Optimizing Target Responses

[6666]

Something is not working right
Part 5 is broken, post #24

The graphics show ok but the Equations are broken in both win7/win10 both ff and chrome

[6666]

Chrome in both Android and Windows 10 show the broken image icon.

The same in MS browser in W10.

I looked at the HTML source and it seems the image URLs are nor url-encoded, that might be the cause.

If I copy the image address and paste it on the address bar then the images show, but still not on the web page even after reloading.

Working ok in win7 with fire fox and chrome, will try win10 later.

[Teleno]

Works fine for me in Firefox on Linux.
What browser and OS are you using?

Clearing the cache may solve the problem, or it may be a browser add-on that's causing problems.

Chrome in both Android and Windows 10 show the broken image icon.

The same in MS browser in W10.

I looked at the HTML source and it seems the image URLs are nor url-encoded, that might be the cause.

If I copy the image address and paste it on the address bar then the images show, but still not on the web page even after reloading.

[Qiaozhi]

The latex hypertext is not working in my browser. It produces broken image links.

Works fine for me in Firefox on Linux.
What browser and OS are you using?

Clearing the cache may solve the problem, or it may be a browser add-on that's causing problems.

[MartinB]

I would guess that wire-wound is fine. I would measure the inductance to ensure it is "in the mud."

It is my recollection that metal oxide has high flicker noise. For the coil series R that doesn't matter but for the damping and clamping R's it does.

Thanks Carl
I was also sold wire wound for the damping resistor, which I know does make a difference.
Looking for a suitably rated metal film resistor now.
Pleased that I had the conversation with my supplier before I boxed everything up.

[MartinB]

If my memory serves me right, I have asked similar questions about this. Dang, cant find em.

Yes, you can use MOX (metal oxide) resistors instead. I only use them now unless in a pinch.
Do check the voltage ratings though. There are quite a few el cheapo brands out there that are not really up to snuff on that.

I used to use the old fashioned dark green glazed wirewound ones. Some of them are bifilar? wound.
I assume you have the light grey/white rectangle box type ones.

Is there a difference? I did not have an oscilloscope back then, so cant definitely say.
You will surely get some sort of "extra effects" with inductive wirewound ones.
I would have to unpack my measuring gear to measure the magnitude of the difference.
Maybe its neglegible, but interesting nontheless.

Maybe Carl or someone else could elaborate on this.

Thanks Polymer
I will see what I can measure.
Carl has also responded.

[Carl-NC]

The latex hypertext is not working in my browser. It produces broken image links.

I'm not sure what to do about that. Copy/paste the following line into your browser URL and see what you get:

https://www.geotech1.com/cgi-bin/mimetex.cgi?i=1

There should be a small image of "i=1".

[Carl-NC]

It think this is the right place for this question.
Last week during casual conversation with my supplier I discovered that the MOSFET protection resistors he had sold me, generally 3R3, were in fact wire wound. Does this affect coil performance in anyway ? 3W metal film resistors are difficult to source - can I used metal oxide instead ?
Very educational thread; thanks guys.

I would guess that wire-wound is fine. I would measure the inductance to ensure it is "in the mud."

It is my recollection that metal oxide has high flicker noise. For the coil series R that doesn't matter but for the damping and clamping R's it does.

[Polymer]

It think this is the right place for this question.
Last week during casual conversation with my supplier I discovered that the MOSFET protection resistors he had sold me, generally 3R3, were in fact wire wound. Does this affect coil performance in anyway ? 3W metal film resistors are difficult to source - can I used metal oxide instead ?
Very educational thread; thanks guys.

If my memory serves me right, I have asked similar questions about this. Dang, cant find em.

Yes, you can use MOX (metal oxide) resistors instead. I only use them now unless in a pinch.
Do check the voltage ratings though. There are quite a few el cheapo brands out there that are not really up to snuff on that.

I used to use the old fashioned dark green glazed wirewound ones. Some of them are bifilar? wound.
I assume you have the light grey/white rectangle box type ones.

Is there a difference? I did not have an oscilloscope back then, so cant definitely say.
You will surely get some sort of "extra effects" with inductive wirewound ones.
I would have to unpack my measuring gear to measure the magnitude of the difference.
Maybe its neglegible, but interesting nontheless.

Maybe Carl or someone else could elaborate on this.

[Teleno]

Yes, better late than never!

