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H-Bridge - diodes (or rather the lack of)

  1. May 28, 2017 #1
    G'day all!

    I've got an H-bridge here from an old motorised toy.. as I want to use it for something else and the RF receiver is no use to me for this project i figured i' strip the H and stripboard it.

    I noticed though that there are no diodes to protect the four transistors (yes they are transistors - looked up the part numbers).

    Now I thought this wasn't supposed to work.. but this toy has survived years.

    Any ideas?

    Thanks,

    Dixo
     
  2. jcsd
  3. May 28, 2017 #2

    davenn

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    what is the type # of the transistors ?

    I have seen the occasional bipolar transistor H-bridges without diodes ... it is surprising that they don't fail

    if they are mosfets, then they have built in diodes by default and don't require external diodes eg .....

    h_bridge_cd4011b.gif

    I should qualify that by saying that not all MOSFETS have built in diode, many do
     
  4. May 29, 2017 #3
    Hi dave

    They are H772 and H882 - bog standard npn and pnp basically.

    Yes I was quite surprised too because i understood that without diodes they were guaranteed to fail.
     
    Last edited by a moderator: May 29, 2017
  5. May 29, 2017 #4

    davenn

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    never heard of them .... cant be very standard

    do you have a link to a datasheet please ?
     
  6. May 29, 2017 #5
  7. May 29, 2017 #6

    davenn

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    cheers :smile:

    definitely ones I have never heard of before
    appear to be a Chinese semi manufacturer

    will be interested in seeing how well they last for you

    you could put diodes across the transistors if you wanted to ... it would give some peace of mind :wink:

    H-Bridge-6.gif

    they used 1N4148 .... I would prefer to see 1N4001 series diodes in there as they are much more robust and commonly used for that and similar purposes
    1N4001 - 4007


    Dave
     
  8. May 30, 2017 #7

    jim hardy

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    I think it's a case of getting away with sloppy design.
    Remember that a bipolar transistor when polarity is reversed will still act like a transistor, just with poor gain.

    HbridgeReversed.jpg
     
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