UC3842 / UC3843 pins: what to measure, what to see
The heart of many adapters, monitors and switch-mode supplies. Tap a pin to see what it does, where and what to measure, and what the scope should show.
Where you'll meet it Flyback adapters, monitor and printer supplies, older TV standby circuits
Pin 1 · Control / signal
COMP
The error amplifier output. The higher this voltage, the longer the pulses (more power). In opto-coupled designs the optocoupler pulls this pin down to reduce power.
🔧 How to measure
DC voltage against the hot-side ground (pin 5).On the scope
📋 What order to measure in?
- Unplugged: is there a short between pins 6–5 and 7–5? (If the MOSFET blew, the IC may be damaged too.)
- VCC (7): does it rise to 16 V (8.4 V on a UC3843) at start-up and stay there, or does it cycle?
- VREF (8): is 5.0 V present? If not, the IC's internals aren't running.
- RT/CT (4): is there a sawtooth on the scope?
- OUTPUT (6): are there square pulses?
- ISENSE (3): does the ramp stay below 1 V?
- COMP (1): is it in the 1.5–3 V range, and does it change with load?
How it works in a circuit and drives the MOSFET
A real circuit using this IC. The line lighting up yellow is the IC's gate output: when the IC drives it HIGH, the MOSFET turns on. Follow the IC's output alongside the circuit's currents and voltages in the slow-motion strip below.
⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)
Each period the IC drives OUT HIGH and the primary current ramps up. When the ramp on CS reaches the threshold set by FB, OUT goes LOW and the energy moves to the secondary. More load raises the threshold and lengthens the pulse.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/2: Q on
The IC turns Q on from its OUT pin. Primary current flows to ground through Rs; the CS pin watches that current.
UC384x OUT pin: HIGH (≈12 V, switch on)
1. Q's drain
The flyback's signature: ≈0 V when on; a spike (limited by the snubber) and ringing when off.
On the scope
🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic
Family differences: 3842, 3843, 3844, 3845
| IC | Start voltage | Stop voltage | Maximum duty cycle |
|---|---|---|---|
| UC3842 | 16 V | 10 V | ≈100% |
| UC3843 | 8.4 V | 7.6 V | ≈100% |
| UC3844 | 16 V | 10 V | 50% |
| UC3845 | 8.4 V | 7.6 V | 50% |
- •All four share the same pinout; they differ in start/stop voltage and maximum duty cycle.
- •On the 3844 and 3845, the output frequency is half the oscillator frequency (every other pulse is skipped).
- •When replacing, use the exact code on the board. Fitting a 3843 in a 16 V design may work but can change its behaviour; fitting a 100% part in place of a 50% one (3844/45) can saturate the transformer.
- •The UC284x and UC184x are the same pinout with wider temperature ranges; other makers' 384x equivalents share the pinout too.