====== How the electrics work ====== How the C5's electrics work: the instrument pod and its ULA chip, the control box, the motor protection layers, the battery gauge, and the charger. This page is based mainly on //The Unofficial Sinclair C5 Service Manual//. It draws on the original engineering paper by **P.J. Milner** (Sinclair Vehicles) and **Perran Newman** (Sinclair Research), who designed the system, and on the redrawn schematics (Figures 1–4) published by C5Alive. **Safety:** the motor circuit carries up to 140 A. Remove the battery, or press the emergency cut-out, before working on the control box or the motor wiring. ===== Design philosophy ===== * The C5 had to be safe for untrained users aged from 14 upwards (the oldest owner recorded during development was 92), and the designers expected **deliberate abuse**. * There is **no proportional speed control**. The motor is simply switched on or off by a relay. Because the permanent-magnet motor has a fixed no-load speed, the 15 mph legal limit is met by the motor itself. * So the electronics are almost entirely about **protection**: the motor against overheating, and the battery against over-discharge and wrong charging. A master switch, fusing and weatherproofing complete the system. ===== Block diagram (Service Manual fig. 2) ===== USER CONTROLS INSTRUMENT POD LAMPS motor switch ──┐ [motor-load bar] [buzzer] [battery bar] front / rear lights ────────┤ │ horn, indicators x4 indicators ────┤ │ │ horn ──────────┴──────────────┼───────── flasher ────────────────┘ │ BATTERY + CONNECTOR CONTROL BOX MOTOR + PROTECTION key master switch current shunt (1 mΩ) thermistor in hollow manual cut-out ───► motor relay ─► commutation diode ─► armature shaft; fuses, 5 V regulator, stall bi-metal thermal cut-out detector (LM358) on motor case The main parts: - **Instrument pod (IP)**: the dashboard under the canopy. It holds the two 5-segment LED bar graphs, a piezo buzzer, and the Ferranti **ULA** chip that does nearly all the logic. - **Control box (CB)**: the sealed box at the rear. It holds the motor relay, the current shunt, the 5 V regulator, the stall detector, the fuses and the harness junctions. - **Motor protection**: a thermistor probe inside the armature shaft, plus a bi-metallic trip on the motor case. - **Battery connector and manual cut-out**: the key-operated master security switch on the battery top. ===== Motor protection: three layers ===== The Sinclair team chose a permanent-magnet motor with **high stall torque**, for hill climbing and brisk acceleration in traffic. That makes it easy to cook the motor, so it is guarded three ways: ^ Layer ^ Sensor ^ Behaviour ^ Reset ^ | 1. Stall / heavy overload | Current shunt in the CB; motor current is integrated over time by the LM358 circuit | A dead stall at ~140 A trips in about **3–4 s**. For heavy loads the sources disagree. //New Scientist// (via Wikipedia) says ~80–90 A trips after 2–3 minutes. //The Engineer// (Jan 1985) says 90 A trips after about 20 s. The latch fakes a "very hot thermistor" so the pod locks out the relay. | Remove power (the +5VA rail) briefly, i.e. switch off and on | | 2. Armature temperature | Close-tolerance **thermistor** in a non-contacting probe that sits in a hole drilled into the end of the armature shaft | Two thresholds. **First level:** buzzer plus flashing warning. **Second level:** motor relay locked out until it cools. Trip points are at nominal thermistor resistances of about **1100 Ω and 900 Ω**. | Automatic, when the motor cools | | 3. Motor case | Resettable **bi-metallic thermal trip** on the motor case | Breaks the relay-coil current at **70 °C ±5 °C** | Automatic | The engineers tried two other approaches first and dropped them. One was an analogue "mathematical model" of motor heating built from integrated circuits. The other was a physical thermal model, a heater on a metal block clamped to the motor. Thermocouple tests showed the shaft-air temperature tracked the deep armature temperature well enough to use in production. Newman later recalled that motor failures were "virtually unknown" before Sinclair Vehicles went into administration. ===== Battery protection and the "fuel gauge" ===== * The right-hand bar graph shows the battery's **compensated terminal voltage under load**. The ULA adds back the voltage the battery loses under load (assuming about 18 mΩ of internal resistance) and uses the shunt current to do it. * The five segments cover **12.0 V down to 10.5 V** in equal steps. Battery voltage