Damper Resistor Reference!

[ivconic]

Hi Ivonic,

I would look at all the power supplies to see if one or more of them are going unstable. I found this happens quite frequently if there is no regulator to regulate the main power supply voltage.

When you try to pulse with a lot of current and then increase the gain and audio levels, there can be an interaction which causes oscillation or instability.

I haven't looked at the schematic of your detector, but I would decouple the audio with a low value resistor and large capacitor, and, if there is no regulator on the main power supply, I would add one. By main power supply, I mean the battery supply.

One of the nice things about using li Ion batteries is the battery voltage of 4 batteries in series is 14.4V which regulates down to 12 volts very nicely with a simple regulator.

Always use quality capacitors, especially around the preamp. In fact, I use tantalums for the best results.

Now, one other thing, some amplifiers will change a little when overloaded or the delay is changed. In other words, all may look ok, but not really be ok. I have found this to be true when using the NE5534. Also, there are differences in NE5534's also. For some reason some brands work better than others. TI's appear to be the worse when trying to sample very early. This is a really hard problem to predict because what happens can vary a little.

Reg

I guess you are right! I totally forgot that there is not any regulator in DP!?
Thank Reg!
Problem will be in accu i am used to use with DP, it is 12v/1.3A...Either to put low drop 10v regulator either to use 2 accu.s in serie, providing 24v and than regulate it down to 15V (in this case might gain some more depth?)...
First occured to my mind is LM338,LM350 or simillar...
Or...to fix some of presets to fixed value with resistors instead pots...
I will see..
Regards!

[1843]

NEW RESULTS!!!

Hi Reg,

Today I changed the damping resistor with a 1K. Although the hump became larger, I could detect a 1m x 1m metal sheet at 1.8m by a 26cm coil!
Excellent detection!
I changed the coil with 1m x 1m. I checked the preamp output and saw that there is an overshoot at about 90us. I set the main sample delay to 100us and detected a 20cm x 20cm metal at 1.5m.

Does the overshoot cause any problem or reduce the sensitivity?
Shall I decrease the damping resistor or just leave it?


Best regards

[bbsailor]

Damping Resistor Value

1843

The value of the damping resistor (Rd) is chosen to be at critical damping or even be slightly overdamped. This damping adjustment is done by the selection of the right damping resistor value. This Rd value is more critical as you try to sample at the least amount of delay for small, low conductive targets.

The way to look at critical damping is to visalize that a damping resistor works like a shock absorber on a car that damps the motion of the wheel springs so that the spring flex returns to a netural point as quickly as possible, without bouncing too many more times.

The optimum damping resistance (Rd) value cannot be stated accurately as every PI design is slightly different and would require a different Rd. Below are many of the things that contribute to the value of Rd.

Coil to control box coaxial cable: The total cable capacitance is defined by pf per foot or meter. Longer cables have higher capacitance. Thinner cables tend to have higher capacitance. Higher cable capacitance means a lower value of Rd needs to be used.

MOSFET COSS Rating: This is the output capacitance of the MOSFET that appears in the coil circuit. Less COSS means a higher resonant frequency and a larger value of Rd can be used.

Coil Self Resonant Frequency: This is based on the following subfactors which can each be analyzed separately.
-Coil Inductance
-Coil Wire Insulation Thickness
-Coil Wire Insulation Dielectric Constant
-Any shielding used on the coil
-Shielding spacer thickness and dielectric constant

TX Pulse Width: If you set optimum damping with a short pulse width and a short sample time, then increase the TX pulse width, you will generate a higher back EMF pulse that would need to have a slightly lower Rd value to provide optimum damping. One thing you can do is to put an optimum value of Rd on the circuit board for the shortest pulse you can use. Then add a SPDT center off switch near the coil connector to add two additional damping resistor values to trim Rd for the mid TX Pulse width and for the high TX pulse width. That way you can optimize Rd for a wider range of pulse width settings. You could also use a small trim pot with some caution about wattage.

It the TX signal is not flat when you sample, you may saturate the front end amplifier and need to wait for it to come out of saturation. If you wait for the TX signal to be flat when sampling, adjusting the value of Rd will have little or no effect.

The value of Rd is best set by observing the output of the first amplifier on an oscilloscope. See my article about how to add an adjustable Rd pot to fine tune the damping. You must select values that fit the range of your particular coil and circuit design.

bbsailor

[1843]

Hi bbsailor,

I agree with all these. Have you read all the posts at this thread?
Damping resistor in my PI is 10W. Any pot will be destroyed if I use...
At the moment there is an overshoot(10us width) at 90us after the Tx off. I want to know that dosn't it cause any problem in sensitivity?
(*I set the sample after that overshoot and on a flat line 100us+)

Best regards