Part 5: Real Responses

So far we've looked at different responses and applied piecewise-linear solutions to much of it. That is, the PI turn-off was a linear ramp that induced a step EMF in the target. This is a good first-order approximation and a good way to do quick-n-dirty comparisons, like what happens when you try to get a sharper & sharper turn-off slew rate (Part 4).

In reality, the PI coil is an RL circuit during turn-on and an RLC circuit during turn-off. Equations can be derived that accurately describe what they do. I won't derive them here but rather just toss them out. ITMD3 will have full derivations.

The turn-on equation is simple:



The turn-off equation is non-obvious but here it is:



where is the peak current at the turn-off event.

Let's look at a realistic example...

Measured coil parameters:
L = 300uH
Rs = 5 ohms (total series R)
SRF = 500kHz (inc. cable etc)

VTX = 10V

The parasitic C for the coil is



The require damping resistor for critical damping is



Now calculate the taus:





VTX (10V) and Rs (5 ohms) tells us the max flat-top current will be 2A. The TX tau os 60us so we would need a TX pulse width of ~300us to reach the flat-top current. A more normal TX pulse width is 100us which makes the calculated peak current 1.62A at turn-off. This becomes the starting point for the turn-off equation.

Here are both equations plotted in Excel. Note that the turn-on curve spans 100us whereas the turn-off curve spans only 3us.



Also note that in the turn-off side the tau_RX is 318ns. If you are expecting the current to be substantially gone in 5*tau (1.59us) you can see it is not. The 0.67% mark occurs at about 2.25ns which is about 7*tau, about 40% more than the usual RL curve.

But that's not the worst part; let's also look at the flyback voltage. Its equation can also be derived:



We have all the variables from above; the curve looks like this:



This curve is also plotted to only 3us. The peak hits ~560v which can be calculated from the derivative of the above equation. Note that the peak occurs at exactly tau_RX. If you were to assume that any kind of 5*tau settling applies here, then you would take 0.67% of 560v (=3.8v) and see that it occurs at about 2.57us, which is about 8*tau.

But this is the raw coil voltage which is then applied to a preamp with a fairly hefty gain, perhaps 500. It's immediately obvious that 3.8v will seriously overload the preamp. To determine a realistic required settling we need to make an assumption: that the preamp must be, say, within 0.5v of settled for the demods to work well. For a gain of 500, this means the coil voltage must be within 0.5v/500 = 1mv of settled. In the curve above, this happens at about 5.42us, or 17*tau.

You might be thinking that 5.42us is pretty darned good. But this is strictly the coil settling; the diode clamps and the preamp overloading have not been accounted for. And everything above assumes the MOSFET never avalanches.

[ATTACH]54867[/ATTACH]
[ATTACH]54866[/ATTACH]

The latex hypertext is not working in my browser. It produces broken image links.

[MartinB]

It think this is the right place for this question.
Last week during casual conversation with my supplier I discovered that the MOSFET protection resistors he had sold me, generally 3R3, were in fact wire wound. Does this affect coil performance in anyway ? 3W metal film resistors are difficult to source - can I used metal oxide instead ?
Very educational thread; thanks guys.

[green]

Another stupid pill https://www.geotech1.com/forums/atta...7&d=1618073135 fun exorcise, just worthless.

Looked at data again. bbailor suggested Tx turn off tau should be 5 times faster than target tau. He was using L/R. If use L/2R, would need turn off tau 10 times faster than target tau. Turn off tau,3 times faster than target tau isn't bad with L/R. Turn off tau, 6 times faster than target tau with L/2R is same. Maybe it wasn't completely worthless.

[6666]

Another stupid pill https://www.geotech1.com/forums/atta...7&d=1618073135 fun exorcise, just worthless.

Green, no such thing as stupid in this hobby its all advancement.

[green]

Used spice and Excel to compare(L/Rd, L/1.4Rd and L/2Rd). Don't know if it's an accurate comparison. Looks like using L/R is better.

Another stupid pill https://www.geotech1.com/forums/atta...7&d=1618073135 fun exorcise, just worthless.

[bbsailor]

Does MOSFET COSS mater if a correct diode(MUR460)or some others is used. Not all diodes work.

Green,

The series diode is just an attempt to reduce the COSS that the coil sees and affects the Rd value. Using low COSS MOSFETS along with a low capacitance diode and minimizing other circuit, cable and coil capacitance will all work to raising the Rd value and making the coil discharge slope steeper.

Joseph J. Rogowski