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Protecting Mosfet from damaging by hi voltage spikes generated by inductive load

Discussion in 'General Electronics Discussion' started by turboturbine, Aug 14, 2013.

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  1. turboturbine

    turboturbine

    10
    0
    Mar 22, 2012
    MOSFET as ignition coil driver in electronic ignition, how to protect the MOSFET

    I am building a electronic ignition for my vintage motorcycle because I had trouble finding springs for my advance unit.

    It seems that nowdays they use IGBT's a lot that is some kind of darlington a mosfet and a transistor, those units are hard to get especially the ones that are dedicated for ignition systems, others lack a protective diode and are not logic level.

    I want to use a 600V mosfet with a small resistance 1 or 0.5 Ohm, but how can I protect this mosfet from damaging while it is in use?
    I have seen zener diodes between the Gate and Drain, do I need somtehing between the Source and Drain or not?

    I tried a so called freewheel diode in parallel with the ignition coil, it could me my fault but it seems to obstruct the coil from working properly :)
     
    Last edited: Aug 15, 2013
  2. turboturbine

    turboturbine

    10
    0
    Mar 22, 2012
    Look at what I found on ebay:

    http://www.ebay.com/itm/11093648478...ries&cmd=ViewItem&hash=item19d4546bae&vxp=mtr

    A DC CDI unit, and it is for a single cylinder engine with the pickup at the crankshaft not camshaft, that is very important...

    I think this is the way to go for me!

    Does soemone have experience with DC CDI units? can you just connect them to a battery and a pickup and you have a spark?

    those units are for gy6 I see
     
    Last edited: Aug 15, 2013
  3. john monks

    john monks

    693
    2
    Mar 9, 2012
    Typically I place a 5.1v or 6.2v Zener from the gate to source to prevent a MOSFET from blowing out when driving and inductor. To prevent over current from damaging the transistor you must rigorously prevent the duty cycle from exceeding a set duty cycle such as 50% and prevent the ON time from exceeding a few milliseconds.
    I can't help you with your second post.
     
  4. turboturbine

    turboturbine

    10
    0
    Mar 22, 2012
    Thank you, yes some mosfets in to-220 package can handle 1000 amps but only pulsed :)

    I found this:

    [​IMG]

    and this:

    [​IMG]

    There is no microcontroller in those units, is this normal? is this some kind of analog advance/retard circuit?

    The unit you see in the higher half of the lower schematic is the HV inverter for charging the high voltage capacitor

    Those DC units are 8 dollars so that is not the point but it will take a month before I can even start I think and it will be winter soon :mad:
     
    Last edited: Aug 15, 2013
  5. john monks

    john monks

    693
    2
    Mar 9, 2012
    I don't see an advance retard circuit circuit for ignition timing. What I do see is that, in the lower schematic, when the pickup coil puts out a positive voltage two unpacked transistors turn on causing the operational amplifier output to go positive causing the SCR to turn ON placing about -200 volts into the primary of the ignition coil and at the same time turning off the 200 volt charging circuit for the capacitor so that the SCR can turn off. The 12 nf capacitor will keep this circuit off for several microseconds so that the 1 uf capacitor can finish discharging so that the SCR can turn OFF.

    I don't see any need for a micro controller in this circuit so I assume this is normal.
    Frankly I have no experience with this type of ignition system so maybe someone else will no better.
     
  6. turboturbine

    turboturbine

    10
    0
    Mar 22, 2012
    Yes it is really strange, but I know because I have replaced a CDI in the past that there are also CDI units which limit the rpm that is because mopeds are normally not allowed here to go faster then 50km/h and I am almost certain now that those are also analog, if those new units are, then that 10 year old unit is certain analog it was installed on a chinese chang jiang, looks like a Honda monkey.

    I do not know if the faster unit I installed did advance but i know that the standard unit did limit the RPM by distorting the ignition at something like 4500rpm.... and really sharp I can tell you that!

    this is a video of a advancing gy6 ignition


    And it is not really strange that those scooter have a advance ignition because that is what you need on a bike which needs performance, only high compression two strokes and low power lawn mower engines can do without maybe.

    and see this, also looks analog:
    http://www.ebay.nl/itm/UNRESTRICTED...arts&hash=item53ea9a0343&_uhb=1#ht_9020wt_956

    There are more similar items....
     
    Last edited: Aug 15, 2013
  7. turboturbine

    turboturbine

    10
    0
    Mar 22, 2012
    I think you where right!
    I now read that not all units do advance, really stupid but true, that does not take away that there have been several other system in the past not only ignitions with microcontrollers, I think really strainge systems with several alternator charging coils and so on.

    Thank anyway.
     
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