connecting a diode to a circuit

Discussion in 'General Electronics Chat' started by Sami Alawadhi, Oct 15, 2015.

  1. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    I bought these zener diodes and I just want to know how to connect them to the circuit breadboard. They have these three really tiny pins. I've posted a link:
    http://www.amazon.com/LM78L05-SOT-8...qid=1443902255&sr=8-8&keywords=5v+zener+diode
    We tried soldering the three pins with wires, but it's very difficult and one of them kept falling off, so I don't think that method would've worked anyway. Any recommendations?

    Also, we're ordering these power FETs from digi-key and we would like to know how they're connected. Which pins go where? We are trying to put an FET between the output of an NE555 timer and a HV ignition coil so that the timer won't burn out. Can someone just tell me how the FETs should be placed. I've attached a schematic of our circuit and the spec of the FET we ordered.
    http://www.digikey.com/product-search/en?KeyWords=IXTH02N250-ND&WT.z_header=search_go

    . Spark1.jpeg
    Thank you
     
  2. pwdixon

    Member

    Oct 11, 2012
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    The 7805 is not a zener it's a voltage regulator, not sure what you are using it for it's not on your schematic.

    Those FETs are really expensive, of course you did realise your schematic is an NPN transistor not a FET. I can see why you might end up with this FET to get the high voltage but the current might not be enough with Rdson is high, depends on the coil. You need to connect the drain to the coil, the source to ground and the gate to the 555 output.
     
  3. Sami Alawadhi

    Thread Starter New Member

    Oct 5, 2015
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    Thank you.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What reason did you select such a high 2500v Mosfet?
    See IRF640.
    Max.
     
    Last edited: Oct 15, 2015
  5. InspectorGadget

    Active Member

    Nov 5, 2010
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    Aren't you going to get one heck of a reverse-emf kick from the ignition coil? You need a high Vds or Vce device and/or a heavy-duty flyback diode across the coil primary.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    And the 3055 as shown doesn't have the breakdown rating to handle the back emf from an automotive ignition coil.

    The ST website shows several bipolar transistors specifically designed for automotive ignition circuits.

    At a pinch - the chopper transistor from a flyback type mains SMPSU should handle the voltage.

    In the days of big MF CRT PC monitors, you could salvage SMPSU MOSFETs rated 9A 900V - these did very nicely as IGN coil drivers and didn't need any clamp zeners for protection.

    6A 600V types are still fairly common which might just about handle the back emf - MOSFETs parallel very easily so making up a 9A rating isn't a problem.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A transorb across the D & S will prevent the kick back, but a IRFP460 could also be used it is 100% avalanche rated so it will clamp D-S voltage near over 500 V itself without any damage so a transorb is not needed.
    There are many designs out there using the lower rated Mosfet.
    Max.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    There's quite a few devices that can handle the primary back emf of an ignition coil, but its as well to remember that the voltage peak is clamped by the spark gap and transformed back to the primary.

    Excessive size plug gaps will increase the peak voltage, and a fallen off HT lead leaves one of the peaks not clamped at all.

    You have to design for the worst case scenario.

    Some manufacturers offer devices specifically designed for driving ignition coils, or you can use a bog standard SMPSU transistor, and if necessary put 4 or more 75V zeners in series across the transistor.
     
  9. InspectorGadget

    Active Member

    Nov 5, 2010
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    I would submit that you don't need a transorb, that a high-current silicon diode would work just fine for the inductive kick back, as long as it could tolerate the peak conditions as Ian suggests. A transorb works when you need to suppress spikes in both directions. Do you have a spike in the same direction as the current flow? I think you only get a reverse spike when the field collapses.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Ironically originally points had a plain .1μf cap.:rolleyes:
    Max.
     
  11. nigelwright7557

    Senior Member

    May 10, 2008
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    Don't put a diode across the coil it will kill the current and you will get a weak output from the coil.

    You need about a 1000 volt mosfet to drive the coil. The required back emf will be at least 400 volts as the mosfet turns off.

    How do I know ? I designed an ECU for a Honda motorbike many years ago.
     
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  12. InspectorGadget

    Active Member

    Nov 5, 2010
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    OH! I get it now. Very good point. If you quench the back EMF it'll quench the secondary.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    They could be anything up to about 0.25uF.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    With the original contact breaker - the main point of the capacitor was to slow down the back emf rise time so the points separated quickly enough to quench the arc on the primary (LT) side.

    It isn't so obvious looking at it - but a spark on the points clamps the back emf and severely attenuates the HT voltage.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would expect the effect would extend the conduction time on the primary.
    Provide a path to common after the points open.
    My understanding it was also to prevent contact burn.
    See Kettering system.
    http://bmwvmca.org/joomla/index.php?option=com_content&task=view&id=244&Itemid=37
    Max.
     
    Last edited: Oct 16, 2015
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