Delta Pulse

[Geo]

Hi spyros. What about the coil. I think it is 1.25x1.25 !!!!

Regards

[rospy]

Can d.pulse work like this machine:

http://www.youtube.com/watch?v=yJ_E8qsmUlg&NR=1

Regards too all!

in this test DP detect at 2,20-2,40 a small tank.This man is 1,90 tol.

http://www.youtube.com/watch?v=PCmk2UmPIjk

Regards

[Belief]

delta

hi all
my name is iman. i iranian.


ibuilt delta pulse ,but it is so so. i mean it detect a metal at first time but when i movethe coil back over that metal its not work ( detect )

un stable.

please help me.
How much in depth can detect with search coil 1*1 ???

my email : i.belief @yahoo.com

Best wishes

Iman boroumand.

[miki73]

Can d.pulse work like this machine:

http://www.youtube.com/watch?v=yJ_E8qsmUlg&NR=1

Regards too all!

[nick_f]

Hi Nick,

Thats the way a test should be presented(Post #799), Thanks for sharing.

The diode is for isolating the Fet's capacitance from the coil therefore increasing the resonate frequency of the coil = faster

Regards Mark

Hi Mark,

You're welcome and you're right about the diode role in circuit.
In the previous messages the diode was considered unnecessary and I wanted to get to a consensus, wether the diode can improve the results.

Regards,
Nicolae

[nick_f]

Input resistor and limiting diodes is part of damping circuits, it is obvious.


It is not so interesting, but ...

Regards,
Nicolae

[/quote]

Hi Andy,

Sorry to hassle you, to me it is interesting, but not the simulation you've done in your post #808. I was refering to a comparative simulation with two circuits, like in my post #807 where you adjust the damping resistor (measuring only on the antiparallel diodes and placing the waveforms on the same graph).

Regards,
Nicolae

[Altra]

Hi Nick,

Thats the way a test should be presented(Post #799), Thanks for sharing.

The diode is for isolating the Fet's capacitance from the coil therefore increasing the resonate frequency of the coil = faster

Regards Mark

[Andy_F]

The 1kohm resistor and the limiting diodes influence the circuit response and to my knowledge, all PI detector are using similar circuits in the Rx stage.

Input resistor and limiting diodes is part of damping circuits, it is obvious.

If you have time, can you simulate this schematics as well?

It is not so interesting, but ...










Regards,
Nicolae[/QUOTE]

[nick_f]

Just to compy with the suggestion from Aziz, I run a simulation with a resistor in the drain (to compensate for the energy loss on the diode). The graph at the antiparallel diodes is quite different from the one above, but it indicates the same thing: the positive effect of the drain diode (green graph goes to zero before blue graph).

Regards,
Nicolae

[nick_f]

Hi,

No! Bad idea. The mosfet's parasitic capacitance's won't be decoupled from the coil properly.

Aziz

That's exactly why I suggested that (for simulation purposes). By moving the diode into the source, I will still have the voltage drop on the diode, while the energy supplied to the coil will be about the same as the one with the diode in the drain. This would be for the purpose of comparing the effect of the diode in the drain circuit.

Measuring the voltage drop is not a precise method, because the voltage drop depends on the current through the diode. And the current is changing permanently.

I agree with you about using resistor in the source of mosfet and measuring the current at switch-off. A 0.1ohm resistor in the source will cause only a small change of the circuit (I run the simulation).

The simplest method in my oppinion is moving the diode to the source and comparing the results. You can see the difference in the voltage levels at coil is very small (more noticeable is some change in the delay), and I consider the energy taken by the two circuits is close enough for our purpose.
On the other side, we can see a clear difference in the signals at the 1N4148 diodes, and it indicates the circuit without 1N4937 diode in drain needs adjustment of the damping resistor.


Regards,
Nicolae

[Aziz]

Hi,

Can I just move the diode in the source of the MOSFET?

No! Bad idea. The mosfet's parasitic capacitance's won't be decoupled from the coil properly.

Can I just decrease the supply voltage with 1V?

You have to measure the voltage drop. Then it could be one way. But the mosfet's Rdson is dependent on the gate drive voltage, in which would be affected.

Another way is adjusting the pulse with. The time base for switch-off should be same.

The easy method is adding a resistance to source of mosfet. Then look at the current through the coil at switch-off. Adjust the resistance to a value, which both systems have the same coil current at switch-off.

forgotten:
The gate drive voltage would also be affected by the source resistance (voltage drop). You could move the resistance to drain path to avoid this.. (coil - diode - resistor - mosfet-drain)

Aziz

[nick_f]

Hi Aziz,

And how exactly can we compensate for this? Can I just move the diode in the source of the MOSFET, or decrease the supply voltage with 1V?

Regards,
Nicolae

[Aziz]

Hi all,

if you want to compare two systems, then keep in mind, that the two systems have the same conditions:
- same transmit pulse energy E=0.5*L*I² (same coil current)
- critical coil damping for both systems
- and other conditions..

The drain diode has the range of appr. 1V voltage drop. The coil current therefore will be affected by the drain diode. So compensate this, that both systems expose the same transmit pulse energy.

Aziz

[nick_f]

Hi Andy,

You said the advantage is considerable when the voltage is 1mV or less. Keep in mind that usually there are high gains in the preamplifier, and that 1mV becomes 1V for a gain of 1000.

I just realised, you are using a different circuit diagram, which does not reflect the real circuit. Your measurements should be performed at the antiparallel diodes, not on the coil! The 1kohm resistor and the limiting diodes influence the circuit response and to my knowledge, all PI detector are using similar circuits in the Rx stage.
If you have time, can you simulate this schematics as well?



Regards,
Nicolae

[Andy_F]

Yes, it agree. If to choose the resistor by criterion of critical damping, the variant with the diode has some advantage. Where voltage of flayback is a millivolt and less - the advantage is considerable.