KEMI Docs · circuit practice
Connector Selection Guide — JST, Pin Headers, Terminal Blocks, USB-C
Last updated: 2026-09-16
Connectors are decided last in the schematic and cause trouble most often in the field — not enough current, cables that don't match, plugged in backwards. This article compares the five connector families small products actually use by pitch, current, mating cycles and price feel, and lists the accessories (housings, crimp terminals) that ordering often forgets.
1
Five families compared
| Family | Pitch | Current/pin | Mating life | Use |
|---|---|---|---|---|
| JST SH | 1.0 mm | 1 A | 50 | sensors, tiny modules |
| JST PH | 2.0 mm | 2 A | 50 | batteries, small motors, general |
| JST XH | 2.5 mm | 3 A | 50 | batteries, fans, general (most common) |
| 2.54 pin header (box header) | 2.54 mm | 1-3 A | 50-100 | debug, ribbon cable, jumpers |
| Screw terminal block | 3.5/5.08 mm | 8-15 A | unlimited (screw) | power, field wiring |
| USB-C receptacle | — | 3 A (5V), 5 A (PD) | 10,000 | power, data, external port |
A 50-cycle life means internal wiring plugged once at the factory. Using JST on a port users plug repeatedly gives contact failures within months — external ports use USB-C, DC jacks or RJ45 rated for thousands of cycles.
2
Current and wire gauge
| Current | Wire (AWG) | Connector |
|---|---|---|
| ≤ 1 A | 28-26 | SH, PH |
| 1-3 A | 24-22 | XH, 2.54 header (two pins in parallel recommended) |
| 3-8 A | 20-18 | VH (3.96 mm), 3.5 mm terminal block |
| 8-15 A | 16-14 | 5.08 mm terminal block, XT30/XT60 |
Per-pin ratings assume a single pin at room temperature. With several pins carrying current together, derate to 60-70%, and lower still inside a sealed enclosure. When the current sits at the boundary, paralleling two pins is cheaper and more reliable than a larger connector.
3
Preventing reverse and wrong insertion
- JST housings are keyed and cannot be reversed. The real accident is plugging into the wrong header of the same pin count — vary pin counts (2/3/4) or housing colours.
- 2.54 headers have no key. A box (shrouded) header fixes orientation; failing that, remove one pin as a key and block the matching cable hole.
- Power connectors in a different family from signal connectors — two XH-4 side by side for power and a sensor will eventually be swapped.
- Silkscreen pin 1 and signal names (VCC/GND). Housing colour alone is not enough.
4
What ordering forgets
- The board-side header (wafer) and the cable-side housing are separate parts. Buying only one is the most common mistake.
- Crimp terminals are separate from housings and have different part numbers per wire gauge. They come on reels of thousands — for small quantities, buy finished cable assemblies.
- SMD headers need part numbers with pick-up caps and reinforcement tabs to run on an SMT line. For hand assembly, through-hole is sturdier.
- USB-C 'power-only 6-pin' and 'data 16/24-pin' are different parts. Even power-only needs two 5.1 kΩ pull-downs on CC — without them a PD adapter delivers nothing.
KEMI defaults
Internal wiring under 3 A → XH; small under 2 A → PH; tiny module links → SH. Power input → terminal block or DC jack; external data → USB-C; debug → 2.54 box header. Staying within five families standardises stock and cables.
FAQ
Are JST XH and PH compatible?
No. Pitch (2.5 vs 2.0 mm) and housing width differ; they do not mate. Cheap look-alikes have different retention and fall out under vibration.
Can a pin header carry 3 A?
Some part numbers are rated 3 A, but 1 A is the practical figure. For 3 A, parallel two or three pins or move to XH/VH.
Board-to-board instead of wire-to-board?
For two fixed boards, header plus socket (stacking) is cheapest. With vibration, use a latching board-to-board connector (DF11 etc.).
See also
This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.