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Thread: Bedini Comparator Cap Dump

  1. #111
    Volty your work is very impressive I love your work

    First sorry about what I said in an early post about the "hysteresis capacitor", after seeing your schematic I can see how this works, It is a very genius way of creating a hysteresis function. I never thought of doing it that way.

    741 Op-Amp Cap Dump.jpg

    I think it is better than my "schmitt trigger" method, in some ways and more simple. Now there are 2 ways of doing this, both with their merits.


    The 15V zener at the back end limits the "gate to source" junction of the mosfets to 15V. This protects from over voltage. A mosfet "gate to source" junction can only withstand about 20V to 23V, typically before it gets damaged.


    I can explain about the slow turn off of your mosfets. Your scope shot is showing you a capacitance discharge curve of a "gate to source" capacitance

    Slow Turn-Off Example_Yellow_is_SG3524-Out Blue_is_H11D1-Out to BD243 to Red MJL21194 or IRFP250.jpg

    This is caused from a parasitic capacitance in the "gate to source" junction of your mosfets. The solution is to discharge this parasitic capacitance as fast as possible for a fast turn off. For a fast "turn on" this capacitance needs to be charged quickly, so the mosfet driver has to be able to provide the necessary current to charge the "gate to source" capacitance.
    The more mosfets in parallel the greater this parasitic capacitance, and this will load your mosfet driver circuit more, especially at high frequency switching.

    There are a number of different ways to achieve a faster turn off.

    1). Passively:
    Have a resistor from the gate to source of each mosfet. The lower this resistance the faster the turn off. With your schematics you only have one 10K resistor for 2 mosfets. You need to have a 10K resister from gate to source for each mosfet to discharge the parasitic capacitance of each mosfet. You can lower these resistances to 5.6k, but doing so, will waste more current, so this would be a trade off.

    2). Actively: This method will give you the fastest turn off. But you will need to modify your mosfet driver circuit. And add a PNP transistor, and have a "PNP and NPN" complimentary pair. Here is a suggested active mosfet driver circuit. You may have to experiment with the value of R1.
    Mosfet Driver_1.jpg

    For the capacitor pulsar a fast "turn on" is important, and for a coil (SSG coil) a fast "turn off" is important. Because capacitors and inductors(coils) are inverse of each other.

    Mosfets "turn on" fast and bipolar transistors "turn off" fast.

    I hope this helps

    Regards
    Nityesh Schnaderbeck
    Last edited by Nityesh Schnaderbeck; 11-24-2014 at 07:18 AM.

  2. #112
    Quote Originally Posted by Ren View Post
    Hi Nityesh,

    Would you care to share the comparator driven mosfet circuit by any chance?

    Regards
    Hello "Ren" Thankyou for your interest.
    Here is a test circuit you can experiment with. I have even made a Mosfet driven SSG with this circuit, which also charged the battery.

    test circuit.jpg


    Regards
    Nityesh Schnaderbeck

  3. #113
    Quote Originally Posted by Nityesh Schnaderbeck View Post
    Volty your work is very impressive I love your work

    First sorry about what I said in an early post about the "hysteresis capacitor", after seeing your schematic I can see how this works, It is a very genius way of creating a hysteresis function. I never thought of doing it that way.

    741 Op-Amp Cap Dump.jpg

    I think it is better than my "schmitt trigger" method, in some ways and more simple. Now there are 2 ways of doing this, both with their merits.


    The 15V zener at the back end limits the "gate to source" junction of the mosfets to 15V. This protects from over voltage. A mosfet "gate to source" junction can only withstand about 20V to 23V, typically before it gets damaged.


    I can explain about the slow turn off of your mosfets. Your scope shot is showing you a capacitance discharge curve of a "gate to source" capacitance

    Slow Turn-Off Example_Yellow_is_SG3524-Out Blue_is_H11D1-Out to BD243 to Red MJL21194 or IRFP250.jpg

    This is caused from a parasitic capacitance in the "gate to source" junction of your mosfets. The solution is to discharge this parasitic capacitance as fast as possible for a fast turn off. For a fast "turn on" this capacitance needs to be charged quickly, so the mosfet driver has to be able to provide the necessary current to charge the "gate to source" capacitance.
    The more mosfets in parallel the greater this parasitic capacitance, and this will load your mosfet driver circuit more, especially at high frequency switching.

    There are a number of different ways to achieve a faster turn off.

    1). Passively:
    Have a resistor from the gate to source of each mosfet. The lower this resistance the faster the turn off. With your schematics you only have one 10K resistor for 2 mosfets. You need to have a 10K resister from gate to source for each mosfet to discharge the parasitic capacitance of each mosfet. You can lower these resistances to 5.6k, but doing so, will waste more current, so this would be a trade off.

    2). Actively: This method will give you the fastest turn off. But you will need to modify your mosfet driver circuit. And add a PNP transistor, and have a "PNP and NPN" complimentary pair. Here is a suggested active mosfet driver circuit. You may have to experiment with the value of R1.
    Mosfet Driver_1.jpg

    For the capacitor pulsar a fast "turn on" is important, and for a coil (SSG coil) a fast "turn off" is important. Because capacitors and inductors(coils) are inverse of each other.

