Chapter 62: Temperature Drift and Margin

13:00. Dormitory. Lower bunk. Hard wooden board. Covered with old newspapers. Against damp. Lin Chen sat down. Canvas bag. On his knees. Zipper open. Metal sound. Cold. He took out graph paper. Drafting pencil. Eraser. Ruler. Compass. Arranged them. On the desk. Old. Paint flaking. Wood grain split. He pulled out a sheet. Smoothed it flat. Pinned it with the ruler. Drew axes. Horizontal axis. Time. Vertical axis. Temperature. Marked scales. Zero degrees. Twenty degrees. Forty degrees. Sixty degrees. Eight hours. Load. Temperature rise. Curve. Estimated.

Practical assembly assessment. Eight hours. Under load. System. Heating up. Components. Parameter drift. Resistors. Temperature coefficient. Positive. Negative. Capacitors. Dielectric loss. Changing. With temperature. Transistors. Beta value. Drifting. Threshold voltage. Offsetting. Unknown. Tolerance. Standard. He. Did not guess. Designed. For the worst case. Margin. Left ample. Fifteen percent.

He. Calculated. The compensation network. Negative temperature coefficient. Thermistor. NTC. In parallel. With the bias circuit. To cancel. Positive thermal drift. Copper foil. Heat spreader. Fitted to. The power transistor. Thermal grease. Filling. The gaps. Lowering. Thermal resistance. Vent holes. Opened. On both sides. Of the chassis. Diagonally. To form. Airflow. Chimney effect.

The storeroom. Had no standard parts. He stopped writing. Looked. Out the window. Sky. Gray-white. Wind. Whipped up. Dead leaves. Scraped. Across. The window glass. Rustling. He stood up. Put on clothes. Movements. Light. Did not touch. The bed frame. Cold water. On his face. Piercing. He wiped it. Dry. Changed the dressing. Iodophor. Cotton swab. Scab. Edges. Whitish. No seepage. Wrapped it. Tight. Put on shoes. Tied the laces. Canvas bag. Heavy. But. Steady. Out the door.

13:25. Old equipment storeroom. Side door. Iron lock. Rusted. Key. Turned. Stiff. Old Zhao. On duty. Cotton coat. Cuffs. Shiny with oil. Glasses. Filmed with white breath. “Back again.” Lin Chen. Handed over. The list. “Thermistors. Copper sheet. Thermal grease. Cooling fan.” Old Zhao. Looked. At the list. Frowned. “Thermistors. Only old parts. From scrap machines. Resistance value. Ten kilo-ohms. B value. Three-nine-five-zero. Copper sheet. Only offcuts left. The grease. Is almost dried out. Two fans. Broken. Pick through them yourself. Don’t. Mess up. The inventory.” Lin Chen. Nodded. Went in. Light. Dim yellow. Forty-watt. Incandescent bulb. Filament. Trembling faintly. In the air. Thick dust. Musty smell. Mixed with rosin. Old paper. Sourness.

He. Squatted down. Dug through. Wooden boxes. Loose parts. Jumbled together. He. Chose. Three. NTC thermistors. Leads. Oxidized. He. Used. Sandpaper. Polished them. Bright. He. Chose. Copper offcuts. Thickness. One millimeter. Length. Five centimeters. Width. Three centimeters. Two pieces. He. Chose. A tube. Of thermal grease. Squeezed it. Half-dry. He. Chose. A cooling fan. Twelve volts. Bearing. Seized. He. Dripped. Sewing-machine oil. Onto it. Turned. The shaft. Smooth again. He. Found. Insulating washers. Mica. Two pieces. He. Found. Cable ties. Nylon. Ten.

