Dampening resistor q's

[Bill]

In my previous experiments with linear motors, where we need to collapse the field in a coil as quickly as possible(sounds familiar ) I used a TVS diode rated just below the voltage rating of the switch, ie 870V for a 900V rated switch. Wouldnt a TVS diode prevent ringing better than a resistor? Or not... If not, then for choosing a specific dampening resistor for a given coil what is a good method for doing so. Scoping the waveform and using a 1K 2 watt pot, adjusting until the best results?

[Bill]

forgot to add

The TVS would allow a much higher flyback Voltage, which is good for collapsing the current, no?

[PhilippeM]

HI,
You need to use really a resistor. As you discribeTVS diode it seems they are use protect the switch.
You need a scope to ajust the resistor perflectly.
If resistor is to large then the decay time increase and if resistor is too small there is some oscillation that need more time to stabilize.

[Unregistered]

(TVS) transient voltage protector or a Zener. These can be used to clamp HV spikes, but in this application it allows the voltage to peak at a high value before clamping which allows the current to fall quickly. Remember the hv is a result of the current in the inductor falling. As current falls voltage increases. These would not allow the inductor to ring. They are just more expensive and I should think this is why you dont see them in commercial units. Carl-is this right? What are youre thoughts.

[PhilippeM]

Pulse induction work like a emeter and a recepteur:
it generate a variation of magnetic' flux and after measure the magnetic flux generate by the "foulcault " current inside the target.
As quicker is the decay of magnetic generate flux , higher is the current inside the target so higher will be the signal generate by the target.
As a clamp voltage system limite the voltage so the current not decay too quickly and so the flux.
It' s a compromise betwen time to decay energie in coil and time to decay current.
Sorry for my bad english!!!

[Bill]

it generate a variation of magnetic' flux and after measure the magnetic flux generate by the "foulcault " current inside the target.
As quicker is the decay of magnetic generate flux , higher is the current inside the target so higher will be the signal generate by the target.
As a clamp voltage system limite the voltage so the current not decay too quickly and so the flux.
It' s a compromise betwen time to decay energie in coil and time to decay current.

For detecting gold, I want the field in the Tx to decay as quickly as possible. This way I can sample with the Rx (DD config) sooner. The eddy currents in gold do not last long and therefore the sooner I sample the better. I see no reason why current decaying in the Tx coil at the fastest rate possible would have any negative effects on the eddy current generation of target metals. Remember, there is a "on time" in which the Tx is energized, this "on time" is what creates the magnetic field a the resulting eddy currents in underlying targets.
William

[PhilippeM]

Hi William,
My english is really bad : you are right, i try to say that we need a system that have the better performance in the decay flux .The eddy current is generate only when the flux is cut down and not when the flux is on: eddy current depend of the variation of the flux and the flux decrease ( power off) more quickly then it increase ( power on).
As <<Unregistered explain us :As current falls voltage increases>> and what i try to explain too is if we cut the flyback voltage at a specidfic value than the current stop to decay during the cutting .

I hope that it's better now

[Bill]

I think you might have misunderstood me. What I am saying is, when you use a switch say the (IRF740) its only rated to 400V and if I remember has a protection diode from S to D. It clamps the voltage of the coil all by itself. What I am saying is use a switch that is rated for a higher voltage than the coil can generate (function of inductance and capacitance) and use a diode across the coil +12V to the Drain of the fet to stop ringing. Kind of like what they do for relays. When I built my accelerators, I had peak currents in the coil of around 1KA. In order to clamp the coil from ringing and get the fastest possible field decay, I used a pulse rated power diode across the coil. kathode to Vcc and anode to the Drain of a very large IGBT brick. I am just thinking out loud. I actually think a higher voltage rated fet(higher than the coil can generate), without internal clamping with low Coss, Crss would be better, and use the status quo resistor along with it.

[Unregistered]

Eric Foster

The IRF740 has a drain/source breakdown voltage of 400V. If the coil generates more voltage, the junction avalanches, and the voltage clips at 400V. Hexfets are designed to work under repetitive avalanche conditions, provided the energy is within certain limits. A higher voltage Mosfet will obviously allow the voltage to rise higher before clipping, and reduce the width of the back emf spike. At low voltage clipping or high voltage clipping, the same amount of energy has to be dissipated, hence low voltage is spread over a longer time than high voltage. A reverse diode across the coil, as in the case of simple relay protection, gives a very long switch off. The diode is fully conducting above, say. 0.7V, so the coil is virtually shorted, except for its associated resistance, which is usually only a few ohms at most. The current decay time constant, L/R would be 150uS down to 0.7V for a 300uH coil of 2 ohms resistance. With a high voltage Mosfet where BVDss is higher than the coil voltage, the current decay will be dependant on the coil resonant frequency and the damping resistor. Under these conditions the current decay can be made very fast. A low Coss seems to have a beneficial effect, although it is hard to see exactly what is happening as it is very voltage dependant and changes dramatically with Vds.

