Need help with "goes high"/"goes low" definitions

Discussion in 'Homework Help' started by felicia, Dec 18, 2012.

  1. felicia

    Thread Starter New Member

    Dec 18, 2012
    I'm taking an online course and some very basic stuff has been left out. In a recent test, I was asked about when a relay was activated by a transistor if the collector goes high or goes low. The problem is that I was never taught these terms.

    The professor for the class defined them as:
    "Basically when a transistor goes high (NPN) that means that it is turned on and conducting. Going low (NPN) means that the transistor is off and not conducting."

    This doesn't explain why the relay pulls in when the collector goes low. Can someone please either explain these terms or point me to a page that does? Thanks so much! I've been trying to get my professor to give me a more clear explanation for about a week, and he just won't do it. I feel like an idiot, but I want to learn this.
    -- Felicia
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    Your professor seems to be quite confusing. A transistor does not "go high" or "go low". As far as the collector going high or low, it depends on how the transistor is connected in a circuit.

    If we are talking about a low-side driver implementation for a NPN transistor, then, when the transistor is turned on, the collector will be brought low as in something like this.

    However, "go high" and "go low" are digital logic terms, dependent on which logic family one is using, TTL, ECL, RTL, ETL, etc. So the collector may be going lower than it was, but that doesn't mean it is logic low. Going low is relative.
  3. MrChips


    Oct 2, 2009
    High and low can mean anything in electronics and has to be defined.

    In boolean algebra, there are two states, TRUE/FALSE, 1/0, HIGH/LOW.

    When translated to electronics, these can refer to any voltage, current, resistance, switch condition such as OPEN/CLOSE, conducting/non-conducting, ON/OFF etc.

    Hence the terms HIGH and LOW must be defined when applied to electronics.

    If a HIGH means logic = TRUE when a relay is activated
    and the relay is on the collector side of an NPN transistor
    then your professor's definition of HIGH would be acceptable.
  4. felicia

    Thread Starter New Member

    Dec 18, 2012
    My issue isn't that I think the professor is wrong. It is just that I don't understand what exactly going low means (when not applied to boolean logic). If we were talking about boolean logic, then the collector going low would not turn on the relay, right? Because the collector would not be conducting.

    Ahh, perhaps this will help. Using my professor's definition, I would think that any part that goes low is reverse biased, as that would keep it from conducting. However, that is not the case (as far as I can tell).

    tshuck, when you say, "So the collector may be going lower than it was", can you explain what about the collector is going lower? I'm guessing that it is relative to the base, but I don't understand what exactly is going low in the collector.

    Thanks so much for your help!
  5. tshuck

    Well-Known Member

    Oct 18, 2012
    I should have said before, I'm referring to the voltage of the collector with respect to ground, but, again, this is dependent on the way the transistor is hooked up...

    ...just ambiguous...
    felicia likes this.
  6. #12


    Nov 30, 2010
    "going high" and "going low" are almost a form of slang. They are an abbreviation for "the voltage on the collector" goes high (or low). If you say the whole sentence, it stands a better chance of being understood.

    In the attached circuit, when the signal voltage on the input to the base "goes high", then the voltage on the collector "goes low".
    felicia likes this.
  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
    Generally speaking, if you're connected to a 5 volt power supply, "LOW" is anything below about 2 volts, and "HIGH" is anything above about 4 volts. A signal between 2 and 4 volts is considered "floating" and can cause undesired operation of the circuit. That is the purpose of pull-up/pull-down resistors. It holds the node beyond the outer limits, and out of the "floating" range. This way, it's either in one state or the other.

    Hope this helps.
    felicia likes this.
  8. RS-232D

    New Member

    Dec 1, 2012
    I agree with MrChips, it has to be defined first and in what regards. in a signal you can think of it as high and low, on or off, binary 1 or 0,etc. you are mentioning a transistor however and the collector going 'high' or 'low' thus allowing current flow. When you apply current to the 'base' ie the center stem of the transistor, electrons will move from the N negative side to the positive side, thus allowing current flow. As for you mentioning bias. with a NPN transistor, the base has to be positive bias to the emitter, and the collector has to be more positively bias then the base. Derstrom8 gave a great definition for practical purposes, imo. oh. and the answer is it goes high, for most intents and purposes, btw.
    Last edited: Dec 18, 2012
  9. vk6zgo

    Active Member

    Jul 21, 2012
    These terms have been around for the best part of 100 years,firstly used for vacuum tubes,then latterly for semiconductors,so have become part of the commonly understood "technical jargon".

    A Professor should make sure his/her students understand the exact meaning,whenever he/she uses "jargon",as they have not been immersed in it for years,as many of us others have.

    When the transistor is turned on "hard",so that it is fully conducting,its collector voltage goes "low",because the transistor looks like a very low resistance.
    As this "resistance" is negligible compared to that of the relay coil,the relay operates.

    When the transistor is not conducting,("cut off"),collector voltage goes "high",because the transistor looks like an open circuit ,so the relay cannot operate.

    It is difficult to describe things without using a little jargon.
    In this case,I deliberately used "turned on hard" instead of the more correct term "saturated",because the latter has another,"special" meaning when describing FET operation.
  10. vk6zgo

    Active Member

    Jul 21, 2012
    I think your Professor's original definition was intended to refer to the input to a transistor circuit.

    Assuming a normal base input for a NPN transistor in "Common Emitter" configuration ,when the input went "high"(say,+5V), the transistor would turn"ON".

    This would cause the collector voltage to fall,so a Common Emitter circuit is an Inverter.

    So,if you applied a logic "high" which was a high enough voltage to turn the transistor "ON",it would operate the relay,which could present a "high" or "low" (or any other voltage level) at the output of its contacts,depending on how they are wired to any external circuit.