13:50. Back to the dorm. Cleared the desktop. He. Spread. Insulating paper. Set down. The copper sheet. Marked lines. Ruler pressed down. Pencil. Cut deep grooves. He took. Scissors. Copper sheet. Soft. But hard to cut. He. Added force. Steady. No slipping. The edge. Cut his hand. Blood. Seeped out. He. Ignored it. Kept cutting. Folded. The edge. Pliers. Bent it. Ninety degrees. Fitted surface. Flat and even. He. Applied. Thermal grease. Thin. Uniform. Covered. The back. Of the power transistor. Pressed. The copper sheet on. Cable ties. Secured it. Tight. Not loose.

He. Connected. The thermistor. Leads bent. Fixed in place. Soldering iron. Heated. Rosin. Smoked. Acrid. Fed solder. Pulled away. Joints. Full. No burrs. No cold solder. He. Measured. Resistance. Multimeter. Reading. Nine point eight kilo-ohms. Room temperature. Twenty degrees. Acceptable. He. Inserted it. Into the bias network. Calculated. The voltage-divider ratio. Matched it. He. Drew. A wiring diagram. On paper. Parameters. Labeled clearly. Rolled the drawing. Secured it. Put it in. The inner layer. Of the canvas bag.

13:55. Laboratory. Iron door. Open. Heat. Not on. Room temperature. Below freezing. Group A. Reinforcement group. Already there. Nine people. Lin Chen. Sat. In the corner. Third bench. Set down the bag. Opened the case. Filter board. Shield cover. Heat sink. Thermistor. Wiring diagram. Arranged them. On the desk. Neat. No clutter.

14:00. Old Li. Pushed open the door. Army coat. Collar turned up. Chalk dust. On his shoulders. He. Knocked. On the blackboard. Three times. The chalk. Snapped. White dust. Drifted down.

“Practical assembly. Assessment. Begins. Load. Already connected. Rated power. Eighty percent. Time limit. Eight hours. Goal. Output parameters. Attenuation. Less than. Five percent. Thermal drift. Curve. Smooth. No steps. No self-oscillation. Instruments. Hook them up yourselves. Scheme. Adjust it yourselves. Record. Data. Every. Thirty minutes. Once. Out of tolerance. Eliminated. Begin.”

Stopwatch. Tick-tick. Metal casing. Reflecting light.

Lin Chen. Started working. Wiring. Banana plugs. Inserted. Tight. Not loose. Wire order. Red. Black. No crossing. Power. On. Indicator light. Lit. He connected. The load resistor. High-power. Wire-wound. Type. He connected. The heat sink. Flush against. The inner wall. Of the chassis. Copper foil tape. Held it fast. Thermal path. Continuous. He connected. The thermistor probe. Close to. The power transistor casing. Not touching. He connected. The ground wire. Alligator clip. Clamped. Onto the lab’s grounding bar. Metal. Cold. Bite. Firm.

He adjusted. The signal generator. Frequency. Ten megahertz. Amplitude. One hundred millivolts. Waveform. Sine. Stable. He looked. At the oscilloscope screen. Green graticule. Lit. Waveform. Appeared. Sine wave. Smooth. No distortion. He measured. Output voltage. Multimeter. Reading. Stable. He measured. Temperature. Thermocouple probe. Against. The heat sink. Reading. Twenty-five degrees.

He switched on. The load. Click. Current noise. Rose. Low. Continuous.

Time. Zero. Recorded. The first set. Of data. Temperature. Twenty-five. Output voltage. Nine point eight volts. Attenuation. Zero. Waveform. No distortion. Signal-to-noise ratio. Forty-two decibels.

He sat. Upright. Back. Against the chair back. But not resting on it. Hands. On his knees. Knuckles. Whitish. Left foot. Stabbing pain. Continuing. Gauze. Slightly wet. Sticky. He ignored it. Shifted his center of gravity. To the right. Breathing. Steady. White mist. Dissipated.

Time. Thirty minutes. Recorded. The second set. Temperature. Thirty-two. Output voltage. Nine point seven five. Attenuation. Zero point five. Waveform. Slight tremor. He adjusted. The bias potentiometer. Knob. Dragging stiffly. He used force. Evenly. Did not strip it. Half a turn. Stop. Measured. Waveform. Stable. No self-oscillation.

