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Current Comparator

Discussion in 'General Electronics Discussion' started by Rishon Benjamin, Jun 23, 2015.

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  1. Rishon Benjamin

    Rishon Benjamin

    Jun 23, 2015

    I am very new to the field of circuits and electronics, so I apologize if I sound very simplistic. I need help with the following:

    I would like to create a current comparator that feeds an NPN transistor. Basically, only if a certain current or above reaches the comparator, it should be allowed to pass through and activate the transistor.

    I am unsure on how to go about this.

    Any help would be appreciated!

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to electronicspoint.

    The answer to your question hinges on the required accuracy. A simple solution is a series resistor (in series with the current, such that R=0.6V/Imax where Imax is the current that triggers the transistor.
    The transistor itself is connected with base and emitter across the resistor, base to the positive pin of the resistor (i.e. where the voltage drop is positive with repect to the other pin). In this way the transistor will start conducting once the curent reaches a value that generates 0.6V voltage drop across the resistor.
    Note that this method will drop a comparatively high voltage across the resistor (0.6V) and produce accordingly lots of energy loss (heat to be dissipated) at high currents.

    More refined methods use a resistor, too, but a much smaller value to reduce losses. The voltage across the resistor is then amplified (e.g. by an operational amplifier) and evaluated to produce the desired action (e.g. turn off a load). Your favorite search engine will throw lots of hits using the keywords "current sense circuit".
  3. Rishon Benjamin

    Rishon Benjamin

    Jun 23, 2015

    Many thanks for the quick reply! I don't really need that much accuracy. As such, I would like to try the simpler solution first. Let me provide a little more information.

    I have a 9V supply that sees a 30k resistor which leads to the base an NPN transistor. I would like to have the current reach the base only if the resistance is less than or equal to 30k. Most likely due to my inexperience, I don't quite see how having an additional 0.6/Imax in series would prevent this?

    I will definitely take a look at current sense circuits! Thanks again!
  4. morphingstar


    Mar 1, 2012
    You need a switch circuit, as I understand your question.
    Using a switching transistor this rule applies: the emitter current will follow the base current. If the base current is at maximum available and the load draws even more current, the emitter collector voltage will rise, this will kill a power transistor. The difference in the 2 currents is the amplification factor.
    To make sure your emitter voltage will not rise out of saturation level it needs sufficient base current. Resistors have no switching function. Depending on your circuitry you might use a zener diode in front (in series) of the base. When the zener source receives sufficient voltage it will conduct and immediately feed the full available current to the base. This current is to be limited by a resistor in series, I would recommend 150% base current to be available as needed by the emitter / collector circuit. So base current = emitter current divided by amplification.
    Of course you can cascade transistors to increase amplification factor (functional series circuit) advice on this is out of my range.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    That's inverse to what a current sense resistor in my first simple approach would do.
    • What exactly do you want to measure? The load resistance (30k), the current through that load resistance, or the voltage across this resistance?
    • What is this load and why does it vary?
    • What do you want to do once "the resistance is less than or equal to 30k"?
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