Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    2
    ReputationRep:
    Hey finding the thevenin equivalent circuit and am wondering if the 10 and 15 ohm resistors are in paralllel or series?Name:  circ.jpg
Views: 140
Size:  9.8 KB
    Offline

    2
    ReputationRep:
    Series as they aren't parallel to each other
    Offline

    14
    (Original post by Squishy®)
    Series as they aren't parallel to each other
    Lol good one.
    Offline

    11
    ReputationRep:
    (Original post by ErniePicks)
    Hey finding the thevenin equivalent circuit and am wondering if the 10 and 15 ohm resistors are in paralllel or series?Name:  circ.jpg
Views: 140
Size:  9.8 KB
    In the Thevenin equivalent circuit, you will have replaced the voltage source by a short circuit. So the 20 ohm resistor will be in parallel with a 0 ohm resistor, giving a resistance of 0 ohms, leaving the 10 ohm and 15 ohm resistors in parallel across the load terminals.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by atsruser)
    In the Thevenin equivalent circuit, you will have replaced the voltage source by a short circuit. So the 20 ohm resistor will be in parallel with a 0 ohm resistor, giving a resistance of 0 ohms, leaving the 10 ohm and 15 ohm resistors in parallel across the load terminals.
    What about when i'm calculating the open circuit voltage? they are still in parallel then as well? I'm not sure how i would calculate the open circuit voltage. Would i just use the voltage divider rule and do (15k/25k)*12V?

    Also are you sure they are in parallel? The other guy said they are in series, i thought they were in parallel as well though. It's just i'm using the voltage divider rule on the calculation i used above for two resistors in parallel, i just thought i could use it because all the current would flow through the 15k resistor.
    Offline

    3
    ReputationRep:
    The 15 and 20 are quite clearly in parallel due to the way the power supply splits them up.

    The 10 and 15 are not in parallel because if you cut out the 20 for example, it becomes a series.

    Therefore, if you were to calculate the entire circuit resistance, the 10 and 15 effectively become one resistor.
    • Study Helper
    Offline

    21
    ReputationRep:
    Study Helper
    (Original post by ErniePicks)
    Hey finding the thevenin equivalent circuit and am wondering if the 10 and 15 ohm resistors are in paralllel or series?Name:  circ.jpg
Views: 140
Size:  9.8 KB
    I've just noticed this question and realised it's why you are having difficulty with series and parallel circuits in another thread/question you asked.

    Serious advice: Forget Thevenin for the time being (come back to it later). It is a different concept to straight forward series and parallel resistances. Concentrate on understanding series and parallel combinations first and only then come back and ask about Thevenin. Otherwise it will confuse the hell out of you.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by uberteknik)
    I've just noticed this question and realised it's why you are having difficulty with series and parallel circuits in another thread/question you asked.

    Serious advice: Forget Thevenin for the time being (come back to it later). It is a different concept to straight forward series and parallel resistances. Concentrate on understanding series and parallel combinations first and only then come back and ask about Thevenin. Otherwise it will confuse the hell out of you.
    lol it seems like people in this thread are confused as well, some people are saying the 10 and 15 are in series whilst others are saying it's parallel.

    it's having the wire in between the 15 and 10 ohm resistor leading to the open circuit which is confusing me. Current would flow through the 10 ohm and then due to the open circuit the rest would flow through the 15 which would make me think it was in series even though it would be in parallel with the load.
    Offline

    11
    ReputationRep:
    (Original post by transformersone)
    The 15 and 20 are quite clearly in parallel due to the way the power supply splits them up.

    The 10 and 15 are not in parallel because if you cut out the 20 for example, it becomes a series.
    He wants to calculate the Thevenin equivalents, not to analyse the circuit as it stands.

    Regardless it isn't correct to say, even for the circuit as shown, that the 15 and 20 are in parallel - as it stands, we have 15 || (10 + 20), if we ignore the voltage source.

    [edit: I'm going to modify this a bit:

    From the POV of calculating the Thevenin equivalent voltage, then we need to calculate the resistance as 10 || (20 + 15) but we need to find V_{\text{th}} across the 15 ohm resistor - however, we still don't have 15 || 20 as a single resistance in the circuit ]
    Offline

    11
    ReputationRep:
    (Original post by ErniePicks)
    lol it seems like people in this thread are confused as well, some people are saying the 10 and 15 are in series whilst others are saying it's parallel.

    it's having the wire in between the 15 and 10 ohm resistor leading to the open circuit which is confusing me. Current would flow through the 10 ohm and then due to the open circuit the rest would flow through the 15 which would make me think it was in series even though it would be in parallel with the load.
    What open circuit are you thinking of? To calculate a Thevenin equivalent, you replace voltage sources with *short* circuits.
    • Study Helper
    Offline

    21
    ReputationRep:
    Study Helper
    (Original post by ErniePicks)
    lol it seems like people in this thread are confused as well, some people are saying the 10 and 15 are in series whilst others are saying it's parallel.

    it's having the wire in between the 15 and 10 ohm resistor leading to the open circuit which is confusing me. Current would flow through the 10 ohm and then due to the open circuit the rest would flow through the 15 which would make me think it was in series even though it would be in parallel with the load.
    Thévenin theory allows us to simplify the most complex circuit to a simple 'equivalent circuit' comprising a single voltage source and a single series resistance connected to a load.

    It's use becomes apparent when there are several voltage sources in the circuit, like several batteries connected in various series and parallel combinations.

    To reduce a circuit using Thévenin:

    1) Label the load nodes A and B.