Time. One hour. Chen Hao. Turned in his work. Stood up. Left. Footsteps. Light. Confident. Door open. Door shut. Wind. Poured in. Cold.

Lin Chen. Did not look up. Kept recording. The third set. Temperature. Thirty-eight. Output voltage. Nine point seven. Attenuation. One. Waveform. Smooth. He. Cross-checked. Formulae. Verified the arithmetic. No errors. He checked. Wiring. No looseness. No short circuit. Probe. Ground clip. Bit tight. Metal. Cold. Shielding. Intact. Ground wire. Single point. No loop. Heat sink. Temperature rise. Normal. Thermistor. Resistance. Falling. Compensation. Taking effect.

Time. Two hours thirty minutes. Temperature. Forty-five. Output voltage. Nine point six five. Attenuation. One point five. Waveform. Stable. He measured. Signal-to-noise ratio. Reading. Forty-one decibels. Up to standard. He drank water. Thermos cup. Cold. Water. Went down. Stomach. Contracted. Tight. He ignored it. Kept staring. At the screen. Graticule. Green line. Steady.

Time. Four hours. Temperature. Fifty-two. Output voltage. Nine point six. Attenuation. Two. Waveform. Slight distortion. He adjusted. The negative-feedback resistor. Paralleled. A small resistance value. Compensation. Gain. Lowered. He measured again. Waveform. Smooth. No self-oscillation. Signal-to-noise ratio. Forty point five decibels. Up to standard. He recorded. The fourth set. Of data. Paper. Flat. Handwriting. Neat.

Time. Five hours. Temperature. Fifty-eight. Output voltage. Nine point five five. Attenuation. Two point five. Waveform. Stable. He measured. Signal-to-noise ratio. Reading. Forty point two decibels. Critical. He did not panic. Route. Switched. Vent holes. Opened wider. Airflow. Accelerated. Heat dissipation. He used. Cardboard. Folded. An air guide. Wedged it. At the side. Of the chassis. Guided. The airflow. Straight onto. The power transistor. He measured again. Temperature. Down. Two degrees. Output voltage. Rose back. Zero point zero five. Attenuation. Two. Waveform. Smooth. Signal-to-noise ratio. Forty-one decibels. Recovered.

Time. Six hours. Temperature. Sixty. Output voltage. Nine point five. Attenuation. Three. Waveform. Stable. He measured. Signal-to-noise ratio. Reading. Forty point one decibels. Up to standard. He recorded. The sixth set. Of data. Paper. Flat. Handwriting. Neat. Left foot. Stabbing pain. Worse. Gauze. Blood seeped through. Dried. Stuck fast. He stopped writing. Took off his shoe. Tore open. The sock. Scab. Split open. Deep crack. Edges. Whitish. No pus. He took. Iodophor. Cotton swab. Dabbed it on. Sharp pain. He bit his lip. Made no sound. Waited. Until dry. Replaced it. With fresh gauze. Wrapped it. Tight. Not too tight. Blood. Stopped. He put the shoe back on. Tied the laces. Tight. Shifted his center of gravity. To the right. Kept watching.

Time. Seven hours. Temperature. Sixty-two. Output voltage. Nine point four eight. Attenuation. Three point two. Waveform. Slight tremor. He adjusted. The thermistor. Parallel resistance. Fine-tuned. The compensation curve. He measured again. Waveform. Smooth. No self-oscillation. Signal-to-noise ratio. Forty decibels. Up to standard. He recorded. The seventh set. Of data. Paper. Flat. Handwriting. Neat. Fingers. Numb with cold. Knuckles. Whitish. He. Breathed warm air on them. White mist. Scattered. Kept. Adjusting. Fine-adjustment potentiometer. Knob. Dragging stiffly. He used force. Evenly. Did not strip it. Fingertips. Sensed. The change in resistance. Half a turn. Stop. Measured. Waveform. Stable. No self-oscillation.

