On the bench

Use these checks after a part is out of the machine. They tell you what the part is and whether it still works. Measure volts while a circuit is powered. Measure resistance, and use the continuity beep, only when the circuit is unpowered. If a whole module still does its job when you plug it in, keep the module. The chips on the board are not a better find.

• Order of work on a discarded machine

  The part you want is usually still a module. Anonymous surface-mount parts are the last resort, not the first.

  Volts: how hard the electricity is pushing, measured with the circuit powered and the probes in parallel. Ohms and continuity: measured with the part unpowered. A beep means a connection. OL or a 1 on the left usually means open, no connection. Amps need a different socket on many meters. Using the amp socket to measure volts blows the meter fuse.

  1. Photograph every nameplate, battery label, and the sticker on the back of any LCD panel before you unscrew anything. Panel model numbers are printed on the metal of the panel, not on the TV bezel.
  2. Unplug. Remove batteries next and put them somewhere non-flammable. Swollen cells go to recycling, not into a drawer.
  3. Discharge known energy storage: primary bulk capacitors, microwave HV capacitors, camera-flash capacitors. Confirm with a meter.
  4. Keep anything that still works as itself. A 19 V laptop brick, a USB webcam, a class-D board, an IR receiver on its three pins, a SATA-USB bridge. A working module is worth more than the chips on it.
  5. Then motors, switches, connectors, magnets, rods, belts, screws. Bag fasteners with a note of which donor and which thread.
  6. Only then desolder parts you can name. A three-pin SOT-23 with no readable marking is not inventory.
  7. Safety-critical parts you do not fully understand — Y capacitors, a sacrificed MOV, a thermal fuse you were planning to omit — do not go into a new mains build.

  Where this usually goes wrong. Stripping a machine into a bin of loose screws and then hoping to reconstruct which magnet came off which keeper. Keepers stay with the magnets.

• Minimum meter checks

  A cheap meter is the difference between a parts bin and a box of guesses. You need DC volts, AC volts, resistance, continuity, and a diode test.

  Volts: how hard the electricity is pushing, measured with the circuit powered and the probes in parallel. Ohms and continuity: measured with the part unpowered. A beep means a connection. OL or a 1 on the left usually means open, no connection. Amps need a different socket on many meters. Using the amp socket to measure volts blows the meter fuse.

  1. Voltage: measure the brick you hope is 12 V. Read it unloaded and then with a modest load (a 12 V lamp or a power resistor). Unloaded switch-mode readings are often a volt or two high.
  2. Polarity: barrel jacks are often center-positive and sometimes not. The symbol is molded on the brick. Confirm anyway.
  3. Continuity: beep a switch, a fuse, a thermal fuse. A thermal fuse looks like a resistor and reads open forever once it has done its job.
  4. Diode test: a bridge rectifier should show about 0.5–0.7 V in the conducting direction of each diode and open the other way. Schottkys read lower, often 0.2–0.4 V.
  5. Isolation: see the isolation method before you trust a salvaged supply's output as 'safe to touch'.

  Where this usually goes wrong. Using the resistance range on a charged capacitor and watching the meter climb while you decide the cap is bad. Discharge first, then measure.

• Identify an unknown motor

  Wire count and a resistance check sort most salvaged motors before you apply power.

  Two wires mean you can try a low, current-limited voltage and it should spin. The case often hides a label. If the only label you can find is on the machine, that voltage is the one to believe, not a hobby guess.

  1. Two wires, low resistance (ohms to low tens): brushed DC or a universal motor. A brief touch from a current-limited supply (or a series lamp on mains motors) tells you if it spins. Universal and shaded-pole motors are often mains-rated — the nameplate decides, not optimism.
  2. Two wires, higher resistance, no rotation on low voltage DC: could be an AC synchronous motor, a solenoid, a valve coil, or a transformer winding. Check the plate.
  3. Three wires: often a brushed motor with a center tap (two speeds) or, on larger appliances, part of a single-phase winding set. It can also be a BLDC with an external controller that you did not notice. Three thick wires and no other connector is often a sensorless BLDC (hard-drive spindle, some fans).
  4. Three thick wires plus a separate 5-pin hall plug: sensored BLDC. Do not power the halls from 12 V. They want 5 V or whatever the print says.
  5. Four or six wires, in coil pairs: a stepper. See the stepper method.
  6. If it sparks hard at the brushes or smells hot in under ten seconds, stop. Wrong voltage or a shorted winding.

