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M. next said:Resistor Rb1 and Rb2? How come?
Yes - that's one way of looking at it but don't jump to the conclusion that it's always different for AC and DC. The word 'parallel' is only a short hand. If you really want to calculate what is going on, you have to do the sums and the sums don't pay attention to the words series and parallel.M. next said:Wow thanks! Especially for AlephZero, I didnt know it made a difference whether u are in AC or DC
As far as the supply rail is concerned, the two of resistors are, of course, in series. But as with any two-resistor potential divider, the pair of resistors can be represented by its Thevenin equivalent circuit, and this amounts to it being a voltage source with an impedance = R1||R2. This analysis applies regardless of the source being AC or DC.M. next said:Resistor Rb1 and Rb2? How come?
Jobrag said:On the diagram it's marked as +15V, suggests that it's DC.
DailyDose said:These resistors are not in parallel, they're essentially a voltage divider.
sophiecentaur said:Stop getting hung up on the definition. If you were to feed that base with an AC signal, via a capacitor, those two resistors would appear as a load equal to the two in parallel (and also in parallel with the transistor input). Just put it down to Engineers' shorthand and 'get over it'. You'll never reform us.
sophiecentaur said:Jeeeez! You guys.
Voltage divider means they are a voltage divider - that's all. Why this obsession about giving things names? Does the word "series" or the word "parallel" appear in the formula for working out the volts out of a potential divider? Just get on and do the sum and have some confidence that it's right.
Just consider this - if you are sitting on that transistor base, looking out, would you see two resistors, both connected to you, or would you see one resistor and then another resistor, connected to the other end of it? Where is there anything at all in series when you look from the base's point of view?
Who cares? If you're dealing with transistors, you have advanced beyond the stuff that nice lady told you about when you were 13 yrs old.
sophiecentaur said:OMG, it's you again.
Hi.
(I really do have a point there, you know!)
psparky said:You do have a point...but since you are so smart...I think you forget the learning process.
Most students struggle with parallel and series...it is quite confusing at first. What is obvious to a man of your infinite wisdom...is not so obvious to us mere mortals.
For aspiring students...remember, for all parallel loads..."the voltage across is the same".
Averagesupernova said:For aspiring students...remember, the resistors that form the voltage divider to bias the base voltage to 5 volts DC will see the same AC voltage. Sooooo, "the voltage across is the same". They are in parallel concerning the AC voltage on the base of the BJT.
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See it now DailyDose?
psparky said:You do have a point...but since you are so smart...I think you forget the learning process.
Most students struggle with parallel and series...it is quite confusing at first. What is obvious to a man of your infinite wisdom...is not so obvious to us mere mortals.
For aspiring students...remember, for all parallel loads..."the voltage across is the same"
sophiecentaur said:I think I see the perils of learning in a non-optimal way. Hanging on to words which you think are well defined but which are not, can't be a good idea. If there is a node in a circuit and you are interested in what is happening at that node then using a word like "series" to describe the way two of the components may not be appropriate. Yes- of course those two resistors are in series with the power supply but they are not in series with the base junction of the transistor. So why use the word at all unless you include all the qualifications about how you are using it?
In so many posts, I read that people want to categorise rather than to understand. This is a prime example. The Sums give you the answer and sometimes the result can be summed up with simple terminology. When it can't, there is no point in trying to make it.
This is a prime example of over-simplification, which is dodgy. There are two paths for current to flow from that node point - round the top and round the bottom and through the base circuit. This is what Kirchoff 2 tells you and Kirchof will give you the right answer.
People who hold on to handy, elementary, terms through thick and thin will always risk falling over. For instance, take a circuit of three resistors, connected in a Delta (not an overly complicated circuit). Are they in series or are they in parallel? Which is which? You can't answer until you supply which two connecting points you are discussing.
sophiecentaur said:@psparky
You have an excellent attitude to this stuff. You must be soaking it up and really enjoying it.
btw, my Dad was a transmission engineer with the old 'Central Electricity Generating Authority' (UK). (also on RAF Radar in WW2) He was full of talk of Bus Bars, Transformers and Trips. I learned to drive a car on the perimeter road of an electricity substation down in Cornwall.
Resistors are used in parallel to decrease the overall resistance in a circuit, making it easier for current to flow. This is especially useful when a circuit needs a specific amount of current but the available power source has a higher voltage than needed.
When resistors are connected in parallel, the total resistance decreases. This is because the current has multiple paths to flow through, instead of being restricted by just one resistor. The overall resistance can be calculated using the formula 1/Rt = 1/R1 + 1/R2 + ... + 1/Rn, where Rt is the total resistance and R1-Rn are the individual resistances.
Yes, resistors in parallel can have different values. In fact, this is often the case in real-world circuits. The total resistance will still be affected by all the resistors, but the calculation will be more complex.
In a parallel circuit, the voltage across each resistor is the same. This is because all the resistors are connected directly to the same power source, so they all receive the same voltage. However, the current may differ for each resistor depending on their individual values.
One disadvantage of using resistors in parallel is that it can be more expensive and take up more space than using a single resistor with the same total resistance. Additionally, if one resistor were to fail, the overall resistance may change, potentially affecting the entire circuit.