Eric.

[Bill]

I did a little research last night on the diode across the coil and I found the same thing, it lengthens the time for the field to collaspe. So thats no good at all. Bear with me I am a cogger (M.E.) trying to learn as much as I can on electronics theory. So what you are saying Eric is, that if I do use a higher voltage fet, higher than the flyback voltage the inductor can generate, and also use the dampining resistor then I would be able to produce as about as fast "turn off" as I can expect too. Providing I use a Fet with as low Capacitance as I can get my hands on. (I have sampled a few canadates from Fairchild and will be testing them shortly) Ok so the resistor is a must, I see that now. Whats the best way to calculate the reisistance needed across the coil? I was thinking about a 1K pot and a scope to find the reisistance that gives the best results. Does this sound like a good method? Thanks again guys for taking the time to answer my stupid questions.

William

[bbsailor]

Damping Resistors

I'm sure Eric will give you a very detailed answer, but here is what I have found with my PI coil experiments.

The calculated value of the critical damping resistor can be found by the following formula R= 2 X the Square Root of L/C (draw it out and put the L/C under the sq root sign).

The self resonance of the coil, shield and cable will yield a result that is very telling about the potential speed quality of the coil. Use a square wave and induce a pulse into your PI coil (with another coil) and observe the undampened pulses on your scope and then measure the resonance of the waveform. Try a few KHz PPS to start. If the frequency is too low there is not enough energy induced in the PI coil to show a good display on the scope.

This method eliminates the scope probe loading/capacitance on the coil which lowers the true self resonance by about 25 to 50 Khz. Fast coils will self-resonant about 550KHz to 600KHz. Keeping coax cable capacitance low and shield capacitance low are the prime contributors to good coil design and this will show up in the final self-resonance measurement. Using thicker insulated wire to wind the coil will add a little more improvement.

Once you know the L of the coil you can calculate the C by using the coil's self resonance in a formula. Then you can plug the numbers into the critical damping formula. However, you have not accounted for the capacitance of the MOSFET, parasitic effects, and the other components. That is why it is most useful to use a 500 to 720 ohm resistor in series with a 1 K pot placed across the coil and adjust to minimize the ringing. Take this measurement after the the first preamp where you can also see how long the preamp takes to settle down. The actual value of the damping resistor set this way will be several hunderd ohms lower than the ideal value calulated from the formula and in some of my early coils is was half. Once you know the value of the pot, then just put in a fixed resistor equal to the combined value of the pot and the series resistor.

I also tried to cheat by suspending the coil in my workshop and waving a gold ring under the coil while adjusting the damping resistor. I could hear the sensitivity change while the ring was at about 15" from the coil. Changing the damping resistance a few 10s of ohms in either direction can actually be heard in the headphones while making the fine pot adjustment. Some of my recent 300uH coils are damped at about 870 ohms.

I've tried lots of different wire and one of my best coils was made with a tip from Eric. He suggested that AWG30 Kynar wirewrap is a good wire because it has a thicker insulation than plain magnet wire. I used a 50 ft roll to make a 10.5" dia, coil to fit inside a Hays Electronics coil housing. The 50 ft roll, from Radio Shack, just makes 18 turns that is a little under 300 uH. Then I tied the wire coil with dental floss and put spiral wrap around the coil to act as a spacer for the shield. The I used what Reg suggested, 3M 1190 copper fabric over the spiralwrap to make the shield. One interesting about the 3M 1190 is that my PI detctor does not detect it, even at the fastest speeds.

Some of my crude experiments suggest the for each 100 pf in capacitance you can shave off the coil and driver circuit, you gain a potential 1 uS in coil speed. Using some new low capacitance MOSFETS can shave about 300 pf or more off compared to the other versions. As you start to approach the optimum coil designs, getting coils to operate faster by reducing capacitance, becomes harder and harder.

I hope this helps.

bbsailor