falls unevenly as it discharges, so the display drops slowly at first and faster near empty. That gives plenty of warning. * The gauge **can only count down**. A segment only goes out after the voltage has stayed below its threshold for a **20-second** delay, so brief dips on hills don't register. The state is held as long as the battery is connected, because the +5VB rail is always live. * When the last segment goes, a **20-second audible warning** sounds before cut-out, so you can finish a manoeuvre. Wikipedia's sources describe the last light as "about ten minutes left". * The cut-out point is set by trimmer **VR1** in the pod. See //Setting up VR1// below. ===== Motor load display ===== * The left-hand bar graph shows motor current, measured as the voltage across the 1 mΩ shunt (1 mV per amp). * Driving guide from the Service Manual's range chart: cruising in the **right-most green** segment gives about 13 miles from one battery. Amber means heavy load and red means very heavy load, so pedal to help. The calibration comes from the Hoover //Service Engineer Training Notes// (1984). Each segment is worth about **15 A**. The yellow segment at each end is always lit when the switch is at D, as a reference. ^ Segment (left to right) ^ Motor current ^ | Green 1 | 0–15 A | | Green 2 | 15–30 A | | Yellow / amber | 30–45 A | | Red 1 | 45–60 A | | Red 2 | 60–75 A | Key figures from the same notes: ^ Condition ^ Current ^ | Motor running free | ~3 A | | Level running | ~15 A | | Climbing a 1-in-15 hill | ~50 A | | Stalled | ~140 A | * **30 A is the maximum continuous current** the motor can take without a destructive temperature rise. Anything in amber or red means the motor is heating up, and the further right, the faster it heats. * **At 70 A, a cold motor overheats in about 10 minutes.** John Wilson's ammeter retrofit notes agree: about 29–30 A on the flat, about 40 A climbing while pedalling, and under 5 A when coasting. ==== What the pod is telling you (Hoover training notes) ==== ^ Display ^ Meaning ^ | One load segment flashes whenever power is drawn, with an intermittent buzzer | Motor temperature nearing its limit (first thermistor threshold) | | Whole load display lit and flashing, no buzzer | Thermal lock-out. It can't be reset by hand; wait until the motor has cooled, possibly up to about 10 min | | Whole load display flashes about **3–4 s** after pressing the power button | **Stall** lock-out. Reset by pressing the red cut-out and turning the key back to D | | Last fuel segment flashes and the buzzer sounds continuously for 20 s | Low battery. The vehicle then locks out and only the end yellow segments stay lit. The display resets when you switch off and back to D | ===== Instrument pod: ULA and circuit (fig. 3) ===== * **IC1 = Ferranti ULA** (uncommitted logic array), marked **5G014 MK4** on the redrawn schematic. It is a semi-custom chip in a socket and it **cannot be repaired**. * Power: **+5VB** comes from the battery through regulator TR3 (a 78L05) and is always present, which is why the gauge remembers its state. **+5VA** comes from the control box and is only present when the C5 is switched on, so the displays only light when powered up. * The two bar graphs share a 5-line multiplexed bus (SD1–SD5). **TR1/TR2 (FST290)** switch the anodes. These are selected transistors with V_ce(sat) < 0.5 V at 100 mA. * There are three internal oscillators: * R10/C6: about 0.4 Hz, divided by 8 to give the **20 s** fuel-gauge delay. * R7/C4: 50 Hz, used for the flash rate and the lock-out timing. * R11/C7: about 9.5 kHz, divided by 2 to drive the piezo buzzer and the multiplexer. * **Motor switch path:** the handlebar button sends 12 V through a 15 kΩ resistor in the CB to ULA pin 21. The ULA then drives the CB relay transistor from pin 13. This is how the pod can refuse to start the motor. * **Thermistor:** a potential divider with R9/R20 across +5VB, feeding comparator pins 5 and 6, which trip at 0.5 × Vcc. ==== How the ULA measures (fig. 1/2) ==== A single 3-bit DAC and comparator are **time-shared** between the two displays. - In **motor-current mode**, a counter ramps the DAC up until it matches the shunt-derived voltage. The count is the load display, which is sampled and held. - In **fuel mode**, the DAC is set to the current fuel count. If the compensated battery voltage stays below it for the full 20 s delay, the fuel count drops by one. The compensation equation from the manual is I2 = 0.5 (I1 − I6 + I4). In words: the reading rises with battery voltage (I1), is offset by VR1/R3 (I6), and is pushed up by motor current (I4). ===== Control box (fig. 4) ===== * **Connectors:** * X9 goes pin-for-pin to X1 on the pod (the **red** pod loom). * X6 carries the thermistor (pins 1–2), the motor over-temperature trip (pins 4–5) and a remote cut-out (pins 7–8). * The heavy-current connections are on 4–5 mm studs inside the user-accessible part of the box. * **Relay:** a specially developed Ital-Amec unit, specified for 10,000 make/break cycles at 140 A. **TR3 (ZTX449)** drives it, and **ZD1** (a BZY88 zener) clamps the coil at turn-off, which speeds contact opening. * **D1, the commutation diode** (1N5401 class, across the motor): this is the critical part that suppresses arcing at the relay contacts. See //Known problems//. * **IC1** is the 5 V regulator for the pod display. * **IC2 (LM358 dual op-amp)** does two jobs. It turns the shunt signal into a current sink for the ULA, through R9. It also amplifies the shunt signal (gain 100/15) into the integrator C3, which drives the latching pair **TR1 (BC212L) / TR2 (BC182L)**, i.e. the stall detector. * **Fuses:** FS1 and FS2 (1 A and 5 A) on the auxiliary circuits. * The two plastic halves are **ultrasonically welded**. One part is a tamper-proof cavity; the other is user-accessible behind a screwed cover. The cable entry points down, with a splash grommet. * All motor wiring is **6 mm²** automotive PVC cable with machine-crimped ring and 6.35 mm spade terminals. ===== Wiring colours (from the accessory fitting instructions) ===== * Battery feed: **brown = +12 V**, **blue = 0 V** (original power loom from the key-switch block to the control box). * Thermistor: **purple and grey**. Thermal cut-out: **orange**. * Indicator/horn harness: Violet/White, Red/White, Blue/White, Black/White, Green/White, Brown/White and Orange/White pairs. The kits use 4-way and 3-way terminal blocks; see [[parts:accessories:start|Accessories]]. * Some C5s have an **alternative harness**. You can recognise it by a cluster of 6 connectors fixed under the central tunnel near the rear brake arm, and by having no horn connectors taped to the steering bars. ===== Charger ===== * A two-stage constant-current then constant-voltage profile, made by a hybrid circuit with laser-trimmed resistors driving **2 SCRs** as a phase-controlled current source. * The whole charger is **potted** in resin for cooling and weatherproofing. Two LEDs show //charging// and //finished//. * It charges the battery in place or removed. Full charge takes about **8 h**. ===== Reliability modifications (Service Manual) ===== These modifications protect against static damage to the ULA and LM358 failures: * **Pod:** fit a **47 kΩ** resistor (the "Known problems" section suggests 10 kΩ) in series with the track from X1 pin 6 to IC1 (ULA) pin 3. Add 1N914 catch diodes to Vcc and GND on IC1 pins 3/4. * **Control box:** power IC2 (LM358) pin 8 from **+5VA instead of +12 V**. ===== Setting up VR1 (battery cut-out point) ===== Follow these steps on the bench, with the pod out of the vehicle. - Connect ULA pin 9 to pin 22 (GND). This forces fuel-gauge-only mode at power-up. - Apply the required cut-out voltage, typically **10.5 V**, to +BATT at the top of R17. - Apply +5 V to Vcc (pin 8). - Set I4 to zero, for example by grounding pin 3. - Using a meter of at least 1 MΩ impedance, measure between pin 23 (+) and pin 27 (−). - Adjust **VR1 for +200 mV**. If it won't reach that, increase R3 by about 1 kΩ. - Remove the pin-9 link and check normal operation. Each segment should drop out after about 20 s, with two hidden segments below the lowest visible one. ===== Adapting for 24 V (original designers' notes) ===== * **Pod:** uprate C12's voltage rating, and double **R17 to 94 kΩ** so the fuel gauge reads a pair of batteries in series. * **Control box:** uprate C2, give IC1 a bigger heatsink, and put a resistor in series with the relay coil. * **Alternative:** run the electronics on 12 V and change only the battery-sense input through R17. Many owners now fit a PWM controller instead. See [[guides:modifications|Modifications]]. ===== Sources ===== * //The Unofficial Sinclair C5 Service Manual//, pp. 4–11, 19–27. * C5Alive redrawn schematics: Fig. 1 ULA block diagram, Fig. 2 ULA front end, Fig. 3 instrument pod, Fig. 4 control box. These were redrawn by Chris Gavin-Egan from drawings supplied by Roy Woodward. * Perran Newman, "Prescott Testing & C5 Motor Protection", c5owners.com. * John Wilson, "Simplified Relay Controls", c5owners.com.