A7 on a SOT-23: diode pair or MOSFET?

What A7 is printed on

In our tables, A7 in SOT-23 is printed on 44 parts from 39 makers. In every package it is 264 parts in 25 packages.

The BAV99, a pair of switching diodes in series, leads the list: 14 makers print it with this code, and one maker's BAV99 replaces another's. The AO3407 is a P-channel MOSFET from Alpha & Omega Semiconductor. The other parts are why you measure before you order.

Every part printed A7 in SOT-23, with its maker, where it came from and its meter test.

Does the letter after A7 count?

Often it does not. Some makers print a letter for the factory or the date after the code, and code lists record the result as if it were a code of its own. In our tables the BAV99 in SOT-23 is printed A7s, A7W and A7p as well as A7, and every one of them is the same part.

Alpha & Omega print date, assembly-site and lot codes beside the A7 on the AO3407, so a character there is one of those, not another part.

But a third character can be part of the code. A76 is a code of its own: in our tables it is printed on 7 parts, among them BSS123, OPA374AI, OPA374AIDBV and PJA3476.

SMD marking codes explained has more on which characters belong to a code.

Look at where the pins go

With the single lead at the top, pin 1 is at the bottom left, pin 2 at the bottom right and pin 3 is the single lead. The BAV99 has the anode of its first diode on pin 1, the cathode of its second on pin 2 and the point between them on pin 3. The AO3407's datasheets give the gate on pin 1, the source on pin 2 and the drain on pin 3.

On a board, a BAV99 used as a clamp has pin 3 on a signal line, pin 1 on ground and pin 2 on the supply. A P-channel MOSFET such as the AO3407 usually switches a supply: its source on the rail, its drain to whatever it powers, and its gate held at the source through a resistor and pulled down to switch it on.

So look at pin 1 first. If it runs straight into the ground plane, the diode pair is the better guess. If it goes to a resistor or a driver, it is probably a gate.

Three readings settle it

Lift the part, or at least one leg: on the board the meter reads every path between the pins. Use the diode range, and short pin 1 to pin 2 with tweezers first, so a charged gate cannot leave a MOSFET switched on.

The second reading is the trap: both parts read a junction from pin 3 to pin 2, so on its own it tells you nothing. The first is the test. A drop from pin 1 to pin 3 means the diode pair; open means a gate.

With the probes the other way round, a working part of either kind reads open on all three. The same low reading both ways, near 0 V with the meter beeping, is a short. Every reading of both kinds is in Testing SMD parts with a multimeter.

Before you fit the new part

If it was a BAV99, another maker's BAV99 in SOT-23 will do. If it was the MOSFET, check the replacement's pinout in its own datasheet before you solder it: parts that share a marking do not always share a pinout.

If the old part failed short, find out what it was protecting or switching before you power the board again.