2

I'm building the circuit described here, but I need to tweak it a little.

I need a stronger magnetic field from the coil to move a bigger 'swing'. So I want to increase the number of turns on the coil, but then I'd also need to increase the voltage in this circuit. I want to use 12V instead of 5V.

I'm confused if this would blow up the transistors (and LED, obviously). All the datasheets from transistors I can find, say the maximum base-emitter voltage is 5 to 6V. Will it ever reach this voltage however? Since the capacitor will activate Q2 when it reaches 0.7V...

Do I need other transistors, or will it keep working? Not sure if I understand transistors at all anymore! Haha... Any help is appreciated.

thomas clssns
  • 23
  • 4

1 Answers1

0

The B-E voltage limit is the reverse voltage limit. In the forward direction, the B-E junction is just a diode, and it clamps the voltage around 0.65V.

You could increase the number of turns without increasing the voltage - it all depends on the parameters of the coil. Or you could use a stronger magnet.

Kuba hasn't forgotten Monica
  • 40,960
  • 1
  • 39
  • 120
  • oh wow... So you're saying this will indeed work without a problem? Is the PNP transistor also safe to use it? Just wondering: when do you get a reverse voltage? – thomas clssns Apr 20 '22 at 15:02
  • The reverse voltage is generated by the coil when its magnetic flux collapses after you turn it off. Many transistors can also be used in avalanche mode as inherent protection: they will absorb a negative voltage spike by acting like a 60-90V Zener diode :) I haven’t looked at the circuit but I encourage you to experiment. Burning stuff up is OK with simple cheap components. I mostly wanted to make sure you understood the VBE rating. – Kuba hasn't forgotten Monica Apr 20 '22 at 21:19
  • you're absolutely right to experiment and learn along the way! thanks for clarifying. – thomas clssns Apr 21 '22 at 13:29