variable pot with a fine trimmer

[dacflyer]

i cannot think tonight..
i have a 50k pot, i need some fine trim on it..i'd like to use a 1k multi turn trimmer pot , but i cannot think tonight on how i'd wire it.
the 50k would be the rough adjust and the 1k would be the fine trim.

in the end there should be just 3 connection to my circuit that i want to use it on.

pin 1 - neg.
pin 2 - adjust
pin 3 - pos.

can someone suggest a schematic...have me from yanking out what short hair i have..

[positronicle]

--Edited by Positronicle--

[Chris Smith]

Pots form two seperate devices.

A variable resistor, and a voltage divider.

Current and voltage are affected by the pot, so before you waste any ones time, consider both.

Which do you want?

[dyarker]

Three connections, looks like potentiometer.



Cheers,

[dacflyer]

Dyarker >>> thanks , i think what i need is the foto on the left.. i will let you know if it works for me..
also had another brain storm,,amazing what some sleep will do for ya.

posted here is my idea,, wonder which one will work better..
see foto

[philba]

If you are trying to get fine control on the voltage from the divider, your diagram won't do it. You need to use the left one from dyarker's post.

[Robert Reed]

In the end, voltage dividers are all related to network input and output impedances. What is your input impedance ? ; what is your load impedance? In your diagram, if the input is from a low impedance power source, then your diagram would work--BUT only for a specified narrow range of load, say like 10K ohm. You need to be more explicit in your posts. Do you want fine control over the whole input voltage range or do you want control over a portion of it-upper end, middle, low end ?
Take a little extra time with your posts and don't leave people half guessing. In the end you will accomplish problems much quicker as we have a variety of people here that can answer almost anything.

[dacflyer]

robert reed >> i explained the best i could, please do not be putting me down..

the project i am using is a LM7107 chip. and the adjustment is for calibrating the meter input
i made me a digital led temperature modual one with 2.7" tall LEDs.
and the calibrating adjustment is touchy..i barely turn the pot,and the reading is too far or not enough.. so i just want to be able to fine tune it.

as far as me needing hi , mid, low range adjust , i am not sure, i just need to fine adjust a portion of the main pot. not the whole range.
the 1k 15 turn pot i have should be enough of what i want.. like i said, i barely touch the adjuster of the main pot, and its too much or too little, cannot get it just right..so thats why i'd like a fine adjuster.

[Robert Reed]

Not putting you down - just thought you would get better replies by going more into detail - all for your benefit. Is your chip an LM7107 or an ICL7107 ? Sounds like something is way out of value here.If the main pot is that touchy, it probably wouldn't hold its setting even with a fine tune pot attached (change with temperature and vibration). When the pot adjust. jumps does it throw the meter reading from one extreme to the other ? If so you may have to reexamine the scaling and reference circuits for proper values. The data sheet has a lot of info on this subject. BTW, do you know what the output voltage range is from your temperature transducer is ?

[Chris Smith]

The 50 k , not the one K will give you fine tunning.

Five times the ratio and movment.

Now which do you need, current or voltage adjustment?

Pots can do both, pots can use two legs or three?

[variable resistors or pots]

Voltage, current or V/C/ D? What?

[MrAl]

Hi there,

Just a few notes...

[1]
Usually a 'fine' adjustment is ok when it is one tenth the size
of the main pot. This would make either the 1k a 5k, or
the 50k should be a 10k. 1k for the 1k may be too fine.

[2]
Consider a 10 percent change in resistance of the 1k pot when
the 50k pot is adjusted near the top and also when the 50k
pot is adjusted near the bottom.
When the 50k pot is adjusted to 1k at its top, the change in voltage
for a 10 percent change in the 1k pot is 0.0188759197195605 volts.
when the source voltage differential is 10v.
But, when the 50k pot is adjusted to 1k at it's bottom, the change
in voltage for the same 10 percent change of the 1k pot and also
with the same 10v source is only 0.000385222851419981 volts.
Thus we see a change in sensitivity of adjustment of the 1k pot
of about 50 times from near one end of the 50k pot to the other, but
it gets worse as the 50k pot gets nearer to it's ends.
This means the better way is to connect the 1k pot to the arm of the
50k pot and hope the load has at least some impedance to work with.
If not, connect maybe a 100k from the 1k pot to ground so the
1k pot has some impedance to work into.
As mentioned above however, you probably want a 5k pot for the
fine adjustment, or else keep the 1k pot and change the 50k pot
to a 10k pot.

Good luck

[dacflyer]

yes it is a ICL7107.....oops
and i am using LM34CAZ for the temp sensor

so i guess i need a voltage adjustment ?
this project is working for the most part. i think i will try maybe using a smaller pot , like a 20k, i just have to see if i have a 20k 15 turn pot in my junk box somewhere. i got pleanty of 1k , 10k , 100k..
i did look more on the data sheet, and a 20k seems to be what i should be using,
my calibrations are only 2/10 - 4/10 of a degree off..compaired to my DVM right now...

i still am trying to find a brighter 7-segment led display 2.7" (70mm)
the one i have right now is only as bright as a alarm clock and thats if you have a good red filter..with out a filter, you barely can see it at all..
so i keep looking too..
thanks

[jwax]

MrAl- Do you think those 18 decimal places are sufficient?

[Chris Smith]

Leg one in.

Leg three to ground.

Leg two out for a voltage variable.

Keep the pots value above the burn out voltage value, [ie in a dead short] and always keep a deliverable current and voltage to the device under variance. [ohms law]

A five watt pot in a dead short [voltage variable] will draw five watts.

Cant over draw, cant burn it out.

[rshayes]

It sounds like you are trying to use the pot to adjust the full scale range of the A/D converter. The trick is to arrange the pot to cover only the range you need.

The LM34 has a scale factor of 10 mV/F. If the potentiometer has 5 volts across it, setting the temperature to within 1 degree will require setting the pot within 10 mV, or about 1/500 of full scale.This will be difficult, to say the least. The backlash on the leadscrew of a 15 turn pot will probably be more than this.

If full scale is chosen to be 200 F, then the LM34 will put out a voltage of 2 volts at full scale. An adjustment range of 20 degrees each side of this point would be reasonable. A 40 degree range will require a voltage across the pot of 400 millivolts. The lower end of the pot would be at 1.8 volts and the upper end would be at 2.2 volts.

If the adjustment pot is 1 K, a current througn the pot of 400 uA will result in a voltage across the pot of 400 mV. This same current will flow through a fixed resistor to ground. This resistor will then be 1.8 volts divided by 400 uA, or 4500 ohms. Use the closest 1 percent resistor.

The resistor on the high side of the pot will depend on the reference voltage available. If the reference voltage is 5 volts, the total voltage across this resistor will be 2.8 volts. The current will still be 400 uA, which results in a resistor value of 7000 ohms.

This should put the voltage at the center of the pots rotation at about 2.0 volts. The totat range of the adjustment is now 40 degrees, so setting the adjustment within 1 degree will require setting the pot to 1/40 of its range. With a 15 turn pot, 1 degree will be represented by about 1/3 of a turn.