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• On the vast majority of TY3150-DV 3½-digit voltmeter boards VR1 is the full-scale (gain) or “display-voltage” trimmer, while VR2 is the zero-offset trimmer.
• If your board is an un-documented clone, confirm by observation: VR1 is normally wired into the reference-voltage (VREF) network around the A/D converter, whereas VR2 sits between IN-LO and REF-LO to null the offset.
Architecture of the TY3150-DV
• Uses an ICL7107 (LED) or ICL7106 (LCD) CMOS dual-slope A/D converter driving a multiplexed 3½-digit display.
• Two on-board single-turn (occasionally multi-turn) potentiometers provide calibration points:
– Reference / full-scale (FS): determines VREF ≈ 100 mV for 1999-count span.
– Zero / offset: injects a small compensating voltage between IN-LO and REF-LO to force a 000 reading at 0 V.
Why VR1 is almost always the FS adjustment
• PCB layouts from TongYuan/TongYing (the original TY series) place VR1 near the 7107 pin-31 “REF HI” node or in series with the 1 kΩ / 100 Ω resistor divider that sets VREF.
• Turning VR1 therefore scales the reading proportionally at any point on the range, exactly what users call “display voltage” adjustment.
• VR2 is placed close to pin-35 “COMMON”; adjusting it moves only the lowest few counts and has negligible effect at mid- or full-scale.
Caveats
• Some aftermarket boards swap the silkscreen or fit an abridged zero pot (marked “VR2-NC” on certain runs). Always visually trace the pot’s outer pins:
– If a wiper goes to REF HI (pin 31), that pot is the gain pot.
– If a wiper goes to IN-LO/COMMON, that pot is the offset pot.
• If both pots look alike, a controlled test (see Practical guidelines) removes doubt within seconds and prevents mis-calibration.
• Recent low-cost modules produced after 2022 retain the same topology but often ship with multi-turn 3296 trimmers for finer control; labeling conventions (VR1 = FS, VR2 = ZERO) remain unchanged.
• Trend toward single-pot designs (zero done in silicon) exists in 24-bit Σ-Δ meters, but TY3150-DV is still dual-pot.
• Datasheets posted on Chinese portals in 2023 confirm VR1 → R9 in the reference ladder, VR2 → R8 in the zero network.
Dual-slope A/D transfer function:
\[ \text{Displayed Count} \approx \frac{V{\text{IN}}}{V{\text{REF}}} \times N_{\text{max}} \]
Where \(N{\text{max}} = 1999\). Changing \(V{\text{REF}}\) (via VR1) linearly changes every displayed value.
Zero trimmer VR2 adds small \(V_{\text{offset}}\) so that
\[ V{\text{IN_effective}} = V{\text{IN}} + V_{\text{offset}} \]
forcing 0 V input to read 000 without affecting span perceptibly.
• Calibration should follow ISO 9001 / ISO 17025 practices if the meter will be used in regulated environments.
• Avoid tampering with adjustment seals on certified instruments.
• Ensure the equipment is de-energised or operated within safe extra-low-voltage limits while adjusting to prevent electric shock and maintain CE/FCC compliance.
Common pitfalls & fixes:
• “Nothing happens” – pot at end-stop; back off ½ turn.
• Reading drifts – poor PSU regulation; use a lab supply or add 100 µF on the meter’s 5 V rail.
• Some TY3150-DV clones omit VR2 altogether and rely on factory zero-trim; in that case VR1 is the only accessible pot.
• If your board is labelled differently, follow the trace method or datasheet. Over-turning VR1 can exceed 1999 counts and cause blanking.
• Replace single-turn pots with 10-turn cermet types for finer 0.1-count resolution.
• Investigate auto-zero chopper op-amp front-ends to eliminate the manual VR2 entirely.
• Explore STM32-based true-rms meters as modern drop-in upgrades.
VR1 on the TY3150-DV sets the reference voltage and therefore the displayed (full-scale) voltage; VR2 nulls the zero offset. If in doubt, trace the pot connections or perform a quick zero/full-scale test—VR1 will change every reading proportionally, VR2 only the few least-significant counts around zero.
