HH2 -- first cut

HammerHead PI

Hi Carl,
I have made 2 HammerHeads and I think its good basic platform for more advanced PI. I intend to implement here some of Pics like 628.877 etc. or some od Atmel low-cost pics. I just finished simulation on SD2000 timings. Zed did nice,excellent job. Timing is qute accurate and precise just he gave us. The most important things are front end receiver, and 3 channel multi-stage, multi-poles filters. I think that kind of filters must be implemented in any advanced PI concept. I study Candy patents and I see that his discoveries are used in all MineLab machines. Conception off all MLab PI machines lie on original Candys artworks.
Carl, could you doing some experimpents on ultra-fast sampling coils. Good and fast sampling machines need such a coil. I think on teflon-fibreglass (Rogers RT/Duroid with an er = 2.2) PCB coils with low dielectic losses. The losses are considerably lower (tan delta = 0.001 to 0.002) from conventional epoxy-fibreglass (tan delta = 0.030 to 0.035). regards

[nick_f]

Hi Carl,

I intend to write some code for a PIC16F84 or similar, to generate just the delays for a PI detector. Based on your experience, could you please provide some general hints about how to structure the program? Should it be written in assembler or C code?
Is it possible to use switches to adjust the delays? What kind of resolution can be achieved by using a 20MHz clock? For the beginning, I will try using some fixed delay values, but it would be good if we can see the values on an LCD and change them from the front panel.
I have some basic experience programming microcontrollers (enough to know that even simple things are not always so simple ).

Regards,
Nicolae

[okantex]

So , you say Vdd of micros are all positive. like Carl's , QED 's ..etc.
okay

if I feed vdd of 18f4550 by +5 ,and put Carl's design's 12F683 's pin 8 , 7 ,6 links to +5 fed 18f4550 and use the right part of 12f683 Carl's design as out of 18f4550 till N4 ,,then connect N4 to Aziz's improved TX circuit. , will everything be okay in the name of transmitter circuit.

excuse me above are wrong , I recently realized.

but Carl's and 1843's tx are almost same and they are driven by pic directly. also Aziz's tx offer looks similar but why does it need (-) drive.?

[okantex]

So , you say Vdd of micros are all positive. like Carl's , QED 's ..etc.
okay

if I feed vdd of 18f4550 by +5 ,and put Carl's design's 12F683 's pin 8 , 7 ,6 links to +5 fed 18f4550 and use the right part of 12f683 Carl's design as out of 18f4550 till N4 ,,then connect N4 to Aziz's improved TX circuit. , will everything be okay in the name of transmitter circuit.

[barry]

Negative voltages can be confusing. When you learn Micro-controllers you only have +5 or +12 volts. You need to think a different way when you have negative voltages. The ground pin of the Micro-controller becomes -5volts, The supply voltage pin of the Micro-controller becomes common ground of the overall circuit.

When the output register is 1 the Micro pulls it "up" to it's supply pin voltage, in this case it is common ground.

When the output register is 0 the Micro pulls is "Down" to it's ground pin voltage, in this case it is -5Volts.

[okantex]

I need to study a little about supply voltage of controllers.

[barry]

Hi Aziz ,
I can not program atmels ,I do not know assembler or C++.so I have to use pic.

I could not understand your below words

"
zero volt on BS250's gate means transmit coil off and a -5V means transmit coil on. So the micro has to support a negative transmit logic: I/O pin=0 => transmit pulse on, I/O pin=1 => transmit pulse off.
"
I/O = 0 => transmit off or on.?

thanks


so if my pic can give -5 when it is on ,I can drive your tx circuit.

micro output register set to 0
voltage at Micro output pin is -5volts
BS250 turns on
Coil Transmit on

micro output register set to 1
voltage at Micro output pin is 0volts
BS250 turns off
Coil Transmit off

[okantex]

Hi Aziz ,
I can not program atmels ,I do not know assembler or C++.so I have to use pic.

I could not understand your below words

"
zero volt on BS250's gate means transmit coil off and a -5V means transmit coil on. So the micro has to support a negative transmit logic: I/O pin=0 => transmit pulse on, I/O pin=1 => transmit pulse off.
"
I/O = 0 => transmit off or on.?

thanks


so if my pic can give -5 when it is on ,I can drive your tx circuit.

[Aziz]

Hi okantex,

Hi Aziz ,

how are you ?

I am sure you remember that I am studying to make TX part for QED.BW's homework to me.
I want to use your improved driver ,but I could not understand how to control it.
in schemetic what is V2 ?
I can understand Carl's and BW 's drawings , but can not understand your simulation drawings.
according to your improved schematic ,how can I control TX.
where Should I put TX timing pin ( 18f4550 's control pin for TX (driver pin))?
Btw I want to use 18F4550 for QED. how can I do connection.
can you explain a little more ,please
regards
Okantex

The mosfet driver circuit is tailored for the negative supply voltage operated micro controller and coil. The coil flyback voltage will give a positive voltage response. If your micro controller and your coil supply has not the same supply configuration, then you can not use the schematics. QED uses a different supply configuration I think. So the schematics must be adapted.

V2 is the transmit trigger pulse from the micro controller and is used only in the simulation model for clearness. The micro is operated from system GND = micro VDD and system -5V = micro GND. The mosfet BS250 is for logic level shifting from 0/-5 into 0/-Ucoil (0=ground). It is also inverting the pulse logic (simple inverter). -Ucoil can be much more (e.g. -12V). A zero volt on BS250's gate means transmit coil off and a -5V means transmit coil on. So the micro has to support a negative transmit logic: I/O pin=0 => transmit pulse on, I/O pin=1 => transmit pulse off.
Giving a higher switching voltage to the IRF740 will lower the mosfets on-resistance. Therefore the full coil voltage rail is fed into IRF740's gate. Care must be taken to the two 2N2907. They should be matched together (= same characteristics). If not, they need an additional seperate resistor on each base. Usually one 2N2907 is enough to discharge the IRF740's gate charge quickly.

