Need help with a basic resistor question

[thpthial]

Hi,

I am new here, and hope that someone can help with my question, it is driving me nuts.

I am currently working on a temperature controller that will have many sensors in it. [all passive components, no pic here ]

These sensors have resistor programmable hi and low trip points. I am trying to set 4 of these chips only using two pots (I would need 8 in total – and I am very limited on space). I have my circuit working using just one chip and two pots. My problem now is that I cannot detach the pots, because obviously this changes the resistance. for that trip point.

the chips are sensitive to 0 - 250kΩ

Can anyone think of anything I can use to 'lock' the resistance, or I suppose the voltage/current whilst I detach the pot and then attach it to something else. I guess it would be like a voltage or amp variable resistor, where pins 1 and 3 are the + - and pin 2 sets the resistance via current/amps/volts.

I did think of this problem before I started this project, but I am sure that I saw a 3 pinned chip that would do this, but I have lost the link for it.

the solution doesnt have to be that elegant but it would be really handy if I could do this with passive components.

I am thinking I am going to have to do somekinda funky feedback thing - I am quite new to electroincs so please go easy on me.

Thanks for reading this far, and any advice is appreciated.

cheers,

th.

 

[dfullmer]

Hello,

It seems you just need to replace the pots with fixed resistors. The pot can be thought of like two variable resitors that devide the voltage it is feed on pin 1 and ground on pin 3. A devided voltage comes out on pin 2. If you know what the value of resistance is between pin 1 and pin 2 you can replace that with a fixed resistor of that value. The same can be said for pin 2 to pin 3. If you have a schematic it would help to answer your question a bit better.


hope this helps

regards

dfullmer

 

[VSMVDD]

the resistors are there to account for the lenght of the wire

a sensible method therefore is simply to place a given resistor
say 10k accross the sensor at the sensor end of the cable

besides this both the sensors and the chips data sheet
will have charts for cable lenght and ideal resistors to use

if this isnt the case you will find it hard to replace them with one offs

a programmable resistor array made by dallas maybe the answer
you then simply step it to the wanted values
and it stays in nvram and restores on power up

all this talk about variable resistors ... hehe

 

[thpthial]

Hi, Thanks for the quick responses

I think my original post is a bit confusing. This is a lot more simplistic, even i cant mess it up... hopefully this a bit more intelligible.

     +||-
      || 
 -----||-------
 |            |
 |            O <--- LED
 |            |
 -/\/\/\-------
    ^
____| POT

This is a very basic dimmer switch for the led.
What I am trying to do is to be able to set this led then remove the pot and the led will stay at the last set value of the pot.

I am not too fussed if re attaching the pot resets the circuit to the current pot level.
I am really struggling to find anything that will allow me to do this and I just dont know enough yet to get the most out of google. any hints at all would be greatly received.

I thought I saw something that had a chip in it [*] that would allow me to do this..

     +||-
      || 
  ----||------
 |            |
 |            O <--- LED
 |    ___     |
  ---|_*_|----
 |     |
 |     |
 |_/\/\/\
      ^
  ____| POT

So that I could remove the pot and it would stay at the level it was last set at, but it seems that this is a bit of a non starter, and i cant seem to find the chip :/ how would i go about looking for chips that

might be able to do this? maybe i could mod another type of temp sensor/other chip with a lock function or something? I just seem to be trawling through datasheets at the moment, not really knowing how to be looking for what i want.

Other thoughts that I have had, have been like creating a timing circuit or maybe a few opamps feeding back into themselves or something. I dont know.. im not good enough to be able to think laterally yet, heh.. I am open to suggestions, but i really dont want to have to program a pic, i work with assembler all day, this is my hobby that is going to take me away from that.

A couple of thoughts that I had that would sorta solve this, in the context of the temperature sensor are,

1 - use a dual pot one for the high and one for the low, then have a configuration pot (screwdriver mini type) that would be used to set a ratio, this seems a little bit too complex though and i dont think it will work as i expect.

2 - Use an external thermistor + resistor to make it + 3 degrees above ambient for the low set, then atleast i will only need 4 pots

if anyone cares, this is the chip I am trying to control.

**broken link removed**

cheers,

th.

 

[thpthial]

Solved it.

I am going to use non volatile digital resistors, some are pretty easy to control using pasive componets, so I wont need a pic .

cheers,

th.

 

[aryajur]

Could you also post the information about the digital resistors you are using? The part number and the company name.

 

[thpthial]

Could you also post the information about the digital resistors you are using? The part number and the company name.

sure, there are many out there, but the ones I am initially looking at are these...
they are by Dallas, here are some links

DS1669 Dallastat Electronic Digital Rheostat
**broken link removed**

DS1809 Dallastat
**broken link removed**

DS1804 NV Trimmer Potentiometer
**broken link removed**

This mainly because of the dip packaging. there are some much better ones but they look a lot harder to solder

Although this solution may force some design compromises on my part. I am going to keep looking for alternatives...

Hope this helps.

th.