A current sense resistor measures load current as a voltage drop across a known low
resistance. Thick film and metal plate are the two constructions. Thick film gives you
far more choice — Walsin Technology (Walsin) lists 10,428 low-ohm thick film parts against
1,188 metal. Metal gives you far better temperature stability. Choose on TCR first.
What is the difference between a thick film and a metal plate current sense resistor?
Thick film screens a resistive paste onto a ceramic substrate. Metal plate uses a solid
alloy element welded to terminations. The metal element has a much lower and flatter
temperature coefficient, handles more power for its size, and survives pulses better.
Thick film is the deeper catalogue, with far more resistance and tolerance combinations.
The divide is sharper at low ohms than anywhere else in the resistor catalogue. To reach
10 mΩ in thick film you make the resistive track as short and wide as the geometry allows,
and sheet resistance is fighting you the whole way. At 3 mΩ it cannot be done in a
practical case size. That is where metal plate takes over.
Catalogue depth reflects this. Our extract of Walsin’s low-ohm current sense data holds
10,428 thick film parts and 1,188 metal parts — roughly nine to one. If your value and
tolerance are ordinary, thick film will have it. If your requirement is single-digit
milliohms or tight TCR, the metal catalogue is the place to look, and it is much narrower.
How much does TCR actually change my current measurement?
Directly and proportionally. TCR is the fractional change in resistance per degree Celsius.
A ±2100 ppm/°C part shifts up to 0.21% per °C, so a 100 °C rise moves the resistance by up
to 21%. A ±50 ppm/°C part moves by up to 0.5% over the same rise. Your current reading
inherits that error one for one.
This is the reason to read the TCR column before the resistance column. A shunt that is
1% accurate at room temperature and 21% wrong at operating temperature is not a 1% shunt.
Here are eight real Walsin parts from our catalogue extract, four thick film and four metal
plate. The reference is WW12XR010FTL. The right-hand column states field by field what
matches and what does not.
| Walsin part number | Series | Case size | Resistance | Tolerance | TCR (ppm/°C) | Rated power | Match / mismatch vs WW12XR010FTL |
|---|---|---|---|---|---|---|---|
| WW12XR010FTL | Thick Film Low Ohm Current Sense | 1206 (3216) | 0.010 Ω | ±1% | ≤ ±2100 | 1/4 W | reference part |
| WW12XR011FTL | Thick Film Low Ohm Current Sense | 1206 (3216) | 0.011 Ω | ±1% | ≤ ±2100 | 1/4 W | match: series, case size, tolerance, TCR, power, Sn-base lead-free termination · mismatch: 0.011 Ω not 0.010 Ω |
| WW12XR012FTL | Thick Film Low Ohm Current Sense | 1206 (3216) | 0.012 Ω | ±1% | ≤ ±2100 | 1/4 W | match: series, case size, tolerance, TCR, power, termination · mismatch: 0.012 Ω not 0.010 Ω |
| WW12XR013FTL | Thick Film Low Ohm Current Sense | 1206 (3216) | 0.013 Ω | ±1% | ≤ ±2100 | 1/4 W | match: series, case size, tolerance, TCR, power, termination · mismatch: 0.013 Ω not 0.010 Ω |
| WW25MR003FTL | Metal Plate Power Low Ohm Current Sense | 2512 (6432) | 0.003 Ω | ±1% | ≤ ±50 | 1 W | match: tolerance ±1%, Sn-base lead-free termination · mismatch: metal plate construction, 2512 footprint, 0.003 Ω, TCR 42× tighter, 1 W not 1/4 W |
| WW25MR004FTL | Metal Plate Power Low Ohm Current Sense | 2512 (6432) | 0.004 Ω | ±1% | ≤ ±50 | 1 W | match: tolerance ±1%, termination · mismatch: metal plate, 2512 footprint, 0.004 Ω, ≤ ±50 TCR, 1 W |
| WW25MR005FTL | Metal Plate Power Low Ohm Current Sense | 2512 (6432) | 0.005 Ω | ±1% | ≤ ±50 | 1 W | match: tolerance ±1%, termination · mismatch: metal plate, 2512 footprint, 0.005 Ω, ≤ ±50 TCR, 1 W |
| WW25MR006FTL | Metal Plate Power Low Ohm Current Sense | 2512 (6432) | 0.006 Ω | ±1% | ≤ ±50 | 1 W | match: tolerance ±1%, termination · mismatch: metal plate, 2512 footprint, 0.006 Ω, ≤ ±50 TCR, 1 W |
The two groups share a tolerance of ±1% and a termination finish. They share nothing else
that matters. Forty-two times the temperature coefficient is not a detail.
