Electronic – Determining Suitable Replacement Transistor

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I have a toy which developed a short, and one of the transistors on the board is now fried.

I've removed the shot transistor, along with the part of the toy which was shorting out, and everything else works fine.

There are no distinguishing markings on the transistor – all I know is it comes in a SOT-23 package – and, I believe, there is a duplicate identical transistor in the toy (for a similar but separate function).

I have a digital multimeter, and a digital oscilloscope at my disposal.

I'm also aware of the type of DC motor this transistor was being used to control (so I should have some idea of the voltage it's required to handle).

Based on this information, and with the tools I have available, how can I determine what a suitable replacement transistor would be?

Best Answer

Summarised_solution:

  • General purpose high current, high gain SOT23 bipolar & MOSFET transistors are available at modest cost. These will work if the part is used as an on/off saturated switch and will probably work in other roles.

  • Required polarity can be determined by a single measurement.

SOT23 small signal transistor is usually -
b c e for bipolar
g d s for MOSFET

as per diagram below.

enter image description here

Measure voltage 2 to 3.
If 2 is +ve wrt 3 then = npn or n channel. If reversed then pnp or p channel.

A high current high beta npn or pnp will work in most bipolar cases.
BC817-30 / BC807-40 are good.
-40 means AVERAG current gan of 400. ie 250-600 range.
50V, 800 mA rated for thisFairchild example sold by Digikey
Rbase of 1k to 10k should be OK.

MOSFET N Channel needs low Vgsth.
CES2310 utterly superb if you can get them. OR

36 cents /1 in stok at Digikey for this
2.6v Vgsth , 60V, 1.A continuous
IRLML2060

But, Better! -->

INFINEON BSS205 N !!!
20v, 2.5A, < 100 mohm Rdson at 2.5V Vgs.
In stock Digikey 37 CENTS /1

No gate resistor needed. Use say 1K if desired.

For P Channel use Digikey selector guide to select out specs of chouce then sort by price ascending in quantity 1.

This is a truly marvellous part for amateur general use. Be ware of 20V Vds max but otherwise marvellous.
eg 0.2V Vds at 1.5A at 2V Vgs.
500 mW disispation and 250 K/W mean due care needed thermally (as in all SOT23)
BUT very low Rdson will keep it OK in many cases.
eg 98% of these have < 80 milliohm Rdson at high temperature.
At say 1A dissipation <= 80 mW for 20 C case rise on modest copper.

If you MUST have 30V there is the very nice 41cents/1 IRLML6346

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