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High Side VS low side

Discussion in 'Electronic Basics' started by Abstract Dissonance, Mar 18, 2006.

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  1. http://www.national.com/pf/LM/LMD18400.html

    I do not understand what the High Side Drive really is doing?

    It says on page 8 that high side drive can protect from being "energized"...
    what is energized? The way I see the circuit is that if the load is to
    high(low resistance) then there will be a lot of current and then the
    circuitry can sense this and turn use some type of circuit limiting method?

    If thats right then I do not get the low side drive. Looks to me, with the
    circuit that is shown, there is no difference. Cause it just shows that the
    load is shorted before the switch but I see the same can happen with the
    high side(if it is shorted to ground before the switch in the high side
    drive circuit)? Although is the current limiting is inside the circuit then
    I do see a different... i.e., no way to short before the circuit limiting
    circuitry.

    Is this making sense?

    Thanks,
    Jon
     
  2. (snip)

    In a system that is enclosed in metal (like an automobile), the most
    common short circuit on any wire is a short to the grounded body. If
    the load is permanently attached to the hot wire, and the power is
    switched in the ground wire (low side switching), any short to ground
    on the wire that gets switched off and on will just look like the
    power switch is on, and the load will be energized (turned on).

    If the load is permanently connected on its ground side, and the power
    is switched through the hot or positive lead (high side switching),
    and the switched wire gets shorted to ground, the high side switch
    circuit will see the load in parallel with the short to ground, when
    it tries to turn the load on, and the excess current can be sensed and
    the switch turned back off.
     
  3. Brian

    Brian Guest

    An energized load, means that the load is turned on.

    On the high side drive, if the wire that is going out to the load is
    shorted, that would be the same as if you put a shorting wire across the
    load. Nothing would happen until the drive tries to turn the load on (in
    which case the drive would see a short) and the + supply would see a short.

    On the low side drive, if the wire that is going out to the load is shorted,
    the short would be doing the same thing that the drive would be doing
    (shorting the ground side of the load to ground). This would turn on the
    load, before the drive turns it on. As far as the load is concerned, it is
    being told to turn on. As far as the + supply is concerned, there is no
    short (only a load being turned on).
    Hope this helps
    Brian
     
  4. Ok guys, I think I got it.

    Thanks,
    Jon
     
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