If the micro has tri-state I/O output on the transmit pulse pin, an additional resistor (e.g. 100k) from the gate of BS250 to source pin must be connected to force switch-off of the coil. Otherwise, it could have an undefined state and could switch the coil on/off arbitrary.

I am not familiar with PIC's. I don't like mature PIC micros. I hate their banking (paging) and inherent inefficiency of coding. The only PIC i could like would be a dsPIC33F..

Aziz

[okantex]

Hi Aziz ,

how are you ?

I am sure you remember that I am studying to make TX part for QED.BW's homework to me.
I want to use your improved driver ,but I could not understand how to control it.
in schemetic what is V2 ?
I can understand Carl's and BW 's drawings , but can not understand your simulation drawings.
according to your improved schematic ,how can I control TX.
where Should I put TX timing pin ( 18f4550 's control pin for TX (driver pin))?
Btw I want to use 18F4550 for QED. how can I do connection.
can you explain a little more ,please
regards
Okantex

[Aziz]

Hi porkluvr,

interesting incident. Your supplied diode models are coming from my living town (Rosenheim). The BYV28-200 is the one I found and did not search for better versions yet. Generally, the higher the breakdown voltage, the higher the drop voltage and thus reducing the power efficiency of the transmit pulse. A bad replacement for the BYV28-200 could be a 1N4004 - 1N4007. So it might be a good task for all, to find better replacements (+ spice models). I don't know, whether the BYV28-200 will withstand 400 V. Any input will be appreciated. The BYV28-400 is also available but I haven't any spice models for it.

The main function of the diode is to reduce the MOSFET's capacitance affecting the transmit coils capacitance. After the avalance breakdown voltage is falling short of, the diode is becoming a capacitor which is connected in series to the MOSFET's capacitance. The total effective capacitance will therefore be lower than the diodes capacitance and has less capacitive load to the transmit coil. The transmit coil can be damped then faster which results in a higher damping resistor.

When I have more time, I will look for better diodes with spice models. But I do not edit or modify the standard files in LTSpice (due to update). So it is better to make external spice models. Thanks for your spice models and hint.

Aziz

[porkluvr]

Aziz, I have a feeling that the BYV28-200 you are using would not stand up very well in a circuit using the IRF740. I had a hard time finding models for 400V diodes, but found models (and suppliers) for 600V (fast recovery) diodes from Vishay and Fairchild.

Some of the models should be added to (LTspice) standard.dio and the subcircuits are to be associated with your x-diode symbol.

These should work with any PSpice compatible simulator.
When I get some more time maybe I'll look for some 400V diodes but it is not a priority.
********************
I might abandon the series diode because I sometimes see a 400uV glitch in my listening period. I haven't determined exactly what circuit combination is causing it but it's something I can't ignore. Rather than pull my hair out trying to analyze it I may just pfft and use a resistor.

What is that series diode there FOR, anyway, do you know? What is the advantage of having a fast recovery diode between the FET and coil?

[Aziz]

Hello friends,

here is the slight improvement of the low-cost PI MOSFET driver. By stacking the 2N2907 (Q1 paralleled with Q2), the circuit is able to switch off faster, be able to drive more paralleled power MOSFET's or a higher input capacitance MOSFET's. If you are using only one power MOSFET, then only one 2N2907 is enough. The resistor R3 gives a soft-on feature. I have reduced the R2 to make the Q1 faster for switch-off. The higher switching current through BS250 is a peanut compared to the potential for reducing the switch-on time due to faster switch-off of the power MOSFET. The switch-on time is delayed due to R3 to a few microseconds. The switch-off delay is just around about 200 ns. The coil current will be switched off in about 70ns until it reaches the avalanced state of the MOSFET.

If you stack the power MOSFET's, then ensure both devices are mounted on the same heat plate to have same temperature. Other PNP transistors for Q1 could be used. Just try it out.

I will enclose the whole spice simulation model for further experiments for you. Look for the improved version in the zip-file.

Why I did this effort?
I couldn't get the MOSFET driver IC's on my local shop or supplier! But all the parts used in the low-cost variant are most common, available and cheap.

Happy simulating..

Aziz

[Aziz]

Hello Stefan,

you have found the soft switch-on solution.

A resistor must be placed in the gate path. The reverse biased diode parallel to this gate resistor gives not much an improvement on the modified low-cost variant without the BJT. Even then, when the drain resistor of BS250 is kept low.

With the MOSFET driver IC's, the switch-off could be fast due to low impedance discharge path. But I want to omit any driver IC's (should be a low-cost design).

Aziz

[Stefan]

Aziz,

If you are using a TCS426/7/8 MOSFET drivers there is a simple solution to give slow turn on and fast turn off.

Slow turn can be adjusted by placing a 10 to 20 ohm resistance in series with the gate drive. This will add a small time constant to the charging of the gate for on time. ie slow turn on.

By placing a diode in parallel with the resistor and biased such than on turn on the current is flowing through the resistor. At turn off the diode is forward biased and will discharge the gate charge quickly. The resistor has no effect at this point. ie fast turn off

I have used this method and it works fine.

Regards,

Stefan