Minipulse Plus Rev My time has come!

[green]

^2 (squared) ^.5(square root) I think

[mushaba]

For critical damping, Rd=(L/4C)^.5 C=L/4/Rd^2. C=390uH/4/1800^2=30pf. Lead can exceed 1pf/inch. A fast 390uH coil will probably exceed 10pf, less than 20inch lead if the circuit doesn't add any capacitance. A MUR460 diode in series with the mosfet reduces the circuit capacitance a lot but still adds a small capacitance for me.

Rd=pi*L*SRF You could remove Rd and the input resistor like Carl suggests and measure SRF with a scope as the coil decays after Tx off.

Green is it possible to post a photo of the above equation because it's getting confusing with symbols...Also SRF?What SRF is?

As godigit1 said some of us are trying just to catch up with you, probably we won't but can't blame someone who's trying...I found thunting and geotech because a friend of mine gave me a gti2500 to repair,now I think im a PI junkie...

[waltr]

^2 (squared) ^.5(square root) I think

Correct.

godigit1,

Here is image of the math symbols done in Word.


SRF = Self Resonant Frequency

[Qiaozhi]

Use latex:

[green]

Green is it possible to post a photo of the above equation because it's getting confusing with symbols...Also SRF?What SRF is?

As godigit1 said some of us are trying just to catch up with you, probably we won't but can't blame someone who's trying...I found thunting and geotech because a friend of mine gave me a gti2500 to repair,now I think im a PI junkie...

I know how you feel. Skippy has corrected my math more than a few times. King JL posted a Spice simulation I tried to look at. I'm a novice with spice, if there is more than one thing to click on I'm in trouble. waltr posted the math symbols in reply#108. Rd*Rd=L/4C and
Rd=pi*L*SRF are the two formulas I use. Carl's suggestion of removing Rd and input resistor to get circuit resonance is probably the best way. MMP Rev E has a MUR460 diode between the mosfet and coil isolating most of the mosfet output capacitance so measuring coil resonance with leads should be close. Clip the scope lead to it's ground lead making a loop. Lay the loop on the test coil and position the test coil near a operating PI coil to get a good scope reading of resonant decay(scope 5mV or 10mV/div). Record SRF. Using a pot to adjust damping is maybe the best way to get Rd but I like to see if calculated Rd is close. Knowing the formulas helps understand what effects Rd, a fast coil with leads has a capacitance of maybe 50pf. Rd=square root(L/4C), Rd=square root(300uH/200pf)=1414ohms. Without the MUR460 in series the mosfet output capacitance would add to the 50pf(maybe 220pf total)Rd=square root(300uH/880pf)=584ohms

[mushaba]

Hello green thanks a lot for one more valuable lesson!!!!Please be patient until theese things comes clear to my damaged brain


In some posts ago you wrote..

Wondering what the coil specs are. A 300uH coil would need 23pf total capacitance to critical damp with a 1.8k resistor. Possible with a short lead.

1)As i understand from the formulas you gave is impossible my circuit (coil - RG58 - MUR) to have only 23pf capacitance,ok,can you think what else could it be?I've never measure capacitance,added 5ohms to limit the spikes and then via pot i adjusted the waveform of amp out not to have ringing,were did i go wrong??

2)What do you mean by short lead???

Rd*Rd=L/4C and Rd=pi*L*SRF

Ok,these formulas are to calculate Rd ONLY or can be used to calculate C and SRF via various Rd values?? Can i use it like C = L / (Rd*Rd*4) or SRF = Rd / (pi*L) ??

Clip the scope lead to it's ground lead making a loop. Lay the loop on the test coil and position the test coil near a operating PI coil to get a good scope reading of resonant decay(scope 5mV or 10mV/div). Record SRF.

The test coil leads are open right? I have another 3dss waiting (340uH 1.9 Ohms made from PTFE 24AWG silver plated)



I will lay on it the loop i made from oscope + and gnd and how close should i put it near the working coil of my MPP...Put the oscope dc coupling and 10mV/div...





What did i measure in this precedure??

[waltr]

Oscope Output.jpg

What did i measure in this precedure??

Nice decaying oscillation-perfect.
Decrease the scope's time base, maybe 2usec/div, to see and measure the time between oscillations (period of oscillation). divide into 1 (1/t) to get the SRF in Hz.

[green]

Looks like your scope can display frequency. Set scope to display frequency and do what waltr suggested, change time base until you get a good reading. Adding a scope picture I posted awhile back.

Ok,these formulas are to calculate Rd ONLY or can be used to calculate C and SRF via various Rd values?? Can i use it like C = L / (Rd*Rd*4) or SRF = Rd / (pi*L) ?? YES

[godigit1]

Thank you guys and thank you Mushaba. That I can understand.
Ps like the eddie avatar.

[mushaba]

Nice decaying oscillation-perfect.
Decrease the scope's time base, maybe 2usec/div, to see and measure the time between oscillations (period of oscillation). divide into 1 (1/t) to get the SRF in Hz.

