wheatstone bridge, how accurate must it get? - Page 1
Forum section: electronics / beginners

author: The Soulman
author_rank: Super Contributor
timestamp: March 30, 2016, 12:03:48 am
content: I've began matching resistors for a wheatstone bridge, just for fun, the next thing would be building a precision voltage divider, and after that hooking it up to a voltage reference.For the purpose I've modded the 300mv scale of my metrawatt ma5d 5 digit bench multimeter to 30mv for 1 micro volt (of almost noise free!)resolution.The resistors being used are cheap 75ohm 0.6watt 1% metal film types, am soldering on high valueresistors (4m7 is now getting to low..) to get them matched but it is a pain in the (...) after soldering the values change slightly easily causing a 20 micro volt change after cooling back down so this way is not going to work (would like to get in the sub 100ppm range) .Should I solder the 75 ohm resistor together with a 20 turn trimpot (paralleled, and specced to trim the maximum 1% error to 0.000x) in placeand adjust afterwards?Is this worth doing or is it going to drift all over the place in time?Not to worried about temperature, that can be controlled.Any input, advice, comments, much appreciated.

author: German_EE
author_rank: Super Contributor
timestamp: March 30, 2016, 08:39:34 pm
content: Accuracy and resolution are two different things and you will be limited by things like thermal drift in your measurements. What I would do initially is set your multimeter on the ohms range and try to find two resistors exactly the same value but:Connect the multimeter to the test jig with reasonably thick cables, good quality plugs and solid connectionsConnect your resistors to the test leads as close to the resistor body as possible and using some form of clamp assembly that has repeatable connection performanceDO NOT handle the resistors with your bare hands as surface contamination and the heat from your fingers will throw off the measurement, use tweezers.You might do this with 1% devices, 0.1% will make your job easier.If you're good then you will get a matched pair, if you're really good (and lucky) you will get three matched resistors, Runknowncan then be the fourth resistor in the set. Everything will be affected by the temperature of soldering so my advice is to use screw down terminals for the bridge components.

author: amspire
author_rank: Super Contributor
timestamp: March 30, 2016, 11:04:54 pm
content: 75 ohms is a very low value for a bridge. Normally you are using 1-10V across the bridge and the 75 ohm resistors will definitely warm up. 0.01%/C drift is common in ordinary film resistors, and you could easily get a 10-20 degree change in the 75 ohm resistor temperature. You may find that your bridge cannot reliably get even 0.1% accuracy.You are much better off with something like a 10K resistor. Find two that match to within 100 ohms, and put a 100 ohm 10 turn trimpot between them. The resistors will drift a bit over time and so you will find it good to be able to easily recalibrate them without a soldering iron.Matching to 1pp is pretty easy if you set up two arms like this and a switch to reverse one of the arms.You adjust the switchable arm so that the offset from a zero volt null is the same in both switch positions. You adjust the non switching arm so that it is zero null. Keep tweaking the two arms until you have a perfect null with both switch positions. Both arms are now exactly matched. The reality is that with cheap resistors, you will see the drift while you are doing this and that is good - you will be able to estimate the accuracy you can expect from the bridge.Check your meter to see if it has a leakage current on the input. Put a 1Mohm resistor across the meter (if you have a lot of noise around, may have to add a 0.1uF cap as well in parallel) and let the meter stabilize. If the meter settles on, say 10mV, then the input of the meter is outputting .01/1000000 = 10nA. 10nA can cause a 100uV error into a 10K impedence. Just something to check. If the meter has a problem you can go to a lower bridge resistor - like 1K. It is not hard to find opamps that have leakages down in the picoamp area and DC offsets in the microvolt area. You can make a simple bridge preamp with a few parts.