Electronics > Projects, Designs, and Technical Stuff
Load cell (Wheatstone bridge) excitation voltage hurdles
wraper:
--- Quote from: V_King on July 12, 2020, 03:23:52 pm ---
--- Quote from: wraper on July 12, 2020, 01:55:58 am ---
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.
--- End quote ---
My knowledge in analogue circuits is very limited. The analog devices equivalent transistor selection guide advises to match the gain in such circuits and MMBTA42LT1G was in the basic JLCPCB parts list (eg available without surcharges) with the same hfe.
--- End quote ---
Something like MMBT4401, BC817, MMBT2222 is better suited for the circuit. Gain specs of TIP29C and MMBTA42LT1G are specified at very different conditions. Also gain does not matter that much since this is an emitter follower. And to get so low current gain spec for small current transistor you need to dig into outskirts, like high voltage part as MMBTA42LT1G.
--- Quote ---Are there any other practical rules of thumb to try to match transistors?
--- End quote ---
At least keep rated voltage in the same ballpark. Going for higher voltage will increase price and likely will negatively affect other specs.
V_King:
Thank you, wraper, for all your help. Transistors do seem to be a black art. Maybe an idea for Dave's videos about practical ins-outs of transistors for young players?!
--- Quote from: wraper on July 12, 2020, 06:20:19 pm ---
--- Quote from: V_King on July 12, 2020, 03:23:52 pm ---
--- Quote from: wraper on July 12, 2020, 01:55:58 am ---
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.
--- End quote ---
My knowledge in analogue circuits is very limited. The analog devices equivalent transistor selection guide advises to match the gain in such circuits and MMBTA42LT1G was in the basic JLCPCB parts list (eg available without surcharges) with the same hfe.
--- End quote ---
Something like MMBT4401, BC817, MMBT2222 is better suited for the circuit. Gain specs of TIP29C and MMBTA42LT1G are specified at very different conditions. Also gain does not matter that much since this is an emitter follower. And to get so low current gain spec for small current transistor you need to dig into outskirts, like high voltage part as MMBTA42LT1G.
--- Quote ---Are there any other practical rules of thumb to try to match transistors?
--- End quote ---
At least keep rated voltage in the same ballpark. Going for higher voltage will increase price and likely will negatively affect other specs.
--- End quote ---
I will order the transistors you are recommending and see if they make any difference. Would it be a good idea to add a small cap of 0.1uF on the output as well? At the moment I added one within load cell wiring and it seem to have reduced the noise levels. But it is more like treating symptoms not causes I guess.
--- Quote from: wraper on July 12, 2020, 05:47:14 pm ---
--- Quote from: V_King on July 12, 2020, 03:23:52 pm ---Yes, the collector is connected to V+, which is +12V with 10uF, 1uF and 0.1uF caps next to MC78L12 V reg.
--- End quote ---
Very likely this is your problem. You want 10V load cell excitation voltage from 12V V+ on top of that using emitter follower. So INA125 needs to output about 10.7V VrefOut voltage for circuit to work properly. Therefore it's marginal at best. Did you measure actual excitation voltage you get? Try powering the circuit from 15V.
--- End quote ---
The voltage measured with transistor was 10.01V sharp, the overheads seem to be sufficient. The INA125 datasheet recommends min of 1.25V above the reference voltage for correct operation.
I would like to keep the 12V rail, so I will get some of the transistors you recommend and see if there is any improvement.
mikerj:
--- Quote from: V_King on July 12, 2020, 03:27:37 pm ---
--- Quote from: mikerj on July 12, 2020, 08:33:40 am ---Noise from the reference will be proportional to the reference voltage you use, e.g. at 10V you can expect 4x the noise compared to 2.5v. At low frequencies you will be fighting against 1/f noise.
However have you ruled out more fundamental issues such as your circuit oscillating?
--- End quote ---
The v ref is internal in the INA125.
--- End quote ---
The noise level from the device will be increased according to the reference multiplier you select. The datasheet does states this.
voltsandjolts:
Maybe this is slightly off-topic since you are looking for analog output but the reference input of SD converters can float (above ground).
So, you can use the voltage across the bridge as the reference which makes everything nicely ratiometric.
As a bonus, you can also measure resistance variation of the bridge itself which might be useful for temperature drift compensation, e.g. in silicon strain gauges.
Refering to the attached schematic: Measuring VR2 (versus GND, single-ended) gives ratio of bridge resistance (perhaps temperature dependant) to some temperature stable resistance R2.
Using the bridge itself to measure temperature provides better compensation than using some external temperature sensor (PRT or whatever), particularly when temperature is changing (no thermal lag).
R1 helps with keeping in the common mode range of the input diff amp.
If you add capacitors for filtering, keep the time constants on the measurand and the reference roughly the same.
So, no accurate voltage references required.
V_King:
--- Quote from: voltsandjolts on July 13, 2020, 09:24:46 am ---Maybe this is slightly off-topic since you are looking for analog output but the reference input of SD converters can float (above ground).
So, you can use the voltage across the bridge as the reference which makes everything nicely ratiometric.
As a bonus, you can also measure resistance variation of the bridge itself which might be useful for temperature drift compensation, e.g. in silicon strain gauges.
Measuring VR2 below gives ratio of bridge resistance to some temperature stable resistance R2.
R1 helps with keeping in the common mode range of the input diff amp.
So, no accurate voltage references required.
(Attachment Link)
--- End quote ---
Thanks for the input.
Following David Hess's comment it got me thinking whether I could use two V-ref outputs from the INA125. The 10V for the load cell excitation and 2.5 to my current ADC.
I just need to understand the implications
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