On an analog weighbridge the junction box is where corner correction happens. It is also where water gets in, where a terminal works loose, and where a drifting reading usually starts. This covers the JB-S box: what it is built from, how to wire it, how to trim the corners, and what to check when the numbers misbehave.
JB-S specification
- Enclosure in 304 stainless steel (1.4301)
- Sealed cable glands in PG9 plastic
- Connects 4, 6 or 8 load cells
- Excitation or signal trimming (±10 turns, ±10 Ω)
- Takes 4-wire or 6-wire load cells
- Output to a 4-wire or 6-wire indicator
- Cable diameter 5.2–6 mm
- Suits cell outputs from 0.6 to 3 mV/V
- Protection IP65
- Screw terminal blocks
Choosing and fitting
Excitation trim or signal trim? Excitation trimming adjusts faster, but where the corner errors are large, use the signal-trimming version instead.
Size to the bridge. J4B-S handles four cells with individual adjustment; J6B-S and J8B-S extend that to six and eight. Unused glands must be closed with the proper plugs to keep dust and moisture out.
Earth the enclosure properly. Stability depends on it.
Operating range is −40 °C to 85 °C. On signal-trimming boxes the potentiometers run up to 20 turns, which allows a much finer trim.
Mounting to the frame
The box fixes through four 8 mm holes. The load cells it serves should be the same model, the same rated capacity and the same rated output in mV/V — mixing them makes the corners impossible to balance cleanly.
When fitting, make sure every cell shield is bonded and the box is firmly attached to the frame, well away from standing water and damp.
Wiring
Remove the lid and land each cell cable on its matching terminal block, then take the board output — marked OUT or INDICATOR — to the indicator. Every block is labelled; read them rather than working by position. Check that all terminals are tight and no conductor is broken.
A JB-S can be used with fewer cells than it was designed for — nothing needs changing. Just make sure the unused glands are properly closed.
Corner trimming
Power up the indicator once every cell is connected. Before touching the corners, confirm the capacity, the division and the repeatability of the scale, and resolve any fault showing on the display — trimming a scale that has another problem just hides it.
The potentiometers leave the factory set neutral, so there is nothing to adjust before calibration.
Step 1 Move the test load to each corner in turn and write down the displayed weight.
Step 2 Average all the readings. That average is your reference value.
Step 3 Go round the corners again, trimming each potentiometer up or down until the displayed weight matches the reference.
Step 4 Repeat steps 2 and 3 until every corner falls inside the permitted error.
Troubleshooting
Reading looks wrong
- Does the scale return to zero with nothing on it?
- Is the load entirely within the platform?
Corners disagree
- Repeat the setup and trim the corners again
- Check whether any cell is damaged
Reading drifts
- Check the box for water ingress
- Check cells and cables for damage
- Disconnect cells one at a time — if the reading steadies, the cell just removed is the suspect
Use a load cell simulator, or a known-good cell, to confirm the indicator itself is working correctly before spending time on the bridge.
Field service note
KALA Technical Notes
A junction box that has taken water usually shows up as drift rather than as an outright failure, which is why it gets blamed on the load cells first. If your readings wander with no load on the bridge, open the box before ordering anything — and send us what you find inside.
● Stainless steel junction boxes for 4, 6 and 8 cell installations.