Base voltage of 2N4401.

Lightium · Jul 14, 2023

I am simulating a circuit and am wondering if a 2N4401 with a max base voltage of 6 volts can handle 7?

Lightium · Jul 14, 2023

OK, how about 6.54 volts?

crutschow · Jul 14, 2023

That's the maximum reverse base-emitter voltage rating, not the forward voltage.
The base-emitter junction forward voltage when the transistor is on, is about 0.7V.

What is the circuit?

danadak · Jul 14, 2023

Typical curve Vbe versus Ib

Regards, Dana.

Lightium · Jul 14, 2023

crutschow said:
That's the maximum reverse base-emitter voltage rating, not the forward voltage.
The base-emitter junction forward voltage when the transistor is on, is about 0.7V.

What is the circuit?

Yes, you need at least 0.7V to turn on, but the max is 6V.

Lightium · Jul 14, 2023

It will be a HV DC-DC converter for investigating resistance of spark gaps.

rjenkinsgb · Jul 14, 2023

Lightium said:
Yes, you need at least 0.7V to turn on, but the max is 6V.

The absolute maximum Emitter-Base voltage is the reverse bias breakdown voltage threshold, NOT forward conduction voltage.

The Base-Emitter forward direction is a maximum current.
50mA is the highest value I can find in the data sheet, which probably occurs somewhat under 1V

Lightium · Jul 14, 2023

So, if I have 6 volts on the base it will fail?

rjenkinsgb · Jul 14, 2023

If you put too much current through the base it will fail. 6V (forward bias) would be rather catastrophic!

I just tested a random NPN (BC550) with 90mA base current and that was still well under 1V forward.

I was curious if the transistor would explode with a directly connected higher voltage, but nothing spectacular with the one I tried - connecting a half charged lithium cell (3.8V) directly across it did nothing visible, but the transistor was quite hot and the base open circuit afterwards.

Lightium · Jul 15, 2023

See, I am too lazy today. I have a breadboard behind me and I don't use it.

Nigel Goodwin · Jul 15, 2023

Lightium said:
So, if I have 6 volts on the base it will fail?

As already said - the base will clamp at around 0.7V, and if you apply 6V (without current limiting) then it will blow instantly (BC550 fuse!).

As also mentioned, the 6V specification is REVERSE Vbe - so applying -6V to the base. This is why you can only use low voltage supplies on multivibrators (unless you add protection) as it effectively switches almost the entire supply voltage backwards across the Vbe junctions.

A 6V multivibrator 'should' be OK, but a 9V one would almost certainly blow at least one of the transistors.

Nigel Goodwin · Jul 15, 2023

rjenkinsgb said:
If you put too much current through the base it will fail. 6V (forward bias) would be rather catastrophic!

I just tested a random NPN (BC550) with 90mA base current and that was still well under 1V forward.

I was curious if the transistor would explode with a directly connected higher voltage, but nothing spectacular with the one I tried - connecting a half charged lithium cell (3.8V) directly across it did nothing visible, but the transistor was quite hot and the base open circuit afterwards.

It would die too easily to be 'spectacular' - 1N4148 diodes across a 240V mains plug makes a better bang

(Apprentice days in the workshop )

I suppose a BC550 would probably blow quite nicely with 240V across it as well?.

Lightium · Jul 15, 2023

Mmm. I must say I thought wrong I'll redesign.

Lightium · Jul 15, 2023

At least the other transistors are in check, it's the osc transistor that's getting six volts.

Nigel Goodwin · Jul 15, 2023

Lightium said:
At least the other transistors are in check, it's the osc transistor that's getting six volts.

Post the circuit (as a picture) so we can see what you're doing.

crutschow · Jul 15, 2023

Lightium said:
Yes, you need at least 0.7V to turn on, but the max is 6V.

What about the word "reverse-bias" do you not understand?

Lightium · Jul 15, 2023

Lightium · Jul 15, 2023

It was late for me this morning, had to get some zzz's.

Lightium · Jul 15, 2023

Lightium said:
Sorry for the delay, it was late for me this morning, had to get some zzz's.

crutschow · Jul 15, 2023

So I see no where in that circuit that any significant reverse bias voltage is being applied to a base-emitter junction.

Base voltage of 2N4401.

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