  Where this usually goes wrong. Connecting a treadmill motor or a washer inverter to a 12 V battery because it 'looks like a DC motor'. Read the plate. Those are often 90 V class or a matched three-phase pair.

• Pair the coils on a stepper

  A junk stepper is usable once you know which wires are a coil. The nameplate current, if you still have it, matters more than the phase count.

  A stepper motor moves a fixed angle each time you pulse it. The wires are the ends of coils. A meter on resistance finds the pairs: two wires that read a low resistance are one coil. Wires from different coils read open, like a broken wire.

  1. Measure resistance between every pair. A bipolar stepper has two coils: four wires, two pairs of equal resistance, open circuit between pairs. Five wires is unipolar with a common. Six wires is unipolar with two commons. Eight wires is two coils each brought out as a pair, which you can series or parallel.
  2. Coil resistance on a printer NEMA-17 is often 2–10 Ω. Very low resistance wants a current-mode driver, not a voltage you guessed.
  3. Turn the shaft by hand. Detent means a permanent-magnet stepper. No detent and almost free spinning can be a reluctance design or a motor with the rotor removed — unusual in printers.
  4. Label the pairs before you unplug anything. 1.8° (200 steps/rev) is the usual hybrid; 7.5° shows up in cheaper appliance steppers and will not drop into a 3D-printer design without new math.
  5. Drive with a proper chopper driver (A4988, DRV8825, TMC, or the driver that came with it). A stepper stalled against DC is just a heater.

  Where this usually goes wrong. Assuming every printer motor is a NEMA-17 bipolar. Many new inkjets are brushed DC with an encoder, and some steppers are 6-wire unipolar.

• Power a salvaged BLDC without magic

  Brushless motors are not 'a bit like DC'. They need a sequence. The easy path is to keep the controller that was born with the motor.

  This is usually a brushless motor. Connecting a battery across two of the wires only makes it twitch, and holding it there heats the winding. Keep the small plug as well. Those thin wires are often the sensors that tell the board where the shaft is.

  1. If the donor had a driver board and the motor still plugs into it, keep that pair together and learn the enable pin. Tool controllers, fan boards, and washer inverters are designed around one stator.
  2. Sensored motors: identify hall wires (often red/black for 5 V and ground, three signal wires). The three phase wires are heavier.
  3. Sensorless (hard-drive spindle, many drones): an RC ESC can start some of them. Many will not, because the winding voltage and the pole count are outside what a cheap ESC expects. A drive spindle is a poor first BLDC.
  4. Never apply steady DC across two phase wires except as a brief identification pulse. It just heats copper.

  Where this usually goes wrong. Cutting the hall cable short because it 'was in the way'. You cannot put those signals back once you have forgotten the order.

• Discharge a capacitor on purpose

  The safe discharge is boring: a resistor, a wait, a measurement. Shorting with a screwdriver is how people get a face full of plasma and a damaged board.

  Unplugging does not empty a large capacitor. The stored energy sits on the two leads. A resistor of a few thousand ohms, held across the leads with insulated tools, bleeds it down. Then measure. A screwdriver spark is not a discharge method.

  1. Unplug. Wait a minute anyway; many designs have bleeders. Do not trust the wait.
  2. Use a resistor in the kilohm-to-tens-of-kilohm range, rated for the pulse, with insulated leads. Clip across the capacitor terminals, not across a random pair of heatsink tabs.
  3. For microwave HV capacitors this is not optional and not a beginner task. If you do not already know which terminals are the HV cap, stop and recycle the oven after pulling only the turntable motor and the door switches.
  4. Confirm voltage has collapsed. Then confirm again, because some supplies recover a little from dielectric absorption.
  5. Primary caps in ATX supplies and UPS units are the ones people forget because the output terminals already read 0 V.