    2) replace the load with an open circuit and calculate the potential difference across that (infinite resistance) open-circuit. (Use ohms law series and parallel combinations to accomplish). This becomes VTH. (in other words VAB)

    3) replace the power sources with a short circuit and calculate the series/parallel equivalent resistance looking back in across the AB nodes. (once again the load is not connected). This becomes RTH.

    4) The Thévenin equivalent circuit connected to the load is then the calculated VTH as the supply and RTH as the resistance in series with the load.

    In essence, the load 'sees' the circuit connected to it, as if it were a battery with an internal resistance.

    Wiki has a good explanation:

    https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theoremtion:
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by atsruser)
    What open circuit are you thinking of? To calculate a Thevenin equivalent, you replace voltage sources with *short* circuits.
    Between nodes A and B, it's an open circuit and has an open circuit voltage right?
    Offline

    11
    ReputationRep:
    (Original post by ErniePicks)
    Between nodes A and B, it's an open circuit and has an open circuit voltage right?
    Assuming that nodes A and B are the two unlabelled terminals, then yes, that is right.

    However, if you want to calculate the Thevenin equivalent resistance, you replace all voltage sources with short circuits, then you calculate the resistance of that new circuit between the terminals. If you do that you will find a total resistance between the terminals of 15 || 20 ohms.

    Maybe I didn't make that clear earlier.

    To calculate the Thevenin equivalent voltage, you just analyse the circuit as it stands, with an open source load. With your circuit, you will have a 10/15 ohm voltage divider in parallel with 10 ohms, so you will have 12 V across 20 || (10 + 15), then you need to find the voltage across the 15 ohm resistance.
    • Study Helper
    Offline

    21
    ReputationRep:
    Study Helper
    (Original post by atsruser)
    Assuming that nodes A and B are the two unlabelled terminals, then yes, that is right.

    However, if you want to calculate the Thevenin equivalent resistance, you replace all voltage sources with short circuits, then you calculate the resistance of that new circuit between the terminals. If you do that you will find a total resistance between the terminals of 15 || 20 ohms.
    Replacing the 12v source with a short circuit and looking back at the nodes AB will produce:

    15||10 ohms = 6 ohms


    (Original post by atsruser)
    To calculate the Thevenin equivalent voltage, you just analyse the circuit as it stands, with an open source load. With your circuit, you will have a 10/15 ohm voltage divider
    Correct

    (Original post by atsruser)
    in parallel with 10 ohms, so you will have 12 V across 20 || (10 + 15), then you need to find the voltage across the 15 ohm resistance.
    The 20 ohm resistor is across the battery. Current through the 20 ohms path is therefore independent of the 10 and 15 ohms voltage divider current path.

    This is why the power source is replaced with a short circuit. Because anything directly in parallel with the supply (other than the node path) can be ignored.

    The Thévenin equivalent voltage appearing at the AB nodes is therefore:

    V_{AB} = V_S(\frac{15}{10 + 15}) = 7.2V

    Making the Thévenin equivalent circuit replacing the original as:

    V_{TH} = 7.2V in series with R_{TH} = 6\Omega
    Offline

    11
    ReputationRep:
    (Original post by uberteknik)
    Replacing the 12v source with a short circuit and looking back at the nodes AB will produce:

    15||10 ohms = 6 ohms
    Right. My 20 should have been 10. That was a typo.

    The 20 ohm resistor is across the battery. Current through the 20 ohms path is therefore independent of the 10 and 15 ohms voltage divider current path.

    This is why the power source is replaced with a short circuit. Because anything directly in parallel with the supply (other than the node path) can be ignored.

    The Thévenin equivalent voltage appearing at the AB nodes is therefore:

    V_{AB} = V_S(\frac{15}{10 + 15}) = 7.2V

    Making the Thévenin equivalent circuit replacing the original as:

    V_{TH} = 7.2V in series with R_{TH} = 6\Omega
    I agree with this, but I'm not sure if you are disagreeing with me.

    All you seem to have done is to perform the voltage divider calculation that I omitted. And I don't understand why you've underlined part of your explanation. It's rather oddly worded - the current through the 20 ohms isn't really *independent* of the other path - it would change if we had 100 and 150 ohms instead. But otherwise it simply seems to repeat what I said (that we have 12 V across 20 || (10+15) ohms).

    I guess that the point that you are trying to make is that the 20 ohm path is irrelevant to the calculation of V_\text{th}, which I possibly didn't make clear.
    • Study Helper
    Offline

    21
    ReputationRep:
    Study Helper
    (Original post by atsruser)
    Right. My 20 should have been 10. That was a typo.



    I agree with this, but I'm not sure if you are disagreeing with me.

    All you seem to have done is to perform the voltage divider calculation that I omitted. And I don't understand why you've underlined part of your explanation. It's rather oddly worded - the current through the 20 ohms isn't really *independent* of the other path - it would change if we had 100 and 150 ohms instead. But otherwise it simply seems to repeat what I said (that we have 12 V across 20 || (10+15) ohms).

    I guess that the point that you are trying to make is that the 20 ohm path is irrelevant to the calculation of V_\text{th}, which I possibly didn't make clear.
    Apologies for not making myself more clear, I'm not disagreeing with you.

    The underlined part is an attempt to clarify to others (who are also reading this thread) why Thévenin methods work the way they do.

    i.e. from a current path perspective, any resistance directly in parallel with the power source (not in the node current path) can be ignored.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Brussels sprouts
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.