Tech Question about BC547 Transistor (and Transistors in General?)

I have a question about the BC547 transistor. Specifically, I stumbled on this video:

https://youtu.be/uqSYnLUq61A

If you check the schematic (shown at about the 3:00 mark), you’ll notice that the circuit doesn’t use the transistor’s base. Everything I know about [BJT] transistors from Sparkfun and elsewhere uses the base to turn the transistor on and off… so how does this blinker circuit work?

My guess is that a “spark” forms across the collector/emitter when the voltage across the capacitor reaches a certain threshold, but I don’t see that threshold info in the spec sheet:

www.sparkfun.com/datasheets/Components/BC546.pdf

Can you please enlighten me? Thanks!

Hello, and thanks for posting on the forums!

There’s no sparks involved, but what’s happening is the circuit is exceeding the reverse voltage between the collector and emitter. When that happens, the transistor turns on until the LED discharges the capacitor and the cycle starts again. There’s actually a good comment in the video that explains this in more detail. I’ve copied that below. The data sheet probably doesn’t reference the reverse CE voltage since this is a condition that is to be avoided and will eventually destroy the transistor.

Let me try to explain how it works. By RC = t, 3.3k resistor and 100u cap forms a time constant around 0.33s. When the cap just be charged, voltage of cap is lower than the acceptable reverse voltage of the VCE of the transistor. CE of transistor keeps open circuit then LED is off. However, once the cap voltage reaches over VCE, transistor becomes short circuit. It turns on the LED and also discharge the cap at the same time to complete one cycle operation. However, this circuit just works in high power supply voltage and the transistor may be damaged after a long time operation. To make this circuit more reliable, the transistor can be replaced by a zener diode, then using a NPN transistor which drives from the net between zener and LED to discharge the cap.

Thanks. What I meant by “spark” (note the quotes) was a reference to a spark gap. But yes… you basically confirmed my suspicion.

That said, at what point does breakdown occur across VCE? Does it occur at the max voltage? If yes, how can that be? Assuming I read the datasheet correctly, the max collector-emitter voltage for the BC547 is 45V, which is well below the 12V power supply voltage.

at what point does breakdown occur across VCE?

The data sheet doesn’t list a figure for VCE reverse breakdown so you’d need to determine it experimentally.