Junction Boxes & Summing Cards

This is the first fork because it changes the price more than anything else on the page. A complete junction box is the summing card already mounted in a sealed housing, with a cable gland for every load cell plus one for the homerun cable, screw terminals landed and a grounding post for the shields. It is what you order for a new installation, and it is what you want if anyone other than you is going to open it later.

A summing card on its own is the same electronics without the housing. It is the right part in exactly two situations. The first is a repair: a board has been damaged, usually by water that got past a gland or by a lightning event on a long cable run, and the enclosure itself is fine. The JB-PCB-4, JB-PCB-6 and JB-PCB-8 drop straight back in. The second is OEM work, where the summing network is being built into a machine frame or a customer's own enclosure rather than into a standalone box.

The size of the board matters when you are fitting it into something that was not designed around it. The standard JB-PCB-4 is 100 × 75 mm, roughly square. The narrow version carries the same four channels on a 124 × 26 × 17 mm strip, which is the part to specify when the available space is a slot rather than a square.

One product on this page belongs to neither group. The Load Cell Splicing Box has no summing card and no trimmers at all: it exists to join or extend a single load cell cable inside a sealed housing, which is the correct alternative to a wire nut and a wrap of tape.

Count the load cells, then buy that channel count or the next one up. Four channels covers the JB-FI-4, JB3V, JB-SS-4 and AJB-015, which is by far the most common requirement. Six is the JB-FI-6 and JB-SS-6. Eight in a single enclosure is the JB-SS-8. Unused channels are not a problem: the JB-FI boards are specified to sum from two load cells upward, so a four-channel box running three load cells is a normal configuration, not a compromise.

The JB-FI boards also daisy-chain. Two JB-FI-4 cards and enclosures sum eight load cells, and two JB-FI-6 cards and enclosures sum twelve. That is worth knowing when a structure grows after the fact, because it means an existing installation can be extended rather than replaced. It is not free of consequence though: two boxes mean two sets of glands, two sets of terminations and a longer interconnect, all of which are places for water and noise to get in. Where a single enclosure will do the job, use one.

The number that actually caps a system is usually not printed on the junction box at all. Load cells wire in parallel, so each one added divides the total bridge resistance and draws more current from the instrument's excitation supply. That supply is the limit. Most of our digital indicators drive four or eight 350-ohm load cells, and the TI-1680 drives twelve. Summing eight load cells into an instrument rated for four does not work, no matter how good the box is.

The other constraint is that the load cells being summed have to be the same capacity and the same nominal output. Trimming corrects small differences in rated output between load cells of the same type. It cannot reconcile a 1,000 lb load cell with a 2,500 lb one, because they are producing fundamentally different millivolts for the same load.

Both methods solve the same problem, which is that two load cells of the same nominal rating do not produce precisely the same output for the same load. The difference is where in the circuit the correction is applied.

Signal trim puts the multi-turn potentiometer in the signal path, attenuating the millivolts returning from the load cell that reads high until it agrees with the others. It is the default on this page and the method almost every field balancing procedure is written around, including the one below. It works with any instrument, and there is nothing about it the instrument needs to know. The JB-FI-4 Signal Trim, JB-FI-6, JB3V, all three JB-SS boxes, the AJB-015 and four of the five summing cards use it.

Excitation trim reduces the excitation voltage delivered to that load cell instead. Because a load cell's output is a ratio of its excitation, dropping the excitation to a high corner scales its output down proportionally, and the correction happens before the measurement rather than after it. Nothing is inserted into the signal path. It is available here on the JB-FI-4 Excitation Trim box and the JB-PCB-4 Excitation Trim card.

In practice the decision is usually made for you. Order excitation trim when your instrument's manual, your scale manufacturer or your customer's specification calls for it, and order signal trim otherwise. On the JB-FI-4 the two versions are the same enclosure at the same $60, so the only thing you are choosing is the card inside, and the two are not interchangeable once the system is calibrated. As a rule, signal trim suits most general-purpose scales, while excitation trim holds linearity better and is the usual choice for high-accuracy systems. Sense wiring is the same on both versions: a 7-position output strip with the sense terminals jumped inside the box.

A junction box is almost always the lowest and least accessible part of the installation, usually bolted under a structure where water collects and where nobody looks until something reads wrong. The housing is therefore worth more attention than the electronics, which are identical across the range.

ABS is the indoor answer. The JB-FI-4 is rated IP65, with glands accepting cable from 4.5 to 8 mm outside diameter and terminals taking 12 to 24 AWG. That covers dust and hose-directed water, which is enough for a bench, a warehouse floor or a dry production line.

IP68 is a different promise: continuous immersion rather than splashing. The JB3V is the model for it, in an ABS housing with stainless self-retaining cover screws and a breather vent. Both details matter more than they sound. Self-retaining screws stay in the lid, so nothing gets dropped into a pit during a service call. The vent lets water vapour out while keeping contaminants and liquid water from getting in, which is what stops condensation forming inside a sealed box that heats up during the day and cools at night. A perfectly sealed unvented enclosure outdoors will eventually rain on its own terminals.

Stainless steel is what food, chemical, pharmaceutical and outdoor coastal installations end up needing. It survives caustic cleaning chemicals and the corrosion that eventually chalks and cracks a plastic housing, and it takes the mechanical abuse of an area where things get hosed and knocked. The JB-SS-4, JB-SS-6, JB-SS-8 and AJB-015 are the stainless options. The AJB-015 is specified for −10 °C to 40 °C at under 95% humidity, non-condensing, which is the range to check against an unheated dock or a cold store before ordering.

Everything the junction box does arrives at the instrument on one cable, and two details about that cable decide whether the installation is stable or intermittently strange.

The first is sense lines. The JB-FI output is a 7-position terminal strip with the sense terminals internally jumped, which means a 6-wire homerun cable can be landed at the box and the instrument's sense lines will read the excitation as it exists at the junction box rather than at the instrument. That is the point of 6-wire: it lets the instrument correct for voltage dropped in the homerun cable, which is otherwise a slow error that changes with temperature and cable length. On a short run in a stable room, 4-wire is fine. On a long run, outdoors, or anywhere the temperature swings, use the sense pair.

The second is the shield. Every load cell cable coming into the box has a shield, and the box provides a grounding post to land them on. The rule that matters is that the shield is grounded at one end only, normally at the instrument. Grounding both ends creates a loop between two points that are not actually at the same potential, and the current that flows around it lands as noise on a signal measured in millivolts. Transcell's load cell wiring guide covers the full colour code and the step-by-step procedure.

Finally, the cable itself. Load cell signal is small enough that cable choice is a real specification, not an afterthought: use shielded load cell cable, keep it physically away from motor leads and variable-frequency drives, and do not run it in the same conduit as anything switching power. The most common noise complaint on a summed platform is not the junction box at all.