I have a problem. I am trying to fix a dead lane switching track piece for slot cars made by Carrera. I measured component on PCB and found out that at least two components have gone bad. I suppose they are transistor which drive the solenoids.

I tried to search that marking from the internet but didn't find anything. I would like to know if those are transistor and what could be used as a substitute.

Those components are marked "T3" and "T4" on circuit board. I assume they are transistors. But aren't transistors usually marked as "Q"? Or only MOSFETs?

enter image description here

I'm sorry for shallow information in the opening post. Here's a proper picture of the circuit board. I have already removed those components from the board. But as I said they are marked T3 and T4 in the upper left corner.


I identified that they are bad by measuring with the multimeter in diode mode. I measured the values between two legs of the component. Two neighboring legs showed low resistance between them. It was about like 13 or 30 Ω. I confirmed that the reading is caused by the components by removing them from the board and measuring the board and the components separately.

Those T3 and T4 measured still oddly and now the corresponding pads on the board measured about 60 kΩ.

On left those four pads are connected to two solenoids.

  • 3
    \$\begingroup\$ What in-circuit measurement did you make to determine those transistors are bad? \$\endgroup\$
    – Andy aka
    Commented Mar 24, 2023 at 16:31
  • \$\begingroup\$ Related: electronics.stackexchange.com/q/6108/2028 \$\endgroup\$
    – JYelton
    Commented Mar 24, 2023 at 18:06
  • \$\begingroup\$ Why do you think these parts are bad? \$\endgroup\$
    – Hearth
    Commented Mar 25, 2023 at 15:09
  • \$\begingroup\$ T and Q are both commonly used for transistors/MOSFETs. \$\endgroup\$
    – Lundin
    Commented Apr 4, 2023 at 6:37
  • \$\begingroup\$ As for why they broke, the SS13 schottky diodes are sufficient to deal with reverse EMF from the solenoids but they can only handle 30V on the anode and also won't do you any good in case of spikes. To solve the problem permanently you might want to switch these for TVS like SM6T33A, roughly the same package. They can work as freewheel and spike elimination both at once. \$\endgroup\$
    – Lundin
    Commented Apr 4, 2023 at 6:46

1 Answer 1


The markings (mostly) match with a IRLML2402 N-channel MOSFET. I say mostly because the number of horizontal lines doesn't match, but if you search for pictures of the part online, there are a few examples with matching line patterns, so I'd say it's still plausible.

enter image description here

Please note, however, that there are a many SOT23-3 parts around and there is no guarantee that this is the right one. Make sure the pin-out and voltage ratings match the circuit.

Edit: The pinout matches with the circuit in the new photo. Whether the voltage and current rating are plausible depends on the solenoid.


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