pull up resistor formula

[Capposteve]

hi, iv been scratching my head all night trying to work this out and was wondering if there was a table or furmula somewhere that can help me work out what wattidge resistor i need for a couple sensors.
how are we ment to be able to tell what resistor and what our final voltage range will be.

many thanks
steve

[Adamw]

Normally a pullup that has a similar resistance to the actual sensor will be about right.
See here for more info:
http://www.race-technology.com/wiki/ind ... ensorToDL1

[Capposteve]

Hi. Thanks for that. Iv seen this already and orders two 220r resistors as my fuel sender is 0-130 ohm and my temp is 0-178 ohms. I take it the higher the resistance resistor you use the less voltage swing you get but also less current draw and visa versa. Thanks steve

[Adamw]

I take it the higher the resistance resistor you use the less voltage swing you get but also less current draw and visa versa.

Yes, that's correct. you can use a voltage divider calculator like this one here to see the effects: http://www.raltron.com/cust/tools/voltage_divider.asp

Input voltage would be 5v, R1 is your pullup, R2 is your sensor resistance, output voltage is what the logger will see at the sensors full scale.
So if for instance we use this for your fuel sender R1=220, R2=130, you are going to see a full scale output of about 1.857v, which is not ideal but probably still quite usable.

The trouble with these low resistance sensors is to get a good output you need a lower resistance pullup, but that then draws a lot of power from the DL1 5v reference, so your pullup will always be a compromise.

[Capposteve]

Hi that's perfect thanks. What size pull up would I need for a decent voltage swing as iv ordered a 2amp regulated 5v supply.

Thanks.
Steve

[Capposteve]

Actualy I have just decided to run it with a 9v regulated supply and I'm getting about a 3-4v swing with a 220r resistor. Is that enough swing?? Cheers.

[osborni]

3-4V swing is good..... depending on what you are measuring and how critical it is.

[Adamw]

I'm not so sure i would do it with a seperate power supply. The DL1 5v references are high precision and probably highly filtered with very low noise. A general regulated supply will not be particularly clean and would likely also introduce ground offsets.

[Capposteve]

Thanks for the replys. Although the regulated supply is ragulated to 98% 9v. As far as the filtering it has a few capacitors and resistors built onto the board So I'm hoping it wi be ok. Iv only decided to do it this way as iv heard of many doing it before. Plus I don't have a dl-1 and the 50mah 5v output on the dash 2 won't be enough. Well I don't think anyway.

Steve

[Support]

It will be fine.

Martin

[Capposteve]

What's that Martin?? The dash2 5v or my 9v power supply?

Steve

[Support]

Your 9v supply. Any instability in the supply voltage will come through to the reading from the sensor, but a 1 or 2% variation in the voltage from the fuel level sender doesn't usually cause an issue. A regulator with a spec of 2% might not be accurate to within better than 2%, but they are generally pretty stable.

Martin

[Capposteve]

Thanks for clearing that up. I Spose also it depends on how the filtering is set up on the dash2 as well.

[osborni]

You are doing a fuel level measurement?

Then you want lots of smoothing and a slow update interval.

The variation in the voltage supply is particular to your device. So when you do a calibration on the actual fuel level, it will take care of itself. i.e., the bias will be compensated for in the calibration equation.

I've measured the ones that I use and IIRC, they are typically 0.05 or so volts off - not enough to matter.

[peterr88]

The DL1 5v references are high precision and probably highly filtered with very low noise.