  Where this usually goes wrong. Trusting the bleeder. Bleeders fail open, which is exactly when the cap stays hot.

• Decide whether an electrolytic is worth keeping

  Age and heat kill electrolytics. They are also the part most likely to embarrass you a month after the repair.

  Unplugging does not empty a large capacitor. The stored energy sits on the two leads. A resistor of a few thousand ohms, held across the leads with insulated tools, bleeds it down. Then measure. A screwdriver spark is not a discharge method.

  1. Look first. A bulged vent, a crust of electrolyte, or a sleeve that has split means scrap. Do not reform it 'to see'.
  2. Primary-side electrolytics from a mains supply are a safety part. If you are building a new mains supply, buy new ones with a known ripple rating. Salvage is for the secondary side and for low-voltage boards.
  3. If you have an ESR meter, compare to a typical value for that capacitance and voltage. High ESR with correct capacitance is a dried-out cap and it will not hold up in a switcher.
  4. A capacitance reading within 20% and a low ESR is good enough for a 5 V or 12 V rail experiment.
  5. Voltage rating must be higher than the rail, with margin. A 16 V cap on a 12 V rail is a maybe; a 10 V cap on a 12 V rail is a no.

  Where this usually goes wrong. Harvesting every cap on a dead motherboard into a jar. The board died of those caps more often than it died of the CPU.

• Sort salvaged lithium cells

  A pack is scrapped because its weakest series group failed. The other cells may be fine. 'May' is not good enough to skip the sort.

  The flat end is negative, the button end is positive. Nominal voltage is about 3.6 to 3.7 volts. Full, for an ordinary cell, is 4.20 volts. Below about 2.5 volts for a long time, or any swelling, means it is for recycling, not for a project. Tape the ends so a bunch of them cannot short in a drawer.

  1. Work one cell at a time on a non-flammable surface. Do not short the tabs. Tape ends the moment they are apart.
  2. Resting voltage: under about 2.5 V and it has been there a while — recycle it. Do not hit it with a 'boost' mode. Between 3.0 and 4.2 V is the normal window for standard Li-ion. Anything that reads 0 V is internally open or the BMS has disconnected; find out which before you prod.
  3. Some LiHV tool cells are 4.35 V full. If the pack label does not say, charge only to 4.20 V.
  4. Internal resistance: a cell that gets hot or sags a volt under a modest load is tired. Match cells that will share a pack by capacity and by resistance, not by voltage alone.
  5. Capacity: a cheap tester or a hobby charger in discharge mode. A 18650 that delivers 800 mAh is not a 3000 mAh cell, no matter what the wrapper someone reprinted says. Wrappers lie.
  6. Parallel only matched cells. Series groups need a BMS that actually balances, or a charger that watches every cell.

  Where this usually goes wrong. Soldering wire directly to the cell can. The heat damages the separator. Tabs are spot-welded.

• Test a speaker or voice coil

  A 1.5 V cell is the whole test set for a cone speaker.

  Touch a 1.5 volt cell to the two terminals for a moment only. A good cone clicks and moves. A scrape means the coil is rubbing, which will sound bad even if it still makes noise. The ohms printed on the magnet, often 4, 6, or 8, tell you what amplifier it expects.

  1. Read the impedance stamp if there is one: 4, 6, or 8 Ω are the usual salvaged drivers. Unmarked phone speakers are often 8 Ω and under a watt.
  2. Briefly touch a 1.5 V cell across the terminals. You should hear a click and see the cone move once. No click and open resistance means a dead coil. A click and a scrape when you gently move the cone means a rubbed voice coil — usable for a buzzer, poor for music.
  3. Do not leave DC connected. Do not connect a speaker to a wall-wart.
  4. A hard-drive voice coil is the same test at lower enthusiasm: it should nudge the arm. Sustained DC just heats it.