    Mosfets "turn on" fast and bipolar transistors "turn off" fast.

    I hope this helps

    Regards
    Nityesh Schnaderbeck
    *********

    Hello Nityesh and Happy Thanksgiving to you and yours ! :-)

    Thanks for kudos, but the hysteresis Cap is not my design, it is supposed to be an earlier design of John Bedini's Comparator Cap Dump Circuit, and was shown as a 1uF or 2uF fixed i recall, before he improved his designs and made it so cool he had to pot it in resin to keep pirates from making cheapo copies. It must be way space-aged to cost so much and not be serviceable per potted.

    The Zener you show to protect FET's i totally failed to consider. I have driven the Dump Caps up to 45V accidentally many times, and the FET's still work somehow. IRFP250 Max Vgs is 20V :-(
    I will try the Zener and check the FET's on Scope compared to some new ones to look for damage signs.

    I am tired from making turkey stock, and am still soaking your Active FET driver circuit suggestion and your LM358 direct Trani' drive. The slow turn-off Scope shot i showed looks same for bipolar MJL21194, where in both the Opto turns off sharply in comparison. I hear you stating Opto not needed. I just built as i saw, and it was the Tesla Switch that made me notice the slow turn off with both. So i will try to adapt this non-inverting LM358 to the 741 Op-Amp or try it after the 741. The 741 can go close to the Supply Rail for single supply operation, and the inverting configuration is needed to sink enough current, or so i have heard ;-). It looks like the 358 single large output voltage swing would be 12V for a 15V single supply.


    How fast can your bipolar MJL21194's (or favorite FET) switch on and off with this LM358 drive? Have you managed to speed up SG turn off times? Have you considered the PNP in series w. NPN for active turn-on and active-turn-off idea?
    1)NPN On (Active Turn-On)
    2)PNP Off (Active Turn-Off)
    3)NPN Off (Relaxation Turn-Off, when PNP already Off, so slowness no matter)
    4)PNP On (Relaxation Turn-On, when NPN already Off, so slowness no matter)

    The SG may be a work of art not to be altered, but the Tesla Switch needs fast turn off for non-resonant inductive loads, at least for Matthew Jones Simple Switch with 3-wire XFRMR to take advantage of BEMF ping-ponging. Thanks for your suggestions,

    Ward

  4. #114
    KOODOS to RS_ for the Schematic on the Bedini Mosfet Comparator circuit. I have built the circuit as per the diagram. There seemed to be an issue in the timing of the Op amp. The Mosfets would spill the caps to the battery only if the 100k pot was manually swung from one side to the other. That suggested that the downstream section of the circuit was functioning but it would NOT automatically sense the buildup of capacitor voltage. Capacitor C2 (.1 uf) was substituted with a number of larger values but the one that was settled on was a 100 uf. After making that change the circuit worked beautifully. I am wondering if the .1 suggestion was simply a typo or is there something I'm missing? My Capture Capacitors are twin 10,000 uf Caps in parallel yielding 20,000 uf total value. Even with the analogue meter indication of the pulsing action, I still wanted an LED visual as well. Initially a 510 ohm resistor in series with an LED was installed from the output of the op amp (pin 6) to the negative ground rail and it did function BUT it glowed during the off part of the cycle instead of the on pulse. That idea was removed in favour of an LED simply being installed in series with the pin 6 output of the comparator and R1 (1.2k) which feeds the front end of the opto-coupler and that solution behaves perfectly. I am very, very pleased with this circuit and with those couple of minor suggestions so far it performs flawlessly under test. Thank you RS_ for the original circuit. Any comments are welcome !
    lampliter14

  5. #115

  6. #116
    How do I match the mosfets???

    Finally I have all the parts for the comparator (RS_ snag 2 12 v). But I was wondering what is the right way to match the irfp260 ???

    tnx

    Happy new year to all!

    best

    Alvaro

  7. #117
    Senior Member Tom C's Avatar
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  8. #118
    Thank you Tom !!! Have a great new year!

    best,

    Alvaro

  9. #119
    rps20150104_152207.jpgrps20150104_152318.jpgrps20150104_152415.jpg

    Just finished the comparator circuit. I had the same issue that lampliter14 pointed out with c2 (.1uf).

    In my case I changed c2 to 2.2uf , with 100uf I had too much "on time".

    My capture cap is small. 35v 3300uf.


    Best

    Alvaro

    Pd: thank you RS for the circuit!!!!
    Last edited by AlvaroHN; 01-04-2015 at 03:29 PM.

  10. #120
    Brand new doubt: Does a 35v cap charges faster to 24v than a 25v cap??? (in the case that the 35v cap have the same capacity at 24v than the 25v cap at 24v)

    I ask this because the cap charge graph starts fast and end slowly... maybe is better to charge the cap until a % of the total charge and it would be faster? or is the same?

    best,

    Alvaro

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