of these could briefly pull the ‘drain’ connection a long way below the 0 Volt line (wrong, it will be a very high positive spike - this would damage the FET) and damage the MOSFET, but the 1N4007 diode switches on and prevents this from happening by clamping the drain voltage to -0.7 volts (wrong, it will clamp it to about +12.7 Volt) if the drain is driven to a negative voltage (misleading, the drain voltage is being switched between about +7.9 Volt and 0 Volt, it does not go negative)."[/I]

I kind of understand this but I'm a real novice where electronics are concerned, hence the request for forum members help.

From my personal perspective, I was hoping to drive the Inductor coil (L) with pulses of varied frequencies via the 555 and the MOSFET and add these to the static magnet field of the (L) coils neodymium magnet core surface. This neodymium magnet core actually has a static gauss strength of 10000 and I was hoping to add the circuits output to this and create a pulsed radiation field. I want to then apply those varying increased pulsing fields to biological matter and view any changes which might take place in such.


John

John,
I've added some comments above in red.

Do you have any info on the neodymium? Is it conductive?

As I said in previous post, if it is conductive it will limit the upper frequency due to eddy currents.

Also, if you want is to resonate, you will have to remove the diode. Note that the capacitor will prevent the hugh spike that would occur if it and the diode were not there.

You could connect the diode between the drain a source (cathode to the drain) to protect the FET from negative peaks.

If you fire up your circuit simulator, you should be able to see what is happening. Try it with and without the cap in parallel with the coil and also, with the cap removed, with and without the diode. Then connect both cap & diode and see what happens.

Also download (if you have not done so already) the data sheet of the MOSFET so you know its maximum ratings.

Originally Posted by johntee

Hi Len,

Thanks for your circuit comments I'll make note of these for future reference. I don't know how to tell if the magnetic core in conductive unless I can use a meter. It just looks like a dark brown material but is extremely powerful.

The diode across the Drain to Source is a most welcomed piece of advice, again it's gone into the old gray matter for future reference.

Many thanks again,

John

John,
You're welcome. I suggest you try to find info on the neodymium magnet, eg. do an internet search.

Measuring it with a meter may be inconclusive as the surface may be non conducting, ie. it may have an oxide or other type of film.

Another way to do it would be to use a ferrite core and a DC current superimposed on the AC current to provide the "DC field" with the AC imposed on it, ie. don't use the magnet. This has the possible disadvantage that you may saturate the core. So you need to know the magnetic properties of the material.

However, as I said in a previous post, the saturation issue also applies to the neodymium magnet, ie. it may be saturated by the AC field when the AC field is in the same direction as the "DC field".