Instrumentation amp and wheatstone bridge question - Page 1
Forum section: electronics / projects

author: technobabble
author_rank: Newbie
timestamp: January 09, 2012, 06:32:42 am
content: So recently I got my first patent, and part of the electronics in the device is a weight scale. I'm new to a lot of this stuff, but with the help of this forum/blog and a few other sites I feel like I can get this stuff mostly worked out. I have a few questions though.To amplify the voltage variation between the two halves of the wheatstone bridge I need an amp, of course. At first I started with an LM358N from Sparkfun. Everything seemed to be doing what was expected, it just didn't amp the signal enough to tell the difference between nothing and a 1KG weight. So, on some advice from a Mouser tech, I picked up a INA126P from them. This one isn't really showing the same behavior. Before I get into walking through step by step on what I'm doing in the setup, is this the right type of ins-amp?The load sensors are here:https://www.sparkfun.com/products/10245I used 2 of them to make both sides of the bridge for the hell of it. Each resistor in the system is 1003 ohms at the 20C room temp here.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: January 09, 2012, 06:50:23 am
content: Using two of these could double your output... or give you no output at all. It depends on where they are in the Whetstone bridge.Consider just one side of the bridge, V+ on one end, output in the center, and ground on the bottom. This looks just like a voltage divider. With strain gages on a cantilever beam, where one is in compression (resistance decreases) and one is in tension (resistance increases) with load, arranged this way the output change is increased.If you put the two Sparkfun sensors in these two positions, and both are compressed equally, the top resistance and bottom resistance both decrease equally with load, and the output doesn't change.If the sensors are in opposite legs of the bridge, the decrease in resistance in load on one side will move the output from that side of the bridge closer to ground. The other sensor, in the top arm of the other side of the bridge will move the output towards V+ with load. The output measured across the bridge is doubled with two sensors.

author: technobabble
author_rank: Newbie
timestamp: January 09, 2012, 06:58:49 am
content: Sorry, I should have made that more clear. I am using 2 of them to make the wheatstone bridge because I have shitty resistors that are like 5% variance. So I used the other one to give me 1003 ohms all the way around. I am NOT putting weight on the second sensor, just the first.

author: DBaron
author_rank: Newbie
timestamp: January 09, 2012, 06:44:47 pm
content: A read through Jim Williams app note 43 could help:www.linear.com/docs/4134There's a high precision scale in there that can measure weight changes due to your heart beat. Fantastic.

author: daedalus
author_rank: Regular Contributor
timestamp: January 13, 2012, 12:29:18 am
content: Hi,What is your measurement range in kg? What resolution do you need in kg? what accuracy are you after? what is your max overload weight in kg? What temperature range does your scale need to hold calibration for?Without those figures its pretty hard to tell if what sparkfun are selling is appropriate for your target use.You also need to consider how you are mounting the load cells to your structure, as doing this incorrectly can lead to very poor results.If you have a structure that is suitable, then directly bonding strain gauges to it would be cheaper and more reliable, check out omega as they have guides on the application process. They also do some easy heat activated glue gauges that are simple to apply (we use these at work).It is much cheaper to get high quality gauges and a cheap amplifier then the other way round.