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Topic: Even voltage output steps with parallel resistors (Read 8938 times) previous topic - next topic

polymorph

Steve Greenfield AE7HD
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larryd

Quote
Or just use two resistors to get the 2R.
The problem was, if they were not within 1% the D/A was not very accurate.
But that was then, 35 years ago.

You can get these:
http://www.bourns.com/pdfs/r2r.pdf
No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

polymorph

Well, the OP must specify how accurate it must be. By anything resembling the method he gives, or any other method for that matter, resistor matching is important.

Much simpler if you are using a whole lot of identical resistors. You can buy them pretty cheap on 1% or better, these days.

That Bourns R2R network is 2%, $1 each.

0.1%, 10k for 46 cents each.
http://www.digikey.com/product-detail/en/MFP-25BRD52-10K/10KADCT-ND/2059114
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Paul__B

The XY problem strikes yet again!


If only we can get the OP to reveal his real problem, it is almost certain that a R-2R resistor network will be the solution.

:D

KenF

I've worked out a scheme that could work, but I suspect you may have trouble obtaining some of the values.

(btw R1 is 1k.)

MarkT

Is it really so that it can't be done ? :/ Has someone an explanation why not?
Your problem is meaningless as it stands since a bunch of resistors in
parallel cannot on their own form a voltage divider at all - they are equivalent to
one resistance.

Perhaps you meant that the set would be fed from a constant current source?
Or perhaps form a divider with another resistor?

You didn't mention this though, so its not clear what you originally meant.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

polymorph

Is it really so that it can't be done ? :/ Has someone an explanation why not?
Are you trying to use one input pin with multiple buttons?

http://forum.arduino.cc/index.php?topic=8558.0

Most of those LCD shields with up-down-left-right pushbuttons work this way.




Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Grumpy_Mike

I think it is pointless guessing what the problem could be and then offering a soloution. It is up to the OP to sate more clearly what the problem actually is because as it stands this is just another noncence. We seem to be getting a lot of these lately, it's 50/50 if we ever here from him again.

polymorph

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Paul__B


DrAzzy

#25
Feb 04, 2015, 12:19 am Last Edit: Feb 04, 2015, 12:20 am by DrAzzy
The OP should really tell us what he is trying to DO.

This sounds like one of those questions where the person is asking a question that isn't normal and doesn't really make sense, when he's got a higher level goal in mind, but has decided he can solve it if he can just solve this other issue, the one he's asking about. However, this isn't the right way to solve his problem, and until we find out the final goal, we're wasting our time (and his). I forget the word for these kind of questions

Edit: Others have beaten me to it, repeatedly. Didn't see the second page.
ATTinyCore for x4/x5/x61/x7/x8/x41/1634/828/x313 megaTinyCore for the megaavr ATtinies - Board Manager:
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Grumpy_Mike

Yes, it has been said it is an X Y problem. Please read:-
http://xyproblem.info/

jtpek

Hello gain and thank you for your great responses! Many of you have said I should explain what I'm aiming for, so here it is :)

I have some blocks I'm stacking and I want to measure which blocks are stacked at the moment of measurement (see picture..in this example I've used "Lego-blocks" to demonstrate the case.



So at the moment I'm doing this project analogically. As you can see in the image I want the output with one block to be ca. 5V. Adding one block the voltage should be about 4.5V..with three blocks about 4V and so on until all the blocks are stacked..at this point the measurement should be about 0.5V.

The exact voltage is not important. I just want the steps to be quite equal, so that I can be certain at any point which blocks are stacked. With equal steps the allowed range can be set as big as possible and the variation in the signal doesn't matter that much.

Of course I'm open to any ideas how to do this in a different way, too. Also if there is some way to do this in a digital way I would love to hear about that kind of options, too.

Thank you guys for all the help so far! I hope this helps :D


KenF

Just a suggestion, You could use capacitors instead.  Capacitors in parallel are simply added.

jackrae

That tends to illustrate what you are attempting, though it doesn't explain how you intend to do so.

Assuming the resistors are all the same value then you come up against the problem that the system response to parallel resistors is exponential (the more you add the less the change).  This can be linearised somewhat by appropriate resistor selection but, but, if you add the wrong resistor in sequence you will get an erroneous "answer" in the equation.   

Perhaps, if you further explained what you are endeavouring to achieve and exactly how you intend to physically add these resistors, you may get an informed solution to the problem

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