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a transistor is a variable resistor

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dragon

Oct 31, 2022
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So when u hit the base of the transistor, the resistance disappears through the collector and emitter.


Is there a way you can get a variable resistor with just passive components?
 

bertus

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Hello,

It can NOT be done with only passive elements.
A JFET can be used as a voltage controlled resistor on a small signals:
Also read the attached PDF.

Bertus

PS did you have a look at the conversation?
 

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Externet

Aug 24, 2009
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That is like what my very first transistor learning lesson said when I was 12.
Transistor = transference resistor.
 

crutschow

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I mean current controlled variable resistor.
In spite of the name the BJT transistor is basically a current-controlled current-source, not a current-controlled resistance.
It's difficult to make a controlled variable ohmic resistance for any significant voltage range.

The FET linear ohmic region, for example, is usually only good for less than a volt pp.
 

Harald Kapp

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Is there a way you can get a variable resistor with just passive components?
Use a potentiometer connected as a rheostat. But this will be manually controlled, not electronically.
A primitive (in today's view) method of electronically controlling a potentiometer or rheostat was used a few decades ago when small electromotors were used to allow e.g. remote control of volume. One could, however, discuss whether an electromotor is a passive device or an active one ...
Seems you and the Op both need a visit to get some help.
A bit unobjective your answer is. Specially in view of the facts.
 

dragon

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If I put a power supply behind a resistor, and I switch the power supply on, the extra voltage supplied will cancel the resistance.

So this is a current controlled variable resistor as well.

When the power supply is off, the resistor counts, when the power supply is on, it will include extra voltage into the system which cancels off the resistor and makes it the same amps.

The power thats already in the system is supposed to connect to this supply in series, and become one single chain, 2 supplies acting as a single supply together.
 

crutschow

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If I put a power supply behind a resistor, and I switch the power supply on, the extra voltage supplied will cancel the resistance.

So this is a current controlled variable resistor as well.

When the power supply is off, the resistor counts, when the power supply is on, it will include extra voltage into the system which cancels off the resistor and makes it the same amps.

The power thats already in the system is supposed to connect to this supply in series, and become one single chain, 2 supplies acting as a single supply together.
That's all such a load of muddled misinformation, I can't really make enough sense of it for a decent reply.
A resistor is still a resistor whether there's voltage applied on not.
Voltage does not "cancel" a resistance.
 
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Harald Kapp

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If I put a power supply behind a resistor, and I switch the power supply on, the extra voltage supplied will cancel the resistance.
What nonsense is this? A voltage doesn't "cancel" a resistance.
So this is a current controlled variable resistor as well.
Not at all. The resistor's value as given by R = V / I stays the same wherever you put voltage sources in your circuit. It does not change.

When the power supply is off, the resistor counts, when the power supply is on, it will include extra voltage into the system which cancels off the resistor and makes it the same amps.
Doesn't sund like you've got your basic electronics skills right.
Apart from that, as has been mentioned in previous posts/threads: use established nomenclature. By inventing new expressions you can be sure of only one thing: nobody is going to undestand you - or at least not the way you expect.
The power thats already in the system is supposed to connect to this supply in series, and become one single chain, 2 supplies acting as a single supply together.
Without a scheatic diagram of the circuit that you seem to have in mind we cannot understand what you mean. There#s more than one way to connect two power supplies.
 

dragon

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5v divided by 5 resistance, is 1 amp.

10v divided by 10 resistance, is 1 amp.

i went through twice the resistance but the amps were the same.

so if I go through a second power supply in series, the resistance gets cancelled out.
 

dragon

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Of course, because you also doubled the voltage.

Not at all, it is still there and it is effective.
The second power supply position will act like a short, and the other wires wont get any current. even if they have 0 resistance! so on the wire the supply is on it is acting like a negative resistor.

the other supply will be diverted in series to this supply, because of the extra voltage on the line. (voltage is inverse resistance.)
 

Harald Kapp

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the wire the supply is on it is acting like a negative resistor
No wire can act as a negative resistor. The best one can achieve with high effort is near 0 Ω resistance with supeconducting wires.
When no current flows this is no sign of zero or negative resistance. It is simple the effect of zero voltage across the resistor. The resistor's value remains unchanged, be it a few mΩ or some MΩ.
Your "theory" is totally flawed.
 

dragon

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You need to open your mind a little bit! These things I'm saying arent just so literal, its being creative!
I'm just saying voltage being inversly opposed to resistance means its like an inverse resistor! so increase the volts, and it causes shorts just as much as decreasing resistance!

But the cool thing, is you can activate and deactivate it solid state, but an ordinary resistor you cant without a relay.
 
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dragon

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No matter how wide open your mind is, you can't alter the experimentally-verifiable physical facts of electronics.
I only care about what works too. If its not in the practical electrical model i'm not interested in it either.
 
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