  Where this usually goes wrong. Wiring an 8 Ω tweeter alone across a salvaged amp that was bi-amped. Tweeters die from bass and from DC offsets.

• Check a supply is actually isolated

  The output of a salvage supply is safe to touch only if a transformer stands between it and the mains. Many cheap LED bulbs do not have one.

  Left: a brick. The label is your spec, and the plastic is the safety barrier. Right: a board taken out of a lamp or a charger. Unless you have checked that the output is isolated from the prongs, do not let it touch a project you will handle while it is plugged in.

  1. Unplug the supply. Measure resistance from each output wire to each mains prong. You want open circuit, megohms, not kilohms.
  2. A capacitive-dropper 'supply' (common in cheap LED bulbs, some clock radios, some night lights) will show a capacitor in that path and the circuit sits at mains potential. Those are not bench supplies. Do not connect their output to a USB port, a microcontroller you care about, or your hand.
  3. Visual check: a real flyback or a 50/60 Hz transformer is a magnetic part you can see between the mains side and the output side, often with an optocoupler beside a slot cut in the board. No slot, no transformer, no isolation.
  4. Laptop bricks and printer supplies are usually isolated. Confirm anyway, once, and then label the brick.
  5. Earth on a three-prong brick is not the same question as isolation. Know which you measured.

  Where this usually goes wrong. Judging safety by 'it is only 5 V'. Five volts sitting on top of the live conductor is still live.

• Start an ATX supply on the bench

  The green wire is a logic input, not a mystery. The purple wire is live whenever the IEC cord is plugged in.

  Yellow is 12 volts, red is 5 volts, orange is 3.3 volts, black is ground. The supply stays off until the green wire is connected to a black one. Purple is a standby 5 volts that is present whenever the cord is plugged in, even if the fan is still. Unplug and discharge before you open the metal box.

  1. Discharge and inspect first. Bulged secondary caps, a burnt smell, or a fan that does not spin are reasons to part it out rather than trust it.
  2. Unplug. Jumper green (PS_ON) to any black (ground) with a switch you can open. Plug in. The fan should start and the rails should come up.
  3. Yellow is +12 V, red is +5 V, orange is +3.3 V, blue is −12 V (low current), purple is +5 V standby, grey is power-good, brown is 3.3 V sense. There is no −5 V on a modern unit.
  4. Older supplies want a small load on 5 V before 12 V regulates. A 10 Ω 10 W resistor or a 12 V automotive bulb on the 5 V rail is the usual dummy. Modern supplies are less fussy and still happier with some load.
  5. Do not pull more current than the 12 V label, and do not believe a no-name 500 W label. If the case says 12 V at 15 A, treat 10 A as the honest number until you know the brand.
  6. The case must stay on if this is a bench tool. The primary caps are inside.

  Where this usually goes wrong. Leaving PS_ON jumpered with the cord plugged in, then opening the case to 'adjust' something. Standby and the primary are still hot.

• Wire a salvaged load cell

  Kitchen scales hide one of the few precision sensors in household trash. The wiring is simple and the colors are not fully standard.

  Red and black feed the sensor a steady few volts. Green and white are the tiny signal, measured between them, not from either one to ground. Pressing the bar changes that signal by millivolts. A normal meter on DC volts can show that it responds. Turning it into a number on a screen wants a small amplifier board, often an HX711.

