Preamp output what am I looking at here.

[Brian Deese]

My preamp wave does not look like the hammerhead article,what is the oscilation after the pulse?

[Brian Deese]

trying to post pics here

a

[DOOLEY]

under damped is my best guess based on what im looking at ,
not sure what the line is after it's settled out ,

but mildly underdamped me thinks.

[DOOLEY]

silly me , the line is a very wide sample pulse isn't it.

lower damper resistor until it DOESNT dip below top rail , but dont go to far.

[Brian Deese]

So underdamped causes ringing? Is that what it is?

silly me , the line is a very wide sample pulse isn't it.

lower damper resistor until it DOESNT dip below top rail , but dont go to far.

Yes it is a sample pulse,I threw an extra pic in.

[Qiaozhi]

What value of damping resistor are you using? If your coil has an inductance below 500uH, then a 680R should be ok. Otherwise, you can easily find the correct value by taking a 1k2 resistor, and placing (in parallel with it) a 10k pot and 10k resistor in series. Then connect this in place of the damping resistor. After adjusting the pot to achieve critical damping, remove the circuit and measure the total resistance. Then fit a damping resistor with this value.

[DOOLEY]

yes , under damped causes "ringing"

here is an idea , most people use 1 resistor for damping resistor , but this way there is
a big jump between resistor choices .

i use 2 or 3 resistors in combination , to fine tune ,

also spreads power disapation across 2 or 3 resistors then they dont get as warm.

so if 470 ohm is over damped , and say 680 ohm rings , then use combinations to gradualy lower ohms to get it spot on ,

so 2 x 1.2 ohms = 600 etc.......
3 x 1.5 ohms = 500 ohm etc.....

this way we don't have to settle for "near enough"

[DOOLEY]

good idea qiaozhi ,

your suggestion even better for initial setup ,

then combinitions of resistors for final build .

[Brian Deese]

Damping

What value of damping resistor are you using? If your coil has an inductance below 500uH, then a 680R should be ok. Otherwise, you can easily find the correct value by taking a 1k2 resistor, and placing (in parallel with it) a 10k pot and 10k resistor in series. Then connect this in place of the damping resistor. After adjusting the pot to achieve critical damping, remove the circuit and measure the total resistance. Then fit a damping resistor with this value.

The damping resistor is 390 ohm,this is my af-108 which is bipolar.I dont have an lcr meter yet,that is very high on my priority list.I think the coil must be center tapped.In the last pic you can see the waveform is from the negative going pulse,then the sample is there,and finally you can see the positive going pulse.So this machine has two damping resistors one for the positive pulse and one for the negative,they are both the same value 390 ohm.What does an over damped wave look like?

[Qiaozhi]

What does an over damped wave look like?

The same as a critically damped, but it just gets to the zero volt line quicker. You need to increase the damping resistor value until you see that it's under-damped, then adjust it down until the ringing stops.

[Brian Deese]

Another question

The same as a critically damped, but it just gets to the zero volt line quicker. You need to increase the damping resistor value until you see that it's under-damped, then adjust it down until the ringing stops.

That leads me to another question,in the schematic vr5 adjusts something on the preamp,I dont know what you call this adjustment but it moves the would-be zero line in relation to the tx pulses(talking amplitude here).Here is where I get confused, if you adjust it to zero then it is not centered between the pulses,and if you adjust it exactly in the center of the pulses then it is not at zero.Half of me wants it to be in the center and the other half wants zero volts.How should it be adjusted? Why cant we just use a large pot to find the damping resistance? I mean the resistor is supposed to be 1/2 watt right? If you use a 1/2 watt pot does it have too much inductance or something?

[Qiaozhi]

That leads me to another question,in the schematic vr5 adjusts something on the preamp,I dont know what you call this adjustment but it moves the would-be zero line in relation to the tx pulses(talking amplitude here).Here is where I get confused, if you adjust it to zero then it is not centered between the pulses,and if you adjust it exactly in the center of the pulses then it is not at zero.Half of me wants it to be in the center and the other half wants zero volts.How should it be adjusted?

VR5 adjusts the null offset voltage of the preamp. With inverting and non-inverting inputs of the pre-amp connected to 0V, the output of the preamp (pin 6) should be zero. An easier method is simply to adjust VR5 so that the settled-out voltage (i.e. after it's stopped ringing) is located at 0V.

Why cant we just use a large pot to find the damping resistance? I mean the resistor is supposed to be 1/2 watt right? If you use a 1/2 watt pot does it have too much inductance or something?

Of course, you could use a suitably high wattage rated pot, but then it's possible to turn it to zero ohms and short out the coil.

[Brian Deese]

back to the scope then

I have vr5 adjusted so the output of the preamp is centered between the pulses now,so I will go back in and set it to zero.I will rig up a pot to check out the ringing and see what I can figure out,I will put a resistor in series with the wiper so I dont short it completely.This machine is coming along nicely,I appreciate the help.I am getting a civil war era button at 12 inches air test now.I will keep improving and see what I can get out of this old dog.

[6666]

Brian somewhere on the forum there is a nice little animated graphic that shows the waveform from the output of the preamp , it show the effect of the damping resistor and the effect on the ringing as the resistor value is changed, it gives a great visualisation of what you are doing with the damping, I dont have a link to the graphic somebody may have it, if they remember the graphic.

[Brian Deese]

Awesome

Brian somewhere on the forum there is a nice little animated graphic that shows the waveform from the output of the preamp , it show the effect of the damping resistor and the effect on the ringing as the resistor value is changed, it gives a great visualisation of what you are doing with the damping, I dont have a link to the graphic somebody may have it, if they remember the graphic.

That would be sweet,I will search for it.Thanks