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Electronics Help me create a switch

Discussion in 'Modding' started by Glider, 26 Nov 2008.

  1. Glider

    Glider /dev/null

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    Hi electronics guru's!

    As you all might have guessed I need some help designing a high speed switch for work. Currently we use specialised gear that can switch up to 15A of current within 0.1 second. This is done using a diode, backed up by some fancy electronic to control it, like this (block wise)

    [​IMG]

    I want to piggyback a relay or thyristor on it so that it can switch currents up to 50A and work bi-directional (current wise). It's critical for our measurements to keep the switching times low (<0.1s). The typical voltages it will switch are up to 48V DC. I know I'll need an external power source, and I'd prefer to use a typical battery pack, because it has to be mobile.

    Can anyone draw me up a diagram with the needed components to achieve this? I'll be handing out e-cookies (or Linux admin favors ;)) to the ones that can help me out.

    PS: I have basic electronics knowledge, but good power electronics knowledge, I'm a MSc in Electromechanics but not electronics ;)
     
  2. Smilodon

    Smilodon The Antagonist

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    Can't you use a standard high power solid state relay?

    What's this for, anyway? High currents and high speeds are always interesting :)
     
  3. Glider

    Glider /dev/null

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    Well, it's for inspecting the coating of a pipeline. We put a specific signal onto the steel tube, which is (in this case) 1.6s on, 0.4s off, but it varies with the location/set up. The current switch (which I linked to) is specialised for this, and 100% configurable, but it uses a diode (current can only flow in 1 direction) and it can only switch 15A.

    We need 50A and bi-directional current flow for a measurement at a specific location, all in a portable package.
     
  4. cpemma

    cpemma Ecky thump

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    This "bi-directional" coupled with "48VDC" is confusing me. AC or just reversed polarity DC occasionally? The problem with triacs on DC is turning them off. For reversed polarity applications a H-Bridge would be the standard solution, and there are several IC drivers available to help switch high currents.
     
  5. Glider

    Glider /dev/null

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    Well, it's easy to illustrate the bi-directional part. Say a pipeline crosses an electrical railway. Under normal circumstances the power source puts a current on the pipeline, at 10 to 48V. But when a train passes, the (>200A) current from the train goes straight into the ground, onto the conducting pipeline underneath it.

    Inside the power source (which isn't managed by us) there is a "draining" mechanism that releases the high current (50A by the time it has reaches the source) onto the railway electrical circuit. But a diode would block the current, and this is quite an influence in our measurements along the pipeline. That's why it has to be bi-directional.
     
  6. SkOTT

    SkOTT New Member

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    What about a Darlington Pair? If you use fairly beefy FETs then there should be no problem in handling that much current.

    Wikipedia knows more than I do.
     
  7. Cinnander

    Cinnander New Member

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    Why not put another of the same diodes in reverse-parallel?
    Or have I missed the point :S
     
  8. Glider

    Glider /dev/null

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    That would only allow 15A, which is still too low
     
  9. chzel

    chzel no comments...

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    How about Mosfets?

    Hi Glider, I am not sure I really understood your requirements, but as I see it, a single N-channel Mosfet would be adequate (something like this one: clicky).
    That mosfet has max drain-source breakdown voltage of 100V so it should handle 48V easily. Max drain-source current is 127A @25C (not a realistic temp) and 90A @100C (more likely).
    There is an internal diode reverse biased to handle the reverse current. You can drive (switch) it with anything from 10-15VDC. A simple transistor should be plenty fast for your timing requirements.
    Since it is a N-chan Mosfet you will use it to switch the low side of the "load", like this:

    Positive of 10-48V-----> pipeline (or whatever) ----->Mosfet ----->Ground (negative)

    I hope I was of some help!
    Christos
     
  10. Glider

    Glider /dev/null

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    This sounds like something useful. I'll look into it when I'm at home (currently away for a week for the job).
     
  11. Glider

    Glider /dev/null

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    I ended up using a 70A car (starter) relay and an external car battery. Works like a charm. It's even a ghetto setup held together with ty-raps!
     
  12. chzel

    chzel no comments...

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    Damn...:duh:
    I always fail to find the simple solution!:blush:
    Glad you got it sorted out!
     

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