Inductors and diode protection of transistor protection - a question.

Discussion in 'General Electronics Chat' started by charliedurrant, Jan 21, 2012.

  1. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Hi,

    I am very new to electronics and am trying to understand things from the ground up. I have been using the 'ebook' which is extremely good!

    My question today is about the arrangement of the protection diode with a transister when the load is a motor or relay (something which I believe has inductance).

    I understand that when the current is suddenly stopped through an inductor the magnetic field will collapse and current will be induced and will flow in the same direction as that of the current that caused the field to grow. Hence inductors resist change in current. This is paraphrasing the ebook.

    This all so far makes sense.

    Now it makes sense to have a diode to protect a transister when the current through the emitter and collector is prevented from flowing and an inductors field is collapsing. My questions are:

    1) why is the protection diode in parrallel, I would have thought it would be in series?
    2) why is the diode only preventing reverse current to the original current flow which seems counter intuitive (to me) with respect to the inductor causing current to flow in the same direction as the inductor charging current on magnetic field collapse.

    A reference circuit

    http://pcbheaven.com/wikipages/Transistor_Circuits/

    The ebook section on inductors

    http://www.allaboutcircuits.com/vol_1/chpt_15/1.html

    Thanks in advance and sorry if this is basic stuff....

    Charlie
     
  2. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Apologies for the repeat use of the word 'protection' in the title - I'm using an iPad and it's not so good for typing and review as the browser refreshes due to lack of ram when switching between tabs and input data is lost!
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    A parallel protection diode allows the current to go on circulating in the coil. The parallel diode restricts the reverse voltage across the coil, so that the total voltage seen by the switch transistor does not get too big.

    The coil current does not necessarily reverse: it decays, although particularly in the presence of larger parasitic capacitance there may be some ringing.

    I can't see how a diode in series with the coil helps: on switch-off, the coil voltage will go flying up above the supply, keeping the diode switched on.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The polarity reverses when power is removed, so if the + was going in the left side and - was on the right side, when power is removed, the left side will be - and the right side will be +

    This is why the diodes work for suppression, if the polarity was the same, a diode would only limit the voltage on the coil to a junction drop (0.6-ish Volts)
     
  5. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Adjuster thanks for the reply.

    Sorry - obvious

    I understand it decays - but my understanding is the direction of current flow is the same as before but there is a very high voltage.

    The following diagrams show my assumptions

    [​IMG]

    and then after the transister is switched off

    [​IMG]


    I think these diagrams represent what you say:

    1) Does the diode simply represent an easier path?
    2) Why use a diode? This is my main confusion? I realise it is a one way route for current but my learning of inductors is that the charge flow is the same direction as that which 'charged' it and the diode isn't stopping anything.

    Quote from
    http://pcbheaven.com/wikipages/Transistor_Circuits/

    "The protection diode Dp is used to protect the transistor from the reverse current generated from the coil of the relay during the switch off time"

    I'm starting with very little knowledge and this is exaclty the sort of conceptual problem that arises without any formal teaching...sorry

    Charlie
     
  6. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Thatoneguy, sorry I posted before your reply. I think this the crux of my issue with understanding. But the current flow is the same direction? I'm in no way disagreeing its just my poor understanding!
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Without the diode, the collapsing field of the coil will induce very high voltage on the collector of the transistor, which will ultimately result in voltage breakdown of the transistor.
    Imagine how the coil tries to keep the current constant, when you interrupt the current the voltage will rise until it finds some way to complete the circuit, the coil acts as a voltage-unlimited current source with no load. The current is "constant", load resistance infinite, so the voltage across the coil is infinite too.
    Without the diode, the circuit is completed either through the transistor or through arcing if the transistor withstands the voltage. With the diode, the current simply goes that way and the voltage rises only to Vf of the diode.
     
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  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The current in the coil cannot change suddenly: that includes not reversing suddenly. Whatever happens after the transistor is turned off, for some time the current has to go somewhere.

    The coil voltage can and does reverse. A diode will limit this reversed voltage to a small value, by providing a safe path for the coil current until it decays.

    The reversed voltage on the coil may be allowed to rise to a larger but still controlled voltage by using a diode in combination with a resistor or with a zener diode. This will make the coil current decay faster.
     
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  9. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Got it - many thanks to all for their replies - really appreciated.
     
    Last edited: Jan 21, 2012
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