Electrical Fundamentals — Bridge, KVL, Series vs Parallel
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Electrical Fundamentals

// Wheatstone bridge · KVL/KCL · Series vs Parallel — field application only
// YOU SEE THIS ON RTDs, LOAD CELLS, AND PRESSURE SENSORS — HERE'S WHAT IT ACTUALLY DOES
WHEATSTONE BRIDGE EXCITATION + EXCITATION − R1 100Ω Rx RTD R2 100Ω R3 100Ω OUTPUT SIGNAL mV output 2 wires in (power) — 2 wires out (signal)
What Is It?
// Four resistors in a diamond — one of them changes with the thing you're measuring
A Wheatstone bridge has four resistors in a diamond pattern. Three are fixed, one is the sensor (Rx). As the sensor's resistance changes (with temperature, pressure, weight), the voltage across the middle of the diamond changes — that small voltage is your measurement signal. Balanced bridge (all equal) → output = 0mV
Rx changes → bridge goes out of balance → output ≠ 0mV The transmitter reads that imbalance and converts it to 4–20mA.
// SHOW HOW Rx CHANGES
Balanced — 0°C / 32°F
All four resistors equal → bridge is balanced → output = 0mV → transmitter outputs 4mA → 0% reading on HMI.
// WHERE YOU SEE THIS IN THE FIELD
RTD Temperature Sensing
PT100 RTD is Rx in the bridge. Temperature rises → PT100 resistance rises → bridge goes out of balance → mV output increases → transmitter scales it to 4–20mA. 4 wires on RTD: 2 excitation leads, 2 signal leads
3-wire: compensates for lead resistance error
2-wire: simpler but lead resistance adds error
// FIELD CHECK
RTD reads max temp or negative? Ohm the RTD terminals directly — OL = open wire, ~0Ω = shorted. The bridge just reported what the RTD gave it.
Load Cells (Weight)
Load cells use strain gauges (tiny resistors that change resistance when flexed). Four strain gauges form the bridge. Weight on the cell flexes the metal → gauges change resistance → bridge outputs mV → weight controller reads it. Your packer load cells work this way
Typical output: 0–30mV at full load
Transmitter amplifies and converts to 4–20mA
// FIELD CHECK
Zero-weight reading drifting? Check excitation voltage first (usually 5–10VDC). No excitation = no bridge output. Also check for moisture in the load cell body.
// ONE PATH OR MANY — WHAT BREAKS AND WHAT KEEPS RUNNING
Series Circuit
// One path — same current through everything
24V R1 R2 R3 ONE FAILS → ALL FAIL
Same current flows through every component. Voltages add up to the source voltage. One break = everything stops. Total resistance: R_total = R1 + R2 + R3
Current: same everywhere in the loop
Voltage: divides across each component
// FIELD FAILURE EXAMPLE
4–20mA loop is series. One broken wire = 0mA = everything reads 0% or faults. One blown fuse in the loop = same result. Find the break.
Parallel Circuit
// Multiple paths — same voltage, currents split
24V SOL 1 ✓ SOL 2 ✗ SOL 3 ✓ ONE FAILS → OTHERS KEEP RUNNING
Same voltage across every branch. Current splits between branches. One branch fails = others unaffected. Voltage: same across every branch
Current: splits — more branches = more total current
One path opens = rest still work
// FIELD EXAMPLE
24VDC solenoids on a common supply rail are parallel. One solenoid coil burns out — it draws no more current but the others keep running. Only that one device fails.
// THE FIELD CLUE — HOW MANY THINGS FAILED?
SymptomCircuit TypeWhat to Look For
Everything on one circuit went dead at onceSeries breakOne fuse, one broken wire, one bad component — find it and fix it
Only one device failed, others on same supply OKParallel — one branch failedThat specific device — coil, wire, connection
Supply voltage dropped but nothing fully failedParallel — high load or partial shortToo many branches drawing current — check total load vs supply rating
4–20mA loop reads 0mA — full deadSeries break in loopAny open in the loop — wire, terminal, fuse, transmitter power
// KIRCHHOFF'S LAWS — FIELD TRANSLATION
KVL — Voltage Law
// "What goes up must come down"
All the voltage that a source puts in must be used up by the loads in the loop. The voltages around a closed loop always add up to zero. Source voltage = Sum of all voltage drops
24V in → devices use exactly 24V total
Voltage doesn't disappear — it converts to heat/work Field meaning: If your 24V supply is putting out 24V but your solenoid only sees 20V — 4V is dropping somewhere upstream. Corroded terminal, undersized wire, loose connection.
// HOW TO USE IT IN THE FIELD
Measure voltage at the supply: 24V. Measure at device: 20V. You know there's a 4V drop. Probe each terminal block and connection between supply and device. The one that shows the 4V drop is your bad connection.
VOLTAGE DROP HUNT 24V SUP −2V bad TB DEVICE 22V ✓ 24V 22V (-2V drop)
KCL — Current Law
// "What goes in must come out"
All the current flowing into a junction must equal all the current flowing out. Current doesn't accumulate anywhere — it's always conserved. Current into junction = Current out of junction
Think of it like water pipes splitting and rejoining
No current disappears or appears from nowhere Field meaning: If your supply says it's putting out 2A but you only see 1.5A in the circuit, current is leaking somewhere — a parallel fault path to ground. Ground fault.
// HOW TO USE IT IN THE FIELD
Supply powering 5 solenoids, each drawing 0.2A: total = 1A. If your clamp meter on the supply reads 1.8A — there's an extra 0.8A going somewhere. Look for a ground fault, shorted coil, or unintended load.
CURRENT SPLITS AT JUNCTION 1.0 A → 0.4A → SOL1 0.3A → SOL2 0.3A → SOL3 0.4 + 0.3 + 0.3 = 1.0A ✓
// APPLY IT — FIELD SCENARIOS
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