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Edit: SOLVED
Hey, I have a question and need some help.
I have a variable resistor, that goes from 50k to 500k Ohm.
How do I "scale" the resistance with some external thing, so that the combined resistance will be 5k to 50k?
+ Show Spoiler [why am I asking] +
We are trying to build a timer circuit that gives off different frequencies with the adjusted resistance. So basically each cycle takes R*C*(ln(2/3)-ln(1/3)) time. So what happened is since the resistance is too huge, our cycle takes very long, and so the frequency is super low. And if we tries to just shrink the resistance with things in parallel, the range becomes terrible because 20k-21k means we're getting a range of 1000-1100 hrz, a near monotone...
MORE information:
We're using a 555 timer, which alternates the output from 0 to 1 depends on 2 of the inputs, trigger and threshold. Depends on the behaviors of trigger/threshold(controlled here by resistor and capacitor), it'll give diff frequencies, changing 0 to 1 and back, driving a speaker and giving us a sound.
I was trying to put things parallel with it but it doesn't work. The original range is 50k to 500k, a 10 times difference, but after putting it in parallel with some other resistor, the value did drop, but now it's more like 20k to 21k, and you only get 5% difference, and I do not want that...
What to do?!
Thanks!
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I'm not a circuit design expert in the least so I'm going to naively ask:
Couldn't just use the resistor you have in series with a current amplifier that amplifies your current by 10? Would a bjt with a gain of 10 do the trick?
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is awesome32277 Posts
When putting resistor in serie, the bigger resistor prevails, when putting them in parallel the smaller one does.
The problem for you is that if you adjut the first resistance to drop from 50k to 5k (say with a 5.5k~ resistor) the opposite value (500k) will drop again near to 5k.
What do you need this for. If it's for electronics, you can get a variable resistor according to your needs.
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My guess is that he is in the lab right now trying to get a demo working thats due tomorrow.
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The only way I can think of is to connect the potentiometer with a resistor in parallel, and then connect both of them in series to another resistor. So basically, the resistor in parallel scales the range of the potentiometer to the range you want, and the resistor in series increases the resistance to the values you want.
edit: I just did the calculations for the resistors need for you case and it seems like you need a negative resistance. My method wont work 
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United States24753 Posts
I'd ask my dad the electrical engineer but he's camping right now 
My guess would be to use two variable resistors that are somehow mechanically linked so that changing the value of one resistor also changes the value of the other in such a way that the combined resistance is what you want. Offhand I'm not sure how viable that is.
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The input resistance of a negative feedback amplifier with a shunt input connection is less than the original input resistance by a factor of (1 + open loop gain * feedback factor) or (1 + beta * A). So R_in effective = R_in / (1 + beta * A). A, the open loop gain, depends on the active device you're using -- a BJT, Op Amp, etc., and beta is determined by the feedback network.
What you want to do in theory is construct some kind of feedback circuit to reduce your effective resistance. I can't figure out how to do it now, though.
As a clarifying note, you want 5k to 50kOhm range at DC? Do you care about the impedance at other frequencies? What about the input voltage range?
edit: I don't have my books with me, so I'm stuck on this unfortunately. I tried doing some simulations of random shunt input feedback amplifier circuits I remembered, but I wasn't able to get close to satisfactory values. One issue was how large the original resistance range is.
To be honest, now I'm kind of interested in the answer too. I could ask my electronics prof, but I don't think I'd get a response by Monday. (School just finished for me, so there'd be no stigma for me to ask.)
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Belgium6774 Posts
Have you checked the manpower thread yet?
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I added why I need to do this, and also I'm willing to try some crazy things like amplifyers or what not.
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Hm now that you're giving an idea of what you really need out of the circuit...I have a lot of different questions for you then. What exactly do you mean by "timer" and what are you using it for? Are you driving some kind of output? You mention 1000-1100 Hz range as near a monotone, so does that mean you're driving a speaker? All you've told us about is a resistor, yet yet you're getting frequency. Are you using a square wave input, or what kind of input signal? What's the use of the resistor? Do you have some sort of RC filter? (It'd help to draw the circuit diagram and what signals you're dealing with.) How is the resistance a variable resistor? Is it a potentiometer, thermistor, or what?
On a more practical level, if you're interested in changing the time constant, why not keep the resistance as is and change the capacitance to 1/10th of the current value instead? (Remember, you can put capacitors in series to reduce the capacitance.) Why not use a resistance or capacitance box? Why not just have a function generator produce your desired output in the first place?
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United States24753 Posts
I was just talking to my dad while he had signal and he made a few comments (but he was kinda running through the woods so it was hard to catch everything he said lol):
He has done this before (scale down a variable resistor). He claims you can do it by setting up a combination circuit with several resistors. However, the resulting variable resistor will be very nonlinear even though it has the correct limits.
So, you start the variable resistor at one end, and you are at 5k. You will adjust the variable resistor only, say, 5% of the travel distance, yet the resistance will change by 50% of the way to 50k. Either that, or you move it 95% of the way, and the resistance has only changed by 50%, or something like that. The limits will work, and you will get a one to one function, but it will be some crazy exponential nonlinear stuff.
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United States17042 Posts
On May 03 2009 04:09 Myrmidon wrote:
Why not just have a function generator produce your desired output in the first place?
Real engineers do this xD
More to the point, if you put enough of them in parallel, with the variable resistors set so that all the resistances are equal then you'll be able to get the range you want. Another way to do it (probably the way they actually want you to do it) is to use a voltage divider/current divider, but that's going to lose a ton of power.
The easiest way to do it off the top of my head (symmetrical reasoning) is to put five of them in parallel and set the resistance to be equal. with the range of your resistors being 50-500, your resistance at that point will be between 10-100 (parallel resistance of 5 resistors set equal is R^5 / ( 5R^4 ) )
then, use a voltage divider to half that. You could also put all 10 resistors in parallel anyway, you'de get R^10/10R^9, which gives you the same answer. There must be some sort of restriction on this though, as that's really a brute force solution to something that can be done smarter with the right equations of resistance and the voltage divider. There's no information given on that part however, so I guess I won't try to solve it.
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The smart solution would be to toss the variable resistor you have in the bin and get one that is in the range you actually want. But I guess this is a school assignment.
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On May 03 2009 07:09 stenole wrote:
The smart solution would be to toss the variable resistor you have in the bin and get one that is in the range you actually want. But I guess this is a school assignment.
haha yeah but we tried very very hard to find a good variable resistor and this one is the best we can find. All the other is eithe rnon-conduction or a conductor TT
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is awesome32277 Posts
On May 03 2009 07:14 evanthebouncy! wrote:Show nested quote +On May 03 2009 07:09 stenole wrote:
The smart solution would be to toss the variable resistor you have in the bin and get one that is in the range you actually want. But I guess this is a school assignment. haha yeah but we tried very very hard to find a good variable resistor and this one is the best we can find. All the other is eithe rnon-conduction or a conductor TT
You just need to use the application note of a 555 to build a timer. Make both R's in the circuit potentiometers, with different values, so one becomes the quick adjuster and the other for fine tuning. 50k and 5k potentiometers are the most common i believe? what do you mean that you can't find a good one?
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EPT
