--- title: "Where do you tap a 0.5 mΩ shunt?" topic: "Shunt taps" answer: "At the middle of the pads’ inner edges. With 10 A through the 0.5 mΩ shunt moteus-x1 uses, sense traces there read 10.1 A. Vias 2 mm out on the copper read 14.1 A." url: "https://nearfieldnotes.com/notes/shunt-taps/" site: "Near Field Notes" author: "Michael Walker" note: 5 family: "Sensing" published: "2026-11-02" revised: "2026-11-02" revision: 1 video: "https://nearfieldnotes.com/videos/shunt-taps/" --- Note 5 · [Sensing](https://nearfieldnotes.com/families/sensing/) · Rev 1 · [Watch the video · 0:19](https://nearfieldnotes.com/videos/shunt-taps/) Shunt taps # Where do you tap a 0.5 mΩ shunt? **Short answer:** At the middle of the pads’ inner edges. With 10 A through the 0.5 mΩ shunt moteus-x1 uses, sense traces there read 10.1 A. Vias 2 mm out on the copper read 14.1 A. **Note 5’s opening figure: The taps, moving out.** From the inner edges to 3 mm out ![Note 5’s opening figure: The taps, moving out](https://nearfieldnotes.com/notes/shunt-taps/fig-opening.png) *What the figure shows:* The 0.5 mΩ shunt moteus-x1 uses, from above, with 10 A through it and a voltage line every 5% of its own voltage. The sense taps step out from the middle of the pads' inner edges across the copper. Taps at the inner edges: 10 A reads as 10.1 A. Taps at the outer edges: 10 A reads as 10.9 A. Taps 1 mm out: 10 A reads as 12.6 A. Taps 2 mm out: 10 A reads as 14.1 A. Taps 3 mm out: 10 A reads as 15.5 A. **Key** - 5% of the shunt’s voltage - Copper in the reading 10 A through a 0.5 mΩ Vishay WSLF2512 on moteus-x1’s pads, 2 oz copper. Each line lit between the part and a tap adds 5% to the reading. ## What a shunt tap is *One of moteus-x1’s phase shunts* A shunt is a resistor in the current’s path, small enough that 10 A drops only 5 mV across 0.5 mΩ. [An amplifier](https://nearfieldnotes.com/glossary/#g-current-sense-amp) reads that voltage through two sense traces, one from each end. Where each trace meets the copper is its tap. At 0.5 mΩ the shunt is about as resistive as 2 oz copper its own size, so any copper between the part and the taps adds to the reading. Where the shunt sits in the bridge is [Note 19](https://nearfieldnotes.com/notes/sense-position/). **Fig. 5: The circuit.** ![Fig. 5: The circuit](https://nearfieldnotes.com/notes/shunt-taps/fig-5.png) *What the figure shows:* Figure 5: the circuit. 10 A runs through the shunt R15, 0.5 mΩ, and drops 5 mV across it. Two sense traces run from the two ends of the shunt to an amplifier's + and − inputs, and the amplifier feeds the ADC. The current runs through the shunt. The amplifier only sees the voltage between its two sense traces. **Fig. 5b: On the board.** ![Fig. 5b: On the board](https://nearfieldnotes.com/notes/shunt-taps/fig-5b.png) *What the figure shows:* Figure 5b: the shunt on the board, from above. Two gold pads sit on copper poured out to each side, and the shunt lies across them, seen through. Two sense traces leave the middle of the pads' inner edges, run in under the shunt side by side and out of its top edge to the amplifier. The current comes in through the copper on the left and leaves on the right. The shunt seen through, from above. Its sense traces leave the middle of the pads’ inner edges and run out under the part side by side. **Fig. 5c: What the taps read.** ![Fig. 5c: What the taps read](https://nearfieldnotes.com/notes/shunt-taps/fig-5c.png) *What the figure shows:* Figure 5c: what 10 A reads as against where both taps are, with the shunt's end drawn side-on to the same scale under it. At the middle of the pads' inner edges it reads 10.1 A. At the pads' outer edges 10.9 A. Vias 2 mm out on the copper read 14.1 A, and the reading keeps climbing with no flat spot past the pad. Both taps moved out together from the middle of the pads’ inner edges. The dots are taps wired as drawn under the chart: a top-layer trace at the inner edge, vias at the outer edge and 2 mm out. Shunt taps ### 2 mm out, 10 A reads as 14.1 A **Field marks.** Taps at the middle of the pads’ inner edges read 10.1 A for 10 A, inside the part’s 1%. **Look-alike.** Taps on the pads’ outer edges already read 10.9 A. The current still has to cross the pad to reach the part, and the copper it crosses is in the reading. **Range.** There’s no flat spot past the pad. Each millimeter out adds about 1.6 A. A 2 mΩ shunt on the same pads reads 11.0 A at 2 mm out, because the copper is a smaller share of it. **Habitat.** The part moteus-x1 uses for its three phase shunts, a Vishay WSLF2512 0.5 mΩ, on KiCad’s 2512 pads with 2 oz copper poured up to them. ## Where on the pad *Middles, not corners* **Fig. 5d: Middles or corners.** ![Fig. 5d: Middles or corners](https://nearfieldnotes.com/notes/shunt-taps/fig-5d.png) *What the figure shows:* Figure 5d: the same shunt from above with a voltage line every 5%. The sense pair leaves the middles of the pads' inner edges, which read 10.1 A for 10 A, and runs in under the part. Taps at the corners of the same edges read 10.7 A. The same shunt with a voltage line every 5%. Beside a pour the lines bend round the pads’ corners, so a tap there picks up copper. Key to shunt taps ### Where should the sense traces connect? 