2n3904. Is it OK to send Base to ground

[Ric046]

Hi. This is a continuation of a query I had last week.

I have an LED setup with a 2n3904 transistor.  I send ground to the base of the transistor which in turn, turns off the led. Which is great. It's exactly what I wanted.

I've done a little research on 2n3904s this week but can't quite find what I'm looking for, which is to know if it's safe to send ground to Base in this fashion? I was hoping some here may be kind enough to enlighten me.

[john_p_wi]

If it is configured normally as one would using a transistor as a switch, grounding the base is no problem.  The more usual configuration would be to use a pull down resistor to ground after the base current limiting resistor, therefore, when the base loses power the pull down resistor pulls the base to ground turning off the transistor.

[Ric046]

OK John, here is the circuit in question. When the boost switch sends ground to the base of Q7 the LED shuts off. Would this be a normal configuration?

[john_p_wi]

The npn transistor as shown doesn't care if its' base is grounded.  Transistors are current devices, the week pullup (330k) shown in combination with the gain of the transistor turns it on, there is only micro amps of current through the 330k r, if the LED is not fully lit lower the resistance value so the transistor is in saturation.

It's a little unorthodox, but IF it works roll with it.  The only downside is the constant parasitic drain to ground IF powered by a battery.

[Ric046]

Hi John, thanks for your reply. I've changed to this configuration now which is more of the regular layout. I've kept the 330k and 4k7 as the LED is perfectly bright enough.

When switched on (base not grounded) the collector voltage is 0.11V (from a 9v supply) and when switched off (base grounded) the voltage of the collector goes up to 8.14V