Walsin’s public resistor product index is at
https://www.passivecomponent.com/products/resistors/ — an index page, not a direct link
to any datasheet PDF. The thick film datasheets we hold locally are
_Walsin_Datasheets/Resistor/Walsin_WR_General_Purpose_Chip_Resistor.pdf and
_Walsin_Datasheets/Resistor/Walsin_MR_Automotive_Thick_Film_Chip_Resistor.pdf.
Flagged for review: we do not hold a local datasheet for the WW low-ohm current sense
family, and terminal configuration — two-terminal versus four-terminal Kelvin — is not in
the parametric data. Confirm both from Walsin before the part is designed in.
The TCR distribution nobody publishes
This is the finding worth taking away from this article. Walsin’s low-ohm thick film
catalogue does not sit in one TCR band, and the milliohm parts sit outside the bands most
people assume.
| TCR band | Thick film parts | Metal plate parts |
|---|---|---|
| ≤ ±50 | — | 612 |
| ≤ ±70 | — | 438 |
| ≤ ±75 | — | 56 |
| ≤ ±100 | 1,501 | 81 |
| ≤ ±150 | 2,798 | 1 |
| ≤ ±200 | 746 | — |
| ≤ ±250 | 612 | — |
| ≤ ±300 | 1,355 | — |
| ≤ ±500 | 1,415 | — |
| ≤ ±600 | 650 | — |
| ≤ ±1000 | 400 | — |
Two things follow from that table.
The metal catalogue is genuinely tight. 1,187 of 1,188 metal parts are at
≤ ±100 ppm/°C or better, and 612 of them are at ≤ ±50. Exactly one metal part in the whole
extract sits as loose as ≤ ±150.
The thick film bands do not add up to the thick film total. They account for 9,477 of
the 10,428 thick film parts. The remaining 951 sit in bands outside that list — and the
1206 milliohm parts in the comparison table above, at ≤ ±2100 ppm/°C, are among them. Filter
the thick film current sense catalogue by the common TCR bands and the lowest-resistance
parts do not appear, because their TCR is worse than any band in the usual list.
That is the trap. An engineer filters for 10 mΩ, gets a result, and never sees that the TCR
on that part is twenty times looser than the thick film parts they have used before.
Venatech Limited, an authorised Walsin distributor in Hong Kong, holds the full parametric
data including the parts outside these bands. Walsin’s own export fails above roughly
10,000 rows, which is why a 10,428-part family has to be pulled in slices at all.
Which construction should I use for battery or motor current sensing?
Metal plate, in almost every case. Battery fuel gauging and motor phase sensing both run
high current through a milliohm shunt, which means real self-heating, and both need the
reading to stay accurate while that happens. A ≤ ±50 ppm/°C metal part holds its value. A
≤ ±2100 ppm/°C thick film part does not.
Use thick film where the current is modest, the resistance is above about 50 mΩ, or
something downstream calibrates out the drift — overcurrent trip thresholds, crude load
detection, non-critical telemetry. Use metal plate where the measurement itself is the
specification: coulomb counting, closed loop motor control, power metering, protection
thresholds that must not drift.
What power rating does a current sense resistor need?