  1. A small digital kitchen scale usually has one aluminum beam and four wires. Bathroom scales usually have four half-bridge sensors, one under each foot, which must be combined before they look like a single cell.
  2. Typical colors, when they are typical: red and black excitation, green and white signal. Blue sometimes replaces green. If the colors are strange, find the pair with the higher resistance as a clue and confirm by watching a millivolt reading change when you press the beam.
  3. Excite with 3–5 V DC, not 12 V. Read the signal differentially. Output is on the order of a millivolt per volt of excitation at full scale, so you need an amplifier (HX711 and its clones exist specifically for this) or a good meter on the millivolt range just to see it move.
  4. Capacity is printed on the beam more often than people notice, or it matches the scale's range. Bend it past that and the zero shifts forever.
  5. Mount it the way it was mounted: the beam wants one end fixed and the load over the other, with the strain gauges on the face they were bonded to. Upside down still works if you recalibrate; twisted does not.

  Where this usually goes wrong. Expecting a 0–5 V signal. You will decide the cell is dead when it is working perfectly at 8 mV.

• Separate hard-drive magnets

  They are stronger than the photo suggests, and they chip.

  The two curved blocks on the steel plates are the magnets, and they are the prize. Slide them apart. Do not pry straight up. The flat coil on the arm is a short-travel motor. The platters are not a useful material, and on some drives they are glass.

  1. Leave the steel keeper on until you actually need the face. The keeper is also what stops two magnets from finding each other across a drawer.
  2. Slide the magnet off the plate sideways. A wooden wedge or a plastic spudger beats a steel screwdriver, which becomes part of the problem.
  3. If two bare magnets snap together, slide them apart. Do not insert a finger in the gap to 'help'.
  4. Store in pairs with a keeper, labeled with the donor size (2.5 in laptop drives are weaker than 3.5 in desktop drives).
  5. Heat and hammering demagnetize them. So does a hard slam that shatters the plating and the brittle alloy underneath.

  Where this usually goes wrong. Putting one in a pocket with a folding knife, or near a mechanical watch, or on the side of a CRT you were sentimentally keeping.

• Tell a thermal fuse from a thermostat

  Both look like a small metal can in series with a heater. One of them resets. The other has already died for a reason.

  Wall power goes through a switch, a one-time thermal fuse, and often a resettable thermostat, then the element. If you remove the fuse because it is open, the element can stay on after something else has already failed. That is how the donor caught fire the first time.

  1. Read the stamp. A thermal fuse has a temperature (for example 128 °C, 192 °C, 240 °C) and does not reset. A bimetal thermostat often has a lower trip point and clicks if you heat it gently and let it cool.
  2. A one-shot fuse that reads open did its job. Replacing it with a wire, or with a fuse of a much higher temperature, is how salvaged heaters burn houses down. The original fault (blocked airflow, failed fan, shorted element-to-chassis) is still there.
  3. Continuity to the metal chassis from either heater lead, on a cold element, means the mineral insulation has failed. Scrap the element.
  4. In a dryer, the fuse opens because the vent was blocked as often as because the element died. Test the element as its own part: resistance should be low ohms and match V²/P from the nameplate within reason.

  Where this usually goes wrong. Bypassing the fuse because the rest of the dryer 'looks fine', then putting the dryer back into service. This index is for parts leaving the machine, not for defeating the cutout.

• Insulation-check a mains element

  A heater that works and also leaks to the chassis is worse than one that is open.

  Wall power goes through a switch, a one-time thermal fuse, and often a resettable thermostat, then the element. If you remove the fuse because it is open, the element can stay on after something else has already failed. That is how the donor caught fire the first time.

  1. Unplug. Disconnect at least one end of the element so you are not measuring through the wiring.
  2. Resistance across the element should agree with R = V²/P from the nameplate. A 240 V, 2 kW kettle element is about 29 Ω. A 120 V, 1500 W element is about 10 Ω. Open circuit means a broken wire. Near 0 Ω means a short, not a prize.
  3. Resistance from either terminal to the metal sheath or the chassis should be open on a normal meter (many megohms). Any low reading is a failed element. Do not put it back in water or in a hand-held dryer.
  4. A proper insulation tester (megger) is the right tool if you have one. A multimeter failing to show a short is not a certificate.

  Where this usually goes wrong. Judging a kettle element only by whether the water still boils. Leakage to the sheath is a shock path through the water.