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Which is the way to choose a resistor for a NPN transistor base?

I want to use P2N2222A as a switch, in a design as shown below. When I have voltage in base (1.8 V), I want a connection to be created between NODE1 and ground.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ It all depends on how much collector current the transistor needs to support when on. \$\endgroup\$ – Olin Lathrop Nov 20 '13 at 15:35
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If you want to conduct up to 150mA of current between node 1 and ground and only drop 1.0V (it's not a perfect switch) then you'll need to assume the current gain is typically 50. If you want a lower saturation voltage then the spec sheets says feed the base with 15mA i.e. the current gain has dropped to only 10 but saturation will only be 0.3V.

So assuming you are happy with 150mA while saturating the transistor to about 1V, you need to push \$\dfrac{150mA}{50} = 3mA\$ into the base.

The base voltage will need about 0.7V so the remainder (1.8V - 0.7V) needs to be across the resistor R1. Ohms law tells us that R = \$\dfrac{1.1V}{3mA} = 366.7\Omega\$.

So choose maybe a 360 ohm resistor.

If this isn't good enough for your needs look for an N channel MOSFET with low \$V_{GS(threshold)}\$ - something like 1V or less.

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  • \$\begingroup\$ It's OK to short-circuit node 1 and ground to measure current? Measurement showed 0.15 mA. The existing circuit (without the transistor) is part of an RC car transmitter (electronics.stackexchange.com/questions/90980/…). \$\endgroup\$ – dempap Nov 20 '13 at 17:01
  • \$\begingroup\$ That sounds fine. \$\endgroup\$ – Andy aka Nov 20 '13 at 17:42
  • \$\begingroup\$ In this case, you think I need less collector current (e.g. 1 mA) or 150 mA is just fine? (node 1 is pin of ic) \$\endgroup\$ – dempap Nov 20 '13 at 19:47
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    \$\begingroup\$ If tthe collector is only grounding less than 1mA then the base current, for a decent switch on will be no more than 20uA but play safe and go for 100uA through R1 which means R1 is approximately 10k ohm. It never hurts to put more current into the base up to a limit of course. \$\endgroup\$ – Andy aka Nov 20 '13 at 22:39
  • \$\begingroup\$ I implemented the circuit described (NPN P2N2222A, 10k Ohm base resistor). I think I burnt the IC (from Datasheet: Operating current, Unload - max: 0.4 mA). How can I confirm if I damaged it? \$\endgroup\$ – dempap Nov 28 '13 at 14:46
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Current

There's a rule of thumb that you make the base resistor such that the base current is a good multiple (5, 10?) of the minimum base current needed to put the BJT into saturation - you can get this from the data sheets.

Voltage

You need to know \$V_{BE(sat)}\$ - but that's in the data sheets too. It's probably safe to assume 0.7 V for most small-signal NPN BJTs.

enter image description here

Resistance

Once you know the voltage across the resistor and the current through it, Ohms law will tell you the answer.

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