PWM with reversed polarity MOSFETS..Please give me some help!

Discussion in 'The Projects Forum' started by AlgoryThm, Oct 9, 2011.

  1. AlgoryThm

    Thread Starter Member

    Apr 23, 2008
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    Hello! Although I'm a member of this forum for some time and I have been helped a lot, it's the first time I need to post something...So, I need your advice for a circuit I'm working on. Any help will be much appreciated!

    What I want to do is to drive two ignition coils in anti-parallel formation. So I found the basic PWM principles and customized it to my needs. The circuit has two different sources (220VAC to 12VAC transformer rectified to 12VDC), a 555 timer with a 393 comparator in order to keep the frequency and the duty independent to each other. The two sources drive two different IRFP460 MOSFETS.

    Now, what I need to know is:

    1)Do I have to reverse the polarity of the second mosfet (the one at the bottom) as well, or I can just connect the coil with reversed polarity?

    2)The switching current (amperes) of the MOSFET or the transformer's max. current has to be larger? In other words, a MOSFET with max 20A switching current and 10A transformer or 15A transformer with 10A MOSFET combination would be better?

    The schematic is this, thank you once again for any help!

    [​IMG]
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    The major problem I see here is that you have only the gate connection of Q2 joining the two halves of the circuit. You need to establish a common ground before this can work.
     
  3. AlgoryThm

    Thread Starter Member

    Apr 23, 2008
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    Hmm...I see your point but what would be the point to have an extra source then? I could connect the two MOSFETS in parallel and avoid the use of the second transformer...So you say it's impossible to work like this?
     
  4. BJT_user

    Member

    Oct 9, 2011
    35
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    As it is, AlgoryThm, it will not work. For the two independent power supplies and drivers to work off a common signal, you MUST connect the two negative supplies together to make them common as well. Remember, current requires a complete path to complete a circuit. Withiout a common ground, the lower mosfet will have no reference for the trigger signal and will be relatively unresponsive to it. It would be like taking a volt meter and connecting the red wire only, to the positive terminal of a battery. The meter won't read any meaningful voltage until you get the negative terminal connected to the negative part of the battery. The gate of the lower mosfet can be likened to the positive terminal of a volt meter. It has to be positive to open up the source-drain path, but it has to be positive with respect to its source. If the source path is not made common to the signal generator's circuit, the gate will have no idea what potential is on the signal wire. Just like connecting only the red wire of a volt meter to a battery.
     
  5. AlgoryThm

    Thread Starter Member

    Apr 23, 2008
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    So both of you are right and now I know why. Thank you BJT for the comprehensive explanation, it helped me a lot. Now, if I connect both grounds (after 4700uF caps) together, what about the other two questions? Do I need to reverse the drain/source of the second MOSFET or just the polarity of the coil? And what about the transformers?
     
  6. AlgoryThm

    Thread Starter Member

    Apr 23, 2008
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    So, I made a few corrections...How does it look now?

    [​IMG]
     
  7. BJT_user

    Member

    Oct 9, 2011
    35
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    My pleasure AlgoryThm. Glad to help out. As for your circuit, it looks sound now. The MOSFET drivers on both coils are fine as they are. You don't need to invert the lower one. It is a mirror of the upper one and should function just the same, as long as the power supply voltages are exactly the same on both.
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    @AlgoryThm. Could I ask what your making? You do know that driving a ignition coil in reverse polarity will cause it to overheat, if its on for a while.

    Back in the days of cars with ignition points wiring a coil backward was the fastest way to burn one out.
     
  9. crutschow

    Expert

    Mar 14, 2008
    12,993
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    Whether on not the ignition points are affected by polarity, the coil isn't. It doesn't care which way the current goes through it and will not overheat more one way than the other. The only effect will be that the output high voltage will be reverse polarity.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Well, the old-style coils WILL overheat if they don't have some kind of current limiting.

    All of the US manufacturers from at least the early 60's up to the early 70's had a solenoid that would bypass a ballast resistor for a really "hot" spark during starting, and once the starter was turned off, the ballast resistor limited the current. Can't remember offhand, but I think the resistor was ~2/3 the resistance of the coil primary.

    A quick Google search found this:
    http://www.imperialclub.com/Repair/Electrical/coil.htm

    Here's one type of ballast resistor for a Chrysler :
    [​IMG]

    That one's for a 1967 CHRYSLER NEWPORT 6.3L 383cid V8

    Ford used a resistive wire instead of an actual resistor. I think GM did as well.
     
  11. AlgoryThm

    Thread Starter Member

    Apr 23, 2008
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    @BJT_user: Man, thank you so much!

    @shortbus: Of course, no problem. Well I'm making a prototype device which uses two ignition coils to give power to two neon/argon tubes. But when the coils are in anti-parallel connection and the tubes at a distance no longer than 10 feet one to another, many strange phenomena occur due to the standing waves...So I want to experiment with it!;)

    @crutschow and SgtWookie: The type I have is the oil one, so don't worry about high temps! :)

    A last question though...I have 2 spare PC PSUs, one exact copy of the other. Same brand, wattage, specifications. Could I use them in parallel with common ground, instead of regular transformers with rectification stage? Because I saw regular transformers at 10A cost a lot of money here...
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The negative pole of the coil secondary is tied internally to the negative pole of the primay. So when wired backward the secondary has to spark internally to complete the circuit. Thats what causes the extra heat in the coil.
     
    Last edited: Oct 11, 2011
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The old oil-filled coils are those that I was referring to. They're basically cylindrical in shape, with a tower, and + and - terminals on either side.

    You can connect the grounds of the two supplies together, but keep the two +12v separate.
     
  14. AlgoryThm

    Thread Starter Member

    Apr 23, 2008
    22
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    @SgtWookie: Hmm...Ok, I'll keep an eye on them...I have tried to feed them with 50W amplifier in the past and I was able to work with them for 20mins or so.They were a little bit warm but nothing to worry about...Never tried them anti-parallel though...

    Thanks for the info btw (about PSU)!I'll give it a try and let you know how it goes...
     
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