Here it is



So Rd=pi*L*SRF --> Rd=3.14*0.00034*1250000=1334.5 Ohms.

So this coil when it will be attached to a PI detector should damp at 1.33 Kilo Ohms ????

I noticed that distance between testing and working coil had to do with signal amplitude not frequency..

Looks like your scope can display frequency. Set scope to display frequency and do what waltr suggested, change time base until you get a good reading.

Yes my oscope measures lots of stuff,trully value for money...I cant understand what is the trigger freq in the upper right of the photo..

Now Rd*Rd= L / 4*C --> C= L / Rd*Rd*4 --> C= 0.00034 / 1334.5*1334.5*4 -->C= 0.00034/7123561 --> C= 4.77 e-11 ...what is that now

Mashuba Thanks for starting your thread Im sorry ive piggybacked on to asking questions but the information here is the missing link for me.
Best Regards

That would be my South African nickname or my name in a next life as a lion (or an elephant because in this life im near 110 kgs..)

Ps like the eddie avatar.

Up The Irons !!!!!!!!

[godigit1]

And the Amador nail recently found on a expidition to recover hot rock samples.
Incased in a conglomerate surly it must be thousands of years old

Ok A little side track got to go to work

[green]

Here it is

[ATTACH]46192[/ATTACH]

So Rd=pi*L*SRF --> Rd=3.14*0.00034*1250000=1334.5 Ohms.

So this coil when it will be attached to a PI detector should damp at 1.33 Kilo Ohms ????

I noticed that distance between testing and working coil had to do with signal amplitude not frequency..



Yes my oscope measures lots of stuff,trully value for money...I cant understand what is the trigger freq in the upper right of the photo..

Now Rd*Rd= L / 4*C --> C= L / Rd*Rd*4 --> C= 0.00034 / 1334.5*1334.5*4 -->C= 0.00034/7123561 --> C= 4.77 e-11 ...what is that now





That would be my South African nickname or my name in a next life as a lion (or an elephant because in this life im near 110 kgs..)

Up The Irons !!!!!!!!

1335ohms should be close if the mosfet capacitance is isolated from the coil during discharge with the MUR460 diode. Lot lower resistance without the diode. Don't know if scope reads frequency better if display shows 5 or 25 cycles, I usually adjust time base for 5 to 10 cycles. Maybe someone knows if should be different.

I use a different formula for calculating C if I know SRF and inductance. C=1/(2pi*2pi*L*SRF*SRF). Calculates the same 47.7pf you got. Total parallel coil capacitance including lead capacitance. Fast coil, one reason easy to get mosfet avalanche. Parallel capacitance stores little energy at Tx off so 1A peak at Tx off would cause about 570V with Rin=1k and Rd=1335

[mushaba]

1335ohms should be close if the mosfet capacitance is isolated from the coil during discharge with the MUR460 diode. Lot lower resistance without the diode. Don't know if scope reads frequency better if display shows 5 or 25 cycles, I usually adjust time base for 5 to 10 cycles. Maybe someone knows if should be different.

I use a different formula for calculating C if I know SRF and inductance. C=1/(2pi*2pi*L*SRF*SRF). Calculates the same 47.7pf you got. Total parallel coil capacitance including lead capacitance. Fast coil, one reason easy to get mosfet avalanche. Parallel capacitance stores little energy at Tx off so 1A peak at Tx off would cause about 570V with Rin=1k and Rd=1335

So with this procedure we can calculate a coils srf and capacitance..But should have in mind capacitance from cable and from diode..

How can I use this coil to it's max capabilities?I mean what tx pulse width delays etc..and if it is a fast coil why can't I sample below 10us?
Tommorow I will wrap a spiderweb just to practice with the thinks I've learned today and finalise my build.

Searching the web for a decent LF357 for my DP Legend but eBay is full of Chinese rubbish..Equivalents are hard because bom says 50V/us slew rate and above grrrrrrrr0

[green]

So with this procedure we can calculate a coils srf and capacitance..But should have in mind capacitance from cable and from diode..

How can I use this coil to it's max capabilities?I mean what tx pulse width delays etc..and if it is a fast coil why can't I sample below 10us?
Tommorow I will wrap a spiderweb just to practice with the thinks I've learned today and finalise my build.

Searching the web for a decent LF357 for my DP Legend but eBay is full of Chinese rubbish..Equivalents are hard because bom says 50V/us slew rate and above grrrrrrrr0

Looked at Rev=E schematic. Rin connected to op amp -input. If op amp was fast enough, shouldn't need Rd(Rin should damp). Wondering what TP3 looks like if a 4148 diode were connected across R8(cathode to amp out) and Rd was adjusted for best decay.

[waltr]

How can I use this coil to it's max capabilities?I mean what tx pulse width delays etc..and if it is a fast coil why can't I sample below 10us?

The main issue is the NE5532 op-amps can not recover from over load (over Voltage) any faster.
This recover time has been discussed much with a few newer op-amps suggested but not much real data on how well they work.

10us is a great short delay for any mono coil and simple front end.
This is fast enough for all but the smaller gold nuggets.