Power FET

Discussion in 'General Electronics Chat' started by Sami Alawadhi, Oct 5, 2015.

  1. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    We are building a circuit for an NE555 timer and our ignition coil's voltage is 20kV. Is anyone aware of any power FETs that go up to 20 kV? If not, are there any that go up to 10 kV? What's the highest? Our issue is that the NE555 timer burns out when we connect it to the spark ignition circuit, and the issue is with the load. We need the FET to rectify this issue.
    Are there any skilled engineers that have worked with ignition coils?
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    First, there are some special mosfets the operate at bout 4500 volts. I am sure these are very pricey. The typical mosfet maxs out at about 1200 volts.

    What are you trying to do? Be more specific.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    No transistor will handle 20kV or even 10kV, but that voltage is at the secondary of the coil. You normally control the primary with the transistor where the voltage is 100 times less.

    Post your circuit schematic.
    It's very difficult to understand what's happening from a written description.
     
  4. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    We are building a spark ignition circuit for a combustion chamber. The battery is 12V and supplies a current of 8 amps. Like I said, the timer burns out when connected to battery, and we need a way for it to not burn out. It doesn't when the load is disconnected. The thing is our ignition coil is 20 kV. If there is a way we could deal with this, please let me know. Our schematic is attached.
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    What is not clear on your schematic is that pin 1 of the 555 needs to be connected to ground/source of the fet (MN1). This must be done with the shortest wires as possible. I suspect that this my be the root of your problem, but not all of it.
     
  6. Bordodynov

    Active Member

    May 20, 2015
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    My advice to you. Use the diode capacitor multiplier. The parasitic transistor and winding parameters overy difficult to obtain high voltages. Another method is the use of resonance (method of Tesla).
     
  7. crutschow

    Expert

    Mar 14, 2008
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    What are the values of Vdd and Vss?
    How are they connected to ground?

    What exactly is L1, the ignition coil? Is that just an inductor as you show?
    I would suggest a real automobile ignition coil there if it's not.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The standard ignition 'coil' is in fact a step-up transformer. Use the FET to switch the primary (low voltage) and connect the spark gap to the secondary (high voltage). The back-emf generated by the primary winding should be of the order of a few hundred volts; not kilovolts.
     
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  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    If your design requires that kind of devices, you have designed it wrong.
     
  10. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    Yes. L1 is the ignition coil.
     
  11. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    Here's a schematic of our spark ignition circuit. The switch will be replaced by the timer so that we can ignite the spark for a very short amount of time. We'll utilize two ne555 timers to achieve a pulse for only 20 microsec. You can check the timer configuration in the previous schematic.

    I've found the link below which shows another schematic for an ignition coil circuit.
    http://www.instructables.com/id/super-simple-ignition-coil-drivers/step6/another-555-timer-driver/

    What is the optimal solution and easiest approach to solve this issue?
     
  12. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    What is a diode capacitor multiplier and what's the use of resonance?
     
  13. Roderick Young

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    Feb 22, 2015
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  14. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    So, is that the issue? Is that what's causing the timer to burn? What remedies could you offer?
     
  15. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    Also, we're working in monostable mode.
     
  16. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Running a simple automotive ignition coil off of a single 555 timer to make a spark generator is easy. Your way over complicating what should be a simple one 555 timer IC circuit and process.

    Also when dealing with driving a highly inductive load like a ignition coil off of the same power source as any IC is powered by you need some degree of input power filtering on the positive side to keep the high voltage inductive pulses from getting to the control circuit.

    That can usually be done with a simple diode in series with a few ohms to tens of ohms resistor followed by a good sized capacitor on the control circuit side. Lastly adding a diode across the part of the circuit that is controlling the coil across the positive and negative leads going into the coil and out of switching device will help keep the coils inductance currents under control and away form the rest of the circuit.

    If done right you should be able to make a highly reliable 20+ KV spark generator with one 555 IC and under 15 components including the ignition coil and switching device. ;)
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Using such a short pulse will not work well with an ignition coil in the flyback mode, as yours is.
    Such a coil has a time constant of several milliseconds and the transistor needs to be ON for at least that amount of time to allow the primary current to build up and store the inductive energy for the spark.
    The spark occurs when the transistor turns off.
    You have no significant control over the spark duration.

    If you want a shorter spark then you should use a capacitive discharge circuit where you charge a capacitor up to 200Vdc or so and then suddenly discharge the capacitor into the coil primary (typically using an SCR) to generate the spark.
    Such a spark is of significantly shorter duration than from the flyback circuit.
     
  18. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    Thank you. What diodes do you recommend I get? I've already placed orders for 5-V zener diodes. Here's the link:
    http://www.amazon.com/LM78L05-SOT-8...qid=1443902255&sr=8-8&keywords=5v+zener+diode

    Also we're ordering 1k, 10k, 100k, and 470k resistors along with .1 uf, 1uf, 1nf, 10nf capacitors for our circuit so that the circuit will automatically shut off after a very short amount of time (microsecond range).
    The link to the power FET we ordered is here; it has a drain-to-source voltage of 2.5 kV:
    http://www.digikey.com/product-search/en?KeyWords=IXTH02N250-ND&WT.z_header=search_go

    Please let me know if I'm on the right track. We are behind on our experiment and we'd like to remedy this situation as soon as possible.

    Thank you
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    If your spark coil is anything like an automotive ignition coil; your transistor only needs to handle about 300 - 600V on the primary winding. You can series connect a few 75V zeners to protect the device from breakdown.

    Back in the CRT era, bipolar transistors rated upto about 2kV were fairly common, but in some cases the gain could be as low as 2.

    MOSFETs rated to about 900 - 1000V aren't exactly rare, they were fairly common in flyback SMPSUs in some of the bigger VGA monitors. A car or motorcycle ignition coil needs somewhere in the region of 8A - I've seen 9A 900V MOSFETs, but 6A types are much more common, MOSFETs are much easier to parallel than bipolar.
     
  20. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    What does CRT era mean?
    Actually, we've ordered 5v zener diodes (15 pieces). Here's the link:
    http://www.amazon.com/LM78L05-SOT-8...qid=1443902255&sr=8-8&keywords=5v+zener+diode

    Is this good?

    Also the FET type we ordered is the MOSFET N-channel metal oxide and its drain-to-source voltage is 2.5 kV. Here's the link. Please tell me if it is sufficient. If so, how exactly do I apply it:
    http://www.digikey.com/product-search/en?KeyWords=IXTH02N250-ND&WT.z_header=search_go

    Thank you.
     
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