The laptop's charger has to know how much current flows from the adapter and how much flows into the battery. It finds out by watching a few millivolts across tiny resistors — current sense resistors, or shunts — placed directly in those paths. When one of them is cracked, has drifted or was replaced with the wrong value, the charger's picture of reality is wrong, and it behaves wrongly: it refuses to charge, charges too slowly, limits the adapter far below its rating or overloads it. This guide explains where these resistors sit, how the charger uses them and how to check them.
Where the sense resistors are
| Position | What it measures | Typical charger pins (names vary) |
|---|---|---|
| Input path, after the input switches | Current from the adapter | ACP / ACN, CSIP / CSIN |
| Battery path, between system and battery | Charge and discharge current | SRP / SRN, CSOP / CSON |
| Other designs | System current, USB-C input, regulator phases | Various |
The values are very low — a few milliohms to a few tens of milliohms — so the voltage across them stays small and wastes little power. They are usually larger packages than ordinary resistors, such as 0805, 1206 or wider, to handle the current. The part is described under current sense resistor.
How the charger reads them
The charger's sense inputs connect to the two ends of the resistor through dedicated lines. Inside, a current-sense amplifier — described under current sense amplifier — measures the tiny voltage difference and converts it into a current reading by dividing by the resistor value the designer chose.
The charger then:
- limits input current so the adapter is not overloaded;
- regulates charge current to the level the battery requests;
- reports currents to the EC on many designs.
How the charger uses these readings is covered in The Laptop Charging IC: How It Works.
Kelvin connections and filters
Because the voltages are so small, the copper around the resistor matters. Designers route the sense lines from the resistor's own pads, separately from the high-current copper — a Kelvin or four-terminal connection — so the charger measures only the resistor. Many boards also add a small filter on the sense lines: series resistors of a few ohms and a capacitor between the lines, to reduce switching noise.
That has consequences for repair:
- a damaged sense line or filter resistor makes the charger read nonsense even when the shunt itself is perfect;
- a leaky filter capacitor between the lines shifts the reading;
- solder bridges or corrosion near the shunt's pads change the effective resistance.
Finding them in the schematic and on the board
In the schematic, sense resistors are easy to spot once you know the pattern: a resistor whose value is written in milliohms or as a fraction of an ohm (0.01, 0.005, 10m), sitting in series with a thick power net, with two thin nets leaving its ends toward the charger. The thin nets usually carry names derived from the charger pins, so searching for those pin names in the boardview leads straight to the resistor's pads.
On the board, look near the charger IC, and between the input switches and the system rail, for a resistor wider than its neighbours, often marked R005, R010 or R020. Some designs use a four-terminal part with separate sense pads; on these, the sense lines connect to the inner pads and the current flows through the outer ones.
What goes wrong and what it looks like
| Fault | Effect on the charger | Typical symptom |
|---|---|---|
| Input shunt drifted high or replaced with a higher value | Thinks more current flows than really does | Limits the adapter early; slow charging, charging only when off |
| Input shunt lower than designed | Thinks less current flows | Can overload the adapter; adapter shuts off under load |
| Input shunt cracked or open | Input path broken or reading invalid | No power from adapter, or charger fault |
| Battery shunt open | Battery disconnected from system | Battery not charging, laptop dies without adapter |
| Battery shunt drifted | Charge current wrong | Charges too fast or too slowly; gauge confusion |
| Sense line or filter damaged | Reading invalid | Charger stops or misbehaves |
The "charging only when off" symptom, which often involves the input shunt, is covered in Laptop Charges Only When Turned Off.
Checking a sense resistor
Visual inspection
Under magnification, look for cracks across the body, darkened terminals and lifted joints. Sense resistors carry the full current and suffer during shorts and surges.
Measuring in operation
The most reliable check is the millivolt method:
- Run the laptop with the adapter connected and a known load.
- Measure the voltage across the resistor's pads with the meter on millivolts.
- Divide by the resistor's value to get the current.
Compare that current with what you expect — for example the current reported by a USB-C meter on the input, or a plausible charge current. The arithmetic is handled by Current Sense Resistor Calculator.
Measuring the sense lines
With the board unpowered, check continuity from each end of the shunt to the charger's corresponding pins, through the filter resistors. Compare the filter capacitor and resistors with the schematic values.
Resistance mode
An ordinary multimeter cannot resolve milliohms accurately, but it can detect an open or badly cracked shunt — a reading of more than a fraction of an ohm across a milliohm resistor means it is broken.
Replacing a sense resistor
Match:
- Resistance — exactly; even a small difference changes every current reading by the same proportion.
- Tolerance — 1 % or better.
- Power rating — at least the original's.
- Temperature coefficient — sense resistors are low-drift parts; ordinary resistors drift with heat.
- Package — the same footprint, including wide-terminal versions.
Read the value from the marking (for example R010 is 10 mΩ) or the schematic, and use a part specified as a current-sense resistor.
Common mistakes during repair
- Bridging the shunt with solder or wire to "test" it. With zero resistance the charger sees no current at all and may overload the adapter or the battery.
- Fitting an ordinary resistor of the right value but with large temperature drift; the readings wander as the board warms up.
- Overheating the sense lines while removing a nearby part; the thin traces lift easily from the board.
- Forgetting the second shunt. Many boards have one in the input path and another in the battery path, and a symptom can come from either.
- Trusting resistance mode for milliohm values; the leads and contact resistance alone exceed the part's value.
When in doubt, compare the millivolt drop with an identical good board under the same load. A difference of more than a few percent between two boards of the same model is worth investigating.
Example: a laptop that would not take full power
A gaming laptop with its correct high-wattage adapter charges only when off and throttles under load, as if the adapter were much smaller. A USB-C power meter is not available on this barrel-jack model, so the technician measures the input shunt in operation: the millivolt reading implies an input current near the charger's limit, yet the adapter's own load suggests the laptop is drawing far less. The shunt, marked as a low milliohm value, had been replaced in an earlier repair with a part of double the original value. The charger saw twice the real current and limited the input at half the adapter's rating. Fitting the correct value restores full power.
FAQ
Why are sense resistors so small in value?
To keep the voltage drop and wasted power small while still giving a measurable signal.
Can I measure a 10 mΩ resistor with my meter?
Not accurately. Measure the millivolt drop in operation and calculate the current instead.
What happens if I fit the wrong value?
Every current reading is off in proportion: the charger limits too early or too late.
Why do sense lines matter?
The charger measures through them. A damaged line or filter part gives a wrong reading even with a good shunt.
Can a sense resistor cause the adapter to shut off?
Yes, if its value is too low the charger underestimates input current and can overload the adapter.
Bottom line
Sense resistors tell the charger how much current flows from the adapter and into the battery. A cracked, drifted or wrong-value shunt — or a damaged sense line or filter — misleads the charger. Inspect them, measure their millivolt drop in operation, check the sense lines, and replace only with the exact value, tolerance and rating.