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Load cell (Wheatstone bridge) excitation voltage hurdles

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V_King:
Hello,

I am playing with my load cells and ran into an issue and wonder if anybody could help with some advice.

I usually use the INA125 load cell amplifier with in-build voltage reference for load cell excitation. For this particular project I want to use 10V to supply to the load cell. The INA125 amplifier can only supply ~10mA, whichis not enough for 350ohm load cell. The data sheet recommends to use TIP29C NPN transistor to increase current. But because I missed the current limit on the amplifier, first PCB I did was without the transistor. Just to get me going, I just connected the load cell excitation wire to the linear voltage regulator from the op-amp rail temporarily until I get the updated PCB. The load cell reading were very good, no unwanted noise.
I used a MMBTA42LT1G transistor as an equivalent for the TIP29C for my new PCB board. Once the board arrived and I connected everything, the first thing I noticed was a low frequency noise which was not there before.   

Am I doing something wrong with the transistor? Adding a 0.1uF capacitor between load cell excitation leads seem to have reduced the noise, but did not remove it completely. That noise was not there when I was using load cells directly driven from the INA125 at 5V or 2.5V. What is more, the PCB with load cell driven of a linear voltage regulator and a few caps seem to be working without any noise issue. That got me thinking whether those precision voltage references for excitation are necessary? I would ideally would like to use a better bandwidth amplifier and this discovery with simple linear voltage regulator got me thinking again. I am not worried about voltage drift, because the load cell is used for short periods of time.

I tried reading the application notes from the usual suspects but I just could not understand the complexity and the need of a number of expensive ICs just to read a load cell. It sounded more like a marketing brochure than advice.

Any input and feedback is much appreciated.

Thanks

David Hess:
The usual and better solution is to make a ratiometric conversion which has the advantage of removing excitation noise and drift.  To do this, the reference for the analog-to-digital converter is made the same as the excitation for the load cell, or the reverse.  With this done, there is no particular need for the load cell excitation and analog-to-digital converter reference to be either noise free or low drift; the output from a common integrated fixed voltage regulator like a 7812 is perfectly adequate.

If you use a separate load cell excitation and load cell reference, then not only does this contribute another gain error term and noise source, but it prevents rejection of flicker noise which now must also be accounted for so a low drift and low noise design becomes more important.

srb1954:
There is no way an MMBTA42LT1G is an acceptable substitute for a TIP29C.

The TIP29C is in a TO-220 package and is rated for up to 30W dissipation (depending on heatsinking) versus 225mW for the MMBTA42 in its SOT-23 surface mount package.

The TIP29C at 1A continuous collector current has twice the rating of the MMBTA42 at 500mA although the useful collector current range of the MMBTA42 is much less as its current gain drops off severely at collector currents more than 20mA.

wraper:

--- Quote from: srb1954 on July 12, 2020, 01:42:04 am ---There is no way an MMBTA42LT1G is an acceptable substitute for a TIP29C.

The TIP29C is in a TO-220 package and is rated for up to 30W dissipation (depending on heatsinking) versus 225mW for the MMBTA42 in its SOT-23 surface mount package.

The TIP29C at 1A continuous collector current has twice the rating of the MMBTA42 at 500mA although the useful collector current range of the MMBTA42 is much less as its current gain drops off severely at collector currents more than 20mA.

--- End quote ---
Pretty dumb train of thought about suitability of replacement. He does not replace it in some device he bought. This is a question of suitability for particular application (driving 350 ohm load), not about what is a drop in substitute for TIP29C in general.
IMHO it should work but it's far from the best choices. There is no reason to use 300V transistor. And probably something with a thermal tab/pad should be used depending on how much voltage drops across it.

wraper:
The question is where you connect collector of a transistor? to V+ of INA125? What is V+ voltage?

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