1. 1a Each end has its own sense pad Anywhere on the sense pads, 5f 2. 1b One pad at each end Go to 2 3. 2a Copper poured beside the pads The middle of the inner edges, 5d 4. 2b Fed by a trace as wide as the pad Anywhere on the inner edges On moteus-x1’s pads with a pour, the corners of the inner edges read 10.7 A for 10 A. Fed by a pad-wide trace they read 10.2 A. Vishay’s datasheet draws the sense traces at the inner edges, and moteus-x1 routes its own pair that way. ## Move the taps yourself *3 coppers, 3 shunt values* **Note 5’s explorer: Shunt taps.** ![Note 5’s explorer: Shunt taps](https://nearfieldnotes.com/notes/shunt-taps/fig-explore.png) *What the figure shows:* What 10 A reads as against where both taps are, for the chosen copper and shunt value, with the shunt's end side-on under the chart. Thinner copper, a pad-wide trace instead of a pour and lower shunt values all climb faster. Both taps moved out together. A 2 oz trace is as wide as the pad, with no pour beside it. Each shunt is read against its own value. **Fig. 5e: What makes it worse.** ![Fig. 5e: What makes it worse](https://nearfieldnotes.com/notes/shunt-taps/fig-5e.png) *What the figure shows:* Figure 5e: what 10 A reads as with the taps on vias 2 mm out, as one thing changes from the board as built: 1 oz copper, 17.6 A, a pad-wide trace, no pour, 17.3 A, copper 60 °C hotter, 15.1 A, as built, 14.1 A, 1 mΩ shunt, 12.1 A and 2 mΩ shunt, 11.0 A. Taps on vias 2 mm out. Each row changes one thing from the board as built, a 0.5 mΩ shunt on a 2 oz pour. Copper and value ### On 1 oz copper, 10 A reads as 17.6 A **Field marks.** Thinner copper has more resistance in the same 2 mm. A pad-wide trace in place of a pour reads 17.3 A. **Look-alike.** Copper 60 °C hotter reads 15.1 A. Copper gains 0.39% of its resistance per °C, so the error grows as the board warms up. **Range.** A bigger shunt drowns the copper out. On the same vias 1 mΩ reads 12.1 A and 2 mΩ reads 11.0 A. **Fig. 5f: A 4-terminal shunt.** ![Fig. 5f: A 4-terminal shunt](https://nearfieldnotes.com/notes/shunt-taps/fig-5f.png) *What the figure shows:* Figure 5f: a 4-terminal shunt, Vishay's WSK2512, from above with a voltage line every 5%. Each end has a small sense pad apart from the current pad, and a sense trace leaves each one through a gap in the copper. At the sense pads 2 oz pour reads 10.0 A, 1 oz copper reads 10.0 A, pad-wide trace reads 10.0 A, copper 60 °C hotter reads 10.1 A for 10 A. Vishay’s WSK2512 0.5 mΩ on its own pads. The sense traces leave through gaps in the pour. 4-terminal shunt ### Its sense pads read within 1% on every board **Field marks.** Each end has a small sense pad beside the current pad. No current flows in it, so it reads the same anywhere along it, to 1 part in 10,000. **Look-alike.** Its current pads, tapped at their outer ends, read 12.7 A. Used that way it’s no better than a 2-terminal part. **Range.** 2 oz, 1 oz, a pad-wide trace and copper 60 °C hotter all read 10.0 to 10.1 A. The part puts its tap point inside itself, where Vishay specifies its 1%. **Habitat.** Vishay WSK2512 in its 0.5 to 0.99 mΩ build, on Vishay’s recommended pads. #### Conditions - **Part:** Vishay WSLF2512L5000FEA, 0.5 mΩ: a manganese-copper element between copper terminals, 6.35 by 3.05 mm. 1 and 2 mΩ parts for comparison. - **Pads:** KiCad’s R_2512_6332Metric, moteus-x1’s footprint: 1.23 by 3.35 mm, 4.7 mm apart - **Copper:** 2 oz, poured up to the pads’ inner edges, with 75 µm of solder under the terminals and a fillet on their ends - **Taps:** Top-layer traces at the middle of the inner edges, and 0.2 mm vias from the outer edges out. The explorer’s stops across the pads are points on the pad. - **Reading:** The voltage between the taps over the current, against the part’s own value - **Element:** 31 µΩ·cm, set so the part reads 0.5 mΩ at the middle of the inner edges on Vishay’s pads. That’s normal for manganese-copper. - **Solver:** A 3D finite-volume DC solver, 149,000 cells and about a second a case - **Check:** Halving the mesh moves the readings by 0.1% or less Simulated only. Nothing on this page has been measured on a board yet. #### Limits - This is the part and the pads moteus-x1 uses, not its copper. On the board the pour wraps under the part, stitched by vias. - DC only. The shunt’s inductance and the amplifier aren’t in the model. - The element’s alloy is taken as fixed with temperature. In the 60 °C case only the copper and the solder warm up. #### Files - shuntsim/fv.py The DC solver - shuntsim/geom.py The parts, their pads and the copper - shuntsim/run.py Every reading on this page #### Sources - [Vishay WSLF datasheet](https://www.vishay.com/docs/30193/wslf.pdf) - [Vishay WSK2512 datasheet](https://www.vishay.com/docs/30108/wsk2512.pdf) - [moteus-x1’s KiCad files, Apache-2.0](https://github.com/mjbots/moteus/tree/main/hw/x1) What should the next note be? [Ask for one](https://nearfieldnotes.com/#ask-for-a-note) --- Source: https://nearfieldnotes.com/notes/shunt-taps/ Cite as: Michael Walker, “Shunt taps”, Near Field Notes, Note 5, Rev 1, 2 Nov 2026. https://nearfieldnotes.com/notes/shunt-taps/ Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/shunt-taps/#fig-5.