Time. Seven hours fifty minutes. Temperature. Sixty-three. Output voltage. Nine point four seven. Attenuation. Three point three. Waveform. Stable. He measured. Signal-to-noise ratio. Reading. Forty point zero five decibels. Up to standard. He recorded. The eighth set. Of data. Paper. Flat. Handwriting. Neat.

Time. Eight hours. Reached. Old Li. Collected the papers. Checked them. Scored them. Chalk. On the blackboard. Wrote. The rankings. Handwriting. Heavy. Thick.

Reinforcement group. Nine people. Lin Chen. First. Chen Hao. Second. Third through ninth. In order.

Old Li. Turned around. “Practical assembly. Passed. Tomorrow. Organize. The data. Submit. The provincial contest. Registration form. And. Travel budget. Deadline. Tomorrow. Seventeen hundred. Late submission. Counted as. Withdrawal.”

He paused. His gaze. Swept over. Lin Chen. “Lin Chen. Stay behind. There’s work.”

The students. Got up. Packed up. Left. Footsteps. Mixed. Door open. Door shut. Wind. Poured in. Cold.

In the laboratory. Only two remained. Old Li. Walked to. Lin Chen’s bench. Set down. A document. Paper. Thick. Printed text. Clear. Provincial Electronic Design Contest. Regional notice. Travel expenses. At your own expense. Room and board. At your own expense. Registration fee. One hundred twenty yuan. Deposit. Fifty yuan.

“Provincial contest. In the city. Three days. Two nights. Ticket. Lodging. Meals. Calculate them yourself. If you can’t produce. A budget. Eliminated.”

Lin Chen. Looked. At the document. Reached out. Touched. The paper. Cold. But. Weighty. He did not hesitate. Opened the bag. Took out. The account book. Edges. Curled. Paper. Rough. Rubbing. Against the skin. He flipped. Pages. Numbers. Dense. No cross-outs.

Assets. One hundred seventy-one point five. Registration fee. One hundred twenty. Deposit. Fifty. Total. One hundred seventy. Shortfall. Negative zero point five. Train ticket. Hard seat. Eighteen. Lodging. Guesthouse. Thirty. Per night. Two nights. Sixty. Meals. Ten yuan. Per day. Three days. Thirty. Total. One hundred eight. Overall. Required. Two hundred seventy-eight point eight. Shortfall. One hundred seven point three.

He stopped writing. Looked. Out the window. Sky. Dark. No stars. Wind. Through the window cracks. Low howl. He did not panic. Route. Switched. Backup plan. Activated.

Old Li. “Tomorrow. Seven o’clock. Gather in the lab. Hand in. The budget. And the registration form. Bring. Cash. Settle it on the spot. Late. We won’t wait.”

“Understood.”

Lin Chen. Slung. The canvas bag. On his back. Went out the door. Corridor. Empty. Footsteps. On terrazzo. Echoes. Short. Cold.

He touched. His pocket. The account book. Edges. Curled. Paper. Rough. Rubbing. Against the skin.

He felt. Neither joy. Nor sorrow. In his mind. Only equations. Routes. Countermeasures. Time windows. Physical limit. Three lines. In the dark. Crossing. Without colliding. Without tangling. Each. Moving forward.

He walked. Stride. Thirty centimeters. Did not step. On the frost line.

Ahead. Bulletin board. Posted. New notice. White paper. Black print. Provincial contest. Training list. Posted. Travel budget. Must include. Detailed breakdown. And source explanation. Deadline. Tomorrow. Sixteen hundred. Submit.

He stopped. Looked. Source explanation. Breakdown. He memorized. The parameter requirements. The route.

He turned. Went back. To the dormitory. Wind. Through the corridor. Low howl.

He closed his eyes. In his head. No curves. Only solder joints. Temperature. Time. Molten solder. Solidifying. Three lines. In the dark. Crossing. Without colliding. Without tangling. Each. Moving forward.

He opened his eyes. Stride. Thirty centimeters. Did not step. On the frost line.

Forward.