So I'm making a simple circuit using a transistor as a switch, I've made these a bunch of times before without any sort of issue but for some reason this time it just doesn't seem to work at all.

I've never used this FZT651 transistor before, datasheet is here. In place of R1 I've used both a 3k resistor and a 510R resistor, measuring B-E I get 0.7V. If I measure between 5V and the collector I don't measure any voltage.

Is there something strange about the transistor I'm using? I don't have any other NPN's to hand but I'm planning to get some 2N3904 and see if they work because from what I've got here I can't seem to see what the problem is.


simulate this circuit – Schematic created using CircuitLab


Options: -

  1. Your transistor (a decent choice) is broken
  2. You haven't wired it up like your schematic
  3. Your power supply is not connected (check volts between 5V and emitter)
  4. Your meter is intermittantly faulty.
  • \$\begingroup\$ Between the 5V and emitter I've got 4.92V. I'll try another FZT651 as I've got a few more. I assumed that as I could measure the B-E drop everything was ok \$\endgroup\$
    – Doodle
    Apr 22 '16 at 17:25
  • \$\begingroup\$ The BE drop may be fine but the transistor could still be toast. \$\endgroup\$
    – Andy aka
    Apr 22 '16 at 17:29
  • \$\begingroup\$ It was a case of my crappy soldering. I think I need some better glasses. My base pin is just slightly hovering over the pin. When I measured the B-E voltage, me pushing down on the pin must have made the contact, now I feel stupid. \$\endgroup\$
    – Doodle
    Apr 22 '16 at 17:50

There is nothing unusual about this NPN transistor. Since you are reading a normal junction voltage on what you say is the base, it's unlikely (but not impossible) it's completely fried.

Please make sure that the B C E order of pins is respected in your wiring (it's not E B C, which would give the symptoms you describe). Also note that the tab is common with the collector so you should make sure it isn't going somewhere it shouldn't.

In a situation where more power dissipation is required, you would want to tie the tab to some copper to draw the heat out, but that's not going to be an issue with your present circuit.


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