Damper Resistor Reference!

[1843]

Hi All,

I want to know why a too low damper resistor in a PI reduces the sensitivity?
Is it for limiting the minimum sample delay? or for decreasing the coil magnetic field?

I don't know how to adjust the damper resistor to its best value.
The Sandbanks with 150us pulse-width has a 100ohms damper while the Goldscan-4 with 250us PW has a 500ohms...
What is the best way to adjusting a damper resistor?

Should be set the PI pulse-width to its maximum value when adjusting the damper resistor?

The peak current in my PI is more than 5A with 1m coil. Does the coil need a low damper?(100-220ohms?)

I think that these questions are the questions of many other guys too.
Any idea will be welcome,

Best Regards,
1843

[chemelec]

Hi All,

I want to know why a too low damper resistor in a PI reduces the sensitivity?
Is it for limiting the minimum sample delay? or for decreasing the coil magnetic field?

I don't know how to adjust the damper resistor to its best value.
The Sandbanks with 150us pulse-width has a 100ohms damper while the Goldscan-4 with 250us PW has a 500ohms...
What is the best way to adjusting a damper resistor?

Should be set the PI pulse-width to its maximum value when adjusting the damper resistor?

The peak current in my PI is more than 5A with 1m coil. Does the coil need a low damper?(100-220ohms?)

I think that these questions are the questions of many other guys too.
Any idea will be welcome,

Best Regards,
1843

Not Only does the damper resistor reduce the inductive kickback, it also reduces the transmit signal.

Each different PI detector and coil combination can have different properties. And resistor values between 100 and 1000 ohms are typical.

But its really impossible to give specifics on this.

[1843]

Not Only does the damper resistor reduce the inductive kickback, it also reduces the transmit signal.

Hi Gary,
Thanks for your reply.

Do you mean the higher the inductive kickback voltage, the more detection depth?
Do you have any idea about the way of adjusting the damper resistor?

Best Regards

[Reg]

Hi 1843,

You have to understand the purpose of the damping resistor which is to dissipate the coil energy and do so without oscillations. Before it oscillates, you will see an obvious dip in the flyback signal. So, as a simple means of trying to figure out what value of resistor you need, you can increase the value of the resistor until you see some obvious dip in the receive signal.

Now, also keep in mind this value of resistor will vary with the coil used. Make a fast small coil and you can use a higher value resistor, which will allow you to sample sooner.

If you make a larger coil (larger coils have more capacitance) or wind it or even shield it such that it has more capacitance, then you will need a lower value resistor to keep it from osicllating.

In other words, there is no single answer to your question as to what value the damping resistor should be. It will depend upon several factors that affect the capacitance of the coil and associated circuitry. Keep the capacitance low and you can sample earlier because you can use a higher value of damping resistor.

Reg

[1843]

Hi Reg,

Sometimes I get a decent curve like pic_1, but when I bring a metal close to the coil the curve becomes like the pic_2.
Does it mean that the damper value is high?

Thanks for your helps

[1843]

You can increase the value of the resistor until you see some obvious dip in the receive signal.

I always use AD844 in my PIs. It doesn't get any dip! I Don't know why?!
But another opAmp like NE5534 gets tens of dip!

So I am confused....

[Reg]

Hi 1843,

You have mentioned one other thing that can greatly influence the decay curve and that is the IC used for the preamp. You may get a great curve with one IC and get oscillations with another. So, the preamp can complicate trying to understand what is the best.

Ideally, you want your signal out of the preamp to start low and rise as quickly as possible to a flat line at 0 volts, like your top drawing. If you increase the damping resistor a little at a time, the time it takes to rise will shorten but the signal should still stop once it gets to 0V if it is not underdamped. If you underdampen, the signal will rise very quickly but overshoot and come back down to 0V.

Your lower drawing shows a target type signal. The hump that occurs before it ramps up is something else and is, most likely caused by the opamp. So, don't worry about that hump.

Now, you don't have to worry about any signal before you sample, so it can do what it wants as long as it stabilizes at 0V at the time of the sampling. Obviously, the signal will not be at 0V if you are trying to detect something, so make sure the coil is not near any metal at all when checking the signal out of the preamp as to what is happening at the time you sample.

Reg

[1843]

Hi Reg,

I still have question. The my 1m coil (1mH) has 17 turns. Although its capacitance is high and my PI makes flow 5A of current
into it, I can increase the damper up to 680-ohms without any ocillations (because of the opAmp).

The my doubt is here and I don't know how to adjust the Damper right. The Hammerhead coil needs a damper about 680-ohms, not such a high capacitance large coil!!!?

I tried various dampers and found that when the damper is high, as I increase the pulse width, the curve gets change in its shape.
These drawing show the problem: (Damper resistor is 560ohms)

[1843]

The hump that occurs before it ramps up is something else and is, most likely caused by the opamp. So, don't worry about that hump.

I observed that by lowering the damper, the hump disappears...?

