# Disc varistor

A leaded metal oxide disc varistor standing on the board: TDK's SIOV StandarD series. The disc stands upright on two straight leads. Lead 1 is at the origin, lead 2 is at +X by the lead spacing e and at -Y by the lead offset a, because the leads are fixed to the disc's two faces. The leads go straight down through the board.

The type names the disc size and the voltage class, so `S10K275` is a 10 mm class disc for 275 V RMS. The part takes them as `size` (`S10`) and `voltage` (`275`). Every type in TDK's dimension table is offered, 149 in all: S05 and S07 from 11 to 460 V, S10 to 680 V, S14 and S20 to 1100 V.

| Size class | Disc Ø w max | Lead spacing e | Lead Ø d | Height h max | Thickness th max |
|---|---|---|---|---|---|
| S05 | 7.0 | 5.0 | 0.6 | 8.5 to 9.0 | 3.3 to 5.7, by voltage |
| S07 | 9.0 | 5.0 | 0.6 | 11.0 to 11.5 | 3.3 to 5.7, by voltage |
| S10 | 12.0 | 7.5 | 0.8 | 14.5 to 15.0 | 3.9 to 7.9, by voltage |
| S14 | 15.5 | 7.5 | 0.8 | 18.5 to 20.5 | 3.9 to 11.0, by voltage |
| S20 | 21.5 | 10.0 | 1.0 | 25.5 to 28.5 | 4.3 to 11.4, by voltage |

`part.py` holds the whole table, type by type, with the ordering code (B722...) for each.

## Source

TDK Electronics, "SIOV metal oxide varistors, Leaded varistors, StandarD series", B722*, March 2018, from TDK's own site: <https://www.tdk-electronics.tdk.com/inf/70/db/var/SIOV_Leaded_StandarD.pdf>.

- Pages 14 to 18, table "Dimensions": for each ordering code, the lead spacing e ±1, the lead offset a (typical), the disc diameter w max, the thickness th max, the height over the seating plane h max, the lead length l min and the lead diameter d ±0.05. The ordering-code tables on pages 2 to 13 give each code's type name (`S14K1000` and `S20K1000` are rated 1100 V RMS, as the sheet's footnote says).
- Page 14, "Dimensional drawings" (VAR0408-C-E): the disc upright on two leads, the leads a apart in the bottom view, and the seating plane (IEC 60717) at most 3 below the disc.

**Proportions, for looks only.** The drawing shows these but doesn't dimension them:

- The disc's rim is rounded with a radius of 0.4 x the thickness, at most 0.1 x the diameter.
- The disc's top is at h max, so its underside is h - w above the board. Each lead runs from its end up into the disc's underside. The coating that runs down the leads (3 max) isn't modelled.
- The leads end 3.0 below the board by default (`tail`). The sheet's lead length, 25 min, is for the loose part before it's cut.
- Materials are for looks and a rough mass. The disc is zinc oxide ceramic (density about 5.6) under a thin epoxy coat. Titanium nitride, the ceramic in the material list nearest in density (5.2), stands in for it, in SIOV's blue. The leads are tinned copper, so copper.

## Catalogue models

The catalogue's `RV_Disc_D<w>mm_W<th>mm_P<e>mm` models name a disc diameter, thickness and lead spacing but no maker. Each one whose three values are a row of TDK's table maps to that size, to the lowest-voltage SIOV type that has it. The types that share a size differ only in the typical lead offset a. 97 of the 100 models map. Measured on three reference models (D7 W3.4 P5, D12 W4.2 P7.5, D21.5 W5.1 P10): the disc's diameter, thickness and lead spacing are the named ones. Their leads are Ø 0.54, 0.72 and 0.9, where TDK gives 0.6, 0.8 and 1.0, and this part uses TDK's leads. The discs sit on the board, where TDK's h max puts the disc's top at w + 1.5 to w + 7.

Three models name a size that isn't in the StandarD table, so they're left out until another table is read (`COVERAGE.md`): D16.5 W6.7 P7.5, D21.5 W6.1 P7.5 and D9 W6.1 P5.