500g weight scale: repair
Small, cheap weight scale with LCD display, 500g range and 0.01g resolution.
Problem: when powered on this weight scale was displaying 0.00g and not reacting in any way to force applied to pan.
This single-chip weight scale is using simplest/cheapest measuring method with DC excitation for the bridge.
Measuring voltage between S+ and S- (with no force applied) was showing 10mV - I guess this offset was beyond the range that all-in-one weight scale chip is able to compensate.
Given that excitation voltage for this bridge is ~2.2V and assuming 2mV/V (*) load cell sensitivity, this would be equivalent to 230% of nominal load.
I don't know what caused this offset, perhaps mechanical bridge overload? This 500g load cell could possibly be damaged with as little force as a weight of 1kg.
Maybe it should be stored in some kind of protecting case.
(*) it looks like many load cells in this range have actually smaller output ratio, 0.7mV/V or 1mV/V.
Usually these weight scales can be calibrated when pressing [T] while turning on, but initially it was displaying only 16384 after this operation.
I've initially connected 250kOhm variable resitor between excitation and signal out bridge pads to find the resistance needed to compensate offset.
Is this the right way or should this weight scale be tossed directly to the trash?
Turns out offset was nearly zeroed with 58kOhms - here with 10kOhms + selected sample of 47kOhms. It is likely that 56kOhms would be also good enough.
After this repair [T] + [ON] key combination was enabling calibration mode as expected.
Quick test with a coin seems to be giving correct result.
I've noticed significant creep even when weighting small object like that coin. In a minute after placing the coin, displayed weight was raising from 6.5g to something like 6.8g. I would not put too much trust in these readings.
Second piecie
Virtually identical device.
This one had higher offset, 20mV and different offset direction. 27kOhms resistor was required.
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