Best regards

[Unregistered]

dd and mono damper

HI all
I finished my HH PI a while ago and seems to be working ok .. but something is puzzling me.
I use a DD coil and Mono coil and am using the DD configuration with 2 x 1.2k damper resistors on board and each of my coils have extra ones in the plug to get the match right.
After calculations are done ( as all resistors are in parallel ) I get for the DD coils a total damper of approx 820 ohms for tx and approx 760 ohms for rx. Now for Mono I have to take into consideration the 2 onboard resistors plus the one in the coil plug and this brings it down to about 370 ohms. I used a scope to get these to the right values.. so why is it that my mono has that lower value for a damper. ??????
But it works....
Henry

[Reg]

Hi 1843,

You are finding out that PI's are not as simple as one might think and about anything you do will change the curves you will see at the output of the preamp.

Now, as for your pic showing the different curves, what you experience is normal. Remember, the longer the pulse width, the more current into the coil, the longer coil signal will take to decay before getting to 0V.

You may be detecting something that isn't obvious as you increase the current also, so make sure there isn't anything metal nearby.

What are you using for shielding on your coils?

What you have to remember is things can change when you change something. Increase the pulse length and it will change the decay curve because of the increased current. This will result in taking longer for the signal to begin to rise and as you noted, cause a different rise shape.

Once again, don't panic about the decay curves. What you are seeing is normal. The key is what is happening at the time you sample. If all is steady at that time, then everything will be ok.

What resistor value do you have for a feedback resistor on the preamp? If you left it a 1 meg, then everything can be touchy. I would lower this value and increase the gain of a later stage to make up any gain loss. If you reduce the gain you should see the signal stabilize faster, meaning you can sample earlier.

Now, as for your earlier post about the hump disappearing, that is possible also. You might take a look to see just what else is happening such as it takes longer or less time before the signal begins to rise towards 0V.

Ideally, you want that signal to be low and rise as fast as possible without much or any overshoot. What is the most critical though, is that the signal is stabilized at 0V at the time of sampling. This assures the signal out of the preamp isn't too great and the next stage doesn't go into saturation.

Reg

[Reg]

Hi Henry,

Again, you are experiencing some of the strange behaviors of a PI that are normal.

Carl made provisions for a DD damping resistor on the pc board, but I don't recommend using it if you plan on using different size coils. If you are only going to use one coil and only one, then using it makes sense.

However, if you are going to use a mono coils and a separate DD or even more coils, then I recommend you simply add the receive resistor inside the coil housing. This makes things a lot simpler to adjust.

Now, you have to keep in mind that the signal you see at the preamp when using a DD coil does not see the intense current that happens on a mono, so the opamp isn't subjected to as much saturation. Also, the receive winding does not have the same capacitance as the mono circuit because in the mono mode, the FET and associated capacitance is there also. So, generally speaking, a mono coil will require a lower value of damping resistor. Your damping resistor is very low indicating there is excessive capacitance involved. This can be coming from the FET, the coil itself, and the shielding. Each can have quite a bit of influence.

What type of wire are you using for the coils? What are you using for shielding, and what FET are you using? Are you using the NE5534 and if you are, what brand is it? What is the value of the preamp feedback resistor (R 13 on the HH). All of these parts make a difference. Some can make a big difference.

Reg

[1843]

Hi Reg,

I don't use any shield for my 1m x 1m coil!

In the drawing_4 I pointed to a tiny hump by green color. If I bring a metal near the coil, the hump becomes like the hump at drawing_2 in the earlier post.

Is damper at this time ok? If I change the damper with 680 ohms, the tiny hump grows.

When the damper is 270ohms, all the curves become like each other; only by increasing the PW, the decay curve takes longer to rise. (without any change in their shapes)

What do you think?

Best regards

[Reg]

Hi,

The hump indicates some instability. Just where it is coming from is what is difficult to determine. There could be a slight oscillation in the opamp, or it could be coil related, or both. I suspect both but again, I wouldn't worry about it. Strange things happen in opamps that we can't see or evaluate. At least, I can't. When a strong signal is applied to an input of an opamp, that amp goes into saturation. Just what is happening internally is something that can't be seen. Now, what you are seeing could the the result of the opamp by itself and what happens when a very fast signal is applied.

The change in the damping resistor will change how the signal is applied so all changes.

The coil could be reasonably close to oscillating and a target alters things just enough that it does. However, I suspect the signal is the result more of what is happening in the amp.

Instead of changing the damping resistor, try a simple cap across the opamp. Check the spec sheets to see if it is recommended. You might have to place a resistor and cap in series across the feedback to obtain the ideal stabilization of that amp.

BTW, which opamp were you using?

I was just looking at the specs of the AD844 and it is somewhat confusing. Maybe Carl will look at the spec sheet and offer his opinion.

Reg

[1843]

Hi Reg,

AD844 is a very low noise and high slew rate and also high price!

I don't need to get sample soon. The minimum sample delay in my PI is 80us. It is a deep seeking PI with 24V battery.

Hi Carl,
Would you please tell us your opinion?
Here is the datasheet:
AD844


Best regards
1843