Compute I²R at the maximum continuous current, then derate for ambient. Walsin’s metal
plate low-ohm catalogue runs from 1/3 W to 3 W: our extract holds 502 parts at 1 W, 320 at
2 W, 156 at 3 W, 155 at 1/2 W, 46 at 3/4 W and 9 at 1/3 W. The thick film 1206 parts in
this article are 1/4 W.
The arithmetic is unforgiving at low ohms. A 5 mΩ shunt carrying 15 A dissipates 1.125 W,
already past a 1 W part before any derating. At 30 A it reaches 4.5 W, beyond the 3 W
ceiling of this catalogue.
Tolerance narrows as power rises. Across the metal catalogue our extract holds 721 parts at
±1%, 435 at ±5% and only 32 at ±0.5%. A design needing ±0.5% is choosing from 32 parts, and
the power rating and resistance you want may not be among them. Establish that early.
Why does my shunt read differently once the board warms up?
Three effects, in order of size. TCR changes the resistance itself. Self-heating raises the
resistor’s own temperature above ambient, which drives TCR further. Thermal EMF at the
junction between element and terminations adds a small offset voltage the amplifier cannot
distinguish from signal.
TCR dominates at low ohms. A 10 mΩ thick film part at ≤ ±2100 ppm/°C can move 21% over a
100 °C rise. The same rise on a ≤ ±50 ppm/°C metal part moves it 0.5%.
Thermal EMF matters most when the sense voltage is small. At 3 mΩ and 5 A the sense voltage
is 15 mV, so a few microvolts is a fraction of a percent — but a fraction that tracks the
thermal gradient rather than the current. Symmetric layout and equal copper on both pads
reduce it. Route the sense traces as a Kelvin pair from the pad edges, keep them
differential and close together, and keep the high-current path out of the sense loop. A
poorly routed 0.5% shunt reads worse than a well-routed 1% one.
Frequently asked questions
What resistance value should a current sense resistor be?
Pick the value that gives your amplifier a usable full-scale sense voltage at maximum
current, then check the dissipation. Typical targets are 50 mV to 100 mV full scale. Lower
resistance means less heat and less burden voltage but a smaller signal, which pushes the
error budget onto the amplifier’s offset.
Is metal plate the same as metal foil?
No. Metal plate uses a solid alloy element welded between terminations and is aimed at
power and low resistance. Metal foil uses a thin patterned foil bonded to a substrate and
targets precision at higher resistance values. Walsin’s low-ohm current sense catalogue in
our extract is metal plate.
What TCR do I need for a 1% measurement over temperature?
Work backwards from your temperature span. For 1% total over a 100 °C rise with no
calibration, the resistor alone must be better than 100 ppm/°C, which leaves nothing for
tolerance or amplifier error. In practice that means a metal part at ≤ ±50 or ≤ ±70 ppm/°C.
How many tight-tolerance metal parts does Walsin offer?
Our extract holds 32 metal plate low-ohm parts at ±0.5%, against 721 at ±1% and 435 at ±5%.
The ±0.5% line is small, so check whether your resistance and power combination exists
there before committing the design to that tolerance.
Do I need a four-terminal Kelvin resistor?
Below about 10 mΩ, usually yes, because solder joint and pad resistance become comparable
to the element. A four-terminal part separates the current path from the sense path at the
component. With a two-terminal part at milliohm values, Kelvin routing to the pads is the
minimum requirement.
Next step
Send us the maximum continuous current, the ambient, the accuracy you need over temperature
and the footprint you can afford. We will return the Walsin thick film and metal plate
candidates with resistance, tolerance, TCR, rated power and case size stated field by field
— including the low-resistance parts whose TCR falls outside the usual filter bands. We
will not tell you a part is equivalent. We will show you where it differs.
Send us your BOM → ·
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Related reading: Chip resistor power rating by case size ·
Thin film vs thick film chip resistors