How to identify a 4‑pin connector type and pinout on a PCB

Short answer: measure the pitch and housing features to identify the family, confirm pin numbering from the board’s pin‑1 mark, then map each pin with continuity and low‑risk electrical tests.

What measurements identify the connector family?

Use calipers. Measure center‑to‑center distance between pin 1 and pin 4, then divide by 3 to get pitch. 7.62 mm ≈ 2.54 mm pitch (Molex KK 254, Dupont); 7.50 mm ≈ 2.50 mm (JST‑XH); 6.00 mm ≈ 2.00 mm (JST‑PH or similar); 5.00 mm ≈ 1.67 mm (less common); 3.75 mm ≈ 1.25 mm (JST‑GH/Hirose). Also note:

• Mounting: through‑hole vs SMT, right‑angle vs vertical.

• Polarization: shroud, asymmetrical key, chamfer.

• Latch style: friction ramp, top latch, side latch.

• Wire gauge in the mating cable (if present): thin (~28–32 AWG) suggests 1.0–2.0 mm families; thicker (~18–22 AWG) suggests 3.96–4.20 mm families (JST‑VH 3.96, Molex Mini‑Fit Jr 4.20).

Use a distributor parametric search with: positions=4, pitch (from measurement), mounting type, locking feature.

How do I find pin 1 and numbering?

Check the PCB first: a square pad on the footprint usually marks pin 1; silkscreen triangle/dot or “1” near the pad also indicates it. On shrouded headers, look for a notch or chamfer on the pin‑1 side. For generic 2.54 mm headers, numbering typically runs left‑to‑right when viewing the board’s silkscreen text upright; for vertical board headers with latch at the top, pin 1 is commonly at the latch end, but verify against the square pad.

Map the pinout without schematics

Start unpowered. Use continuity mode on a DMM:

• Ground: the pin that beeps to mounting holes, shielding cans, or large ground pours is GND.

• Power: the pin feeding an inductor, fuse, or the input of a regulator is likely V+; series diodes/polyfuses are common tells.

• Signals: pins that run to MCU/SoC pins via small resistors (22–100 Ω) or to pull‑ups (2.2–10 kΩ to V+) are digital I/O. Two adjacent lines with 2–10 kΩ pull‑ups often indicate I²C (SCL/SDA). A single line to a UART pin (RX/TX) often has 0 Ω or 22 Ω in series; check for a matching pair.

Power next with limits: current‑limit the bench supply (e.g., 50–100 mA to start), insert a series resistor (100–330 Ω) on suspected V+, and monitor voltage droop and device behavior. Use a scope/logic analyzer to spot protocol signatures: UART shows start bits at 9600/115200 bps; I²C shows 100/400 kHz SCL and ACKs; a constant‑frequency square wave ~25 kHz on one pin with another steady high suggests 4‑wire fan PWM/tach.

Common 4‑pin patterns you may see

• GND, V+, SCL, SDA: small sensors and daughtercards (1.25–2.54 mm pitch)

• GND, V+, TX, RX: debug/program (often 1.25–2.54 mm)

• A+, A−, B+, B−: 2‑phase stepper motor (2.54–3.96 mm)

• GND, +12 V, Tach, PWM: 4‑wire fans (2.54–3.00 mm)

• +V and RGB channels: 12 V RGB LED strips (2.54–2.50 mm)

Can I distinguish JST XH vs PH vs GH vs Molex quickly?

Yes—by pitch and latch geometry. XH is 2.50 mm with a tall, blocky housing and friction ramp; PH is 2.00 mm, lower profile, side walls thinner; GH is 1.25 mm with a positive side latch and crisp shroud; Molex KK 254 is 2.54 mm with optional polarizing key; Micro‑Fit 3.0 is 3.00 mm with robust dual‑beam crimp terminals; Mini‑Fit Jr is 4.20 mm, bulky with strong latch for higher current.

Safety and non‑destructive test tactics

Avoid guessing power. If you must probe, lift the suspected V+ pin pad or insert a Schottky diode and a 100 Ω resistor in series during first power. Use ESD protection on logic lines. Never hot‑plug unknown connectors on a live system; transient spikes can exceed small‑pitch contact ratings.

Documentation breadcrumbs to check

Look for embossed maker marks on the plastic (JST, MOLEX, HRS). Scan the board for a reference like J1/CN4 and any silk legends (+, G, SCL, SDA). If you have the cable, crimp style and terminal shape narrow the family quickly. Photos with a ruler, top and side views, and the PCB footprint help a lot with identification.

Manufacturing notes if you’re redesigning the header

Keep pin‑1 marking unambiguous (square pad + triangle silk). Maintain courtyard clearance for latch access. For 2.00 mm and finer, specify hole tolerances tightly to avoid insertion issues. If the assembly will be inspected to IPC Class 2 or 3, call that out on fab/assembly notes; RoHS/REACH material control may affect plating and solder choices.

About HyperPCB capabilities relevant to this work

We can build to IPC Class 2 or Class 3 acceptance criteria and support RoHS and REACH compliance. Our quotes are typically returned within 24 hours, and we provide DFM feedback within 1 business day. Our quality management system is ISO 9001:2015 certified (SGS, certificate CN-2023-Q1156, valid through June 2027).