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?


2 Answers 2



  • 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

But, Better! -->

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


From the DC motor you can guess the collector current and collector voltage. The most important paramter that you are missing is the HFE. Can you guess the base-collector current? If it is a bipolar transistor (and the design is not too goofy) there should be a resistor in the base path. If not, big chance that it is a FET.

  • \$\begingroup\$ There is a resistor... but using my multimeter or oscilloscope, shouldn't I be able to determine precisely what the trigger is and go from there? \$\endgroup\$
    – Steve
    Jan 8, 2012 at 19:03
  • 1
    \$\begingroup\$ You need the base current and collector current to get an idea of what the HFE must be. What is the value of the base resistor? And the motor parameters? (voltage, stall current) \$\endgroup\$ Jan 8, 2012 at 19:26
  • 1
    \$\begingroup\$ Since hfe varies widely in production transistors anyway, circuits are usually intended to be operable over a wide range of hfe. (And from the description, it sounds like the transistor may be operated in saturation anyway.) Which is to say, @Steve can probably get the circuit working again with a general-purpose transistor of the right gender. \$\endgroup\$
    – markrages
    Jan 8, 2012 at 21:25
  • \$\begingroup\$ Probably, but some indication of at least the collector current might be a good idea (even if only as a reality check). \$\endgroup\$ Jan 8, 2012 at 21:53

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