12V volts being reduced through a PN2222 [duplicate]

Question

I'm trying to build a circuit for controlling the speed of a fan from an ESP8266 MCU. I have a PN2222 transistor with the base connected through a 1K resistor to a digital out of the MCU. The voltage at the digital out is +3.3V when it's high and I verified that I can successfully set it high and low on the MCU reading it with a DMM. So that part seems to be working.

The power supply for the fan that is being switched by the transistor reads ~18V unloaded using a DMM. However when I read the voltage of the emitter of the transistor using a DMM it reads 8V no matter whether the base is high or low.

^{simulate this circuit – Schematic created using CircuitLab}

I was expecting it to read 18V when the base of the transistor was high and 0V when it was low.

Clearly there is something I am misunderstanding here with my first foray into transistors being very new to electronics in general.

Answer 1

The drive should look like this:

^{simulate this circuit – Schematic created using CircuitLab}

I have reduced the base resistor value to allow driving a fan of >200mA. If your fan is only 50mA or less the 1K is fine. D1 is to absorb any inductive spike from the fan motor when the transistor turns off.

Note the ground connection between the ESP and the power supply.

Answer 2

You are using the transistor in emitter follower configuartion. That provides current gain, but no voltage gain. The emitter will be one diode drop below the base, which is at about 2.6 V in this case.

If you are seeing 8 V on the emitter, the you have already blown up the transistor. It probably dissipated too much power.

Connect the emitter to ground, and put the fan between the supply and the collector. Find how much current the fan draws. That divided by the gain of the transistor will be how much base current you need to supply. Size the base resistor to allow at least that much current.

The above is assuming that transistor can handle the fan current, and that this ESP thing can deliver the base current. If the transistor isn't beefy enough, use a different transistor.

A logic-level FET like the IRLML2502 would be a good choice here. It turns on well with 3.3 V gate drive, and it can handle up to 20 V.

I would add a reverse diode across the fan to make sure that whatever transistor you choose doesn't get fried when you shut off the fan.

Answer 3

You were close.

The simplest arrangement is to use the transistor as a "low-side switch". When you inject some current into the base it will allow a much higher current to flow from the collector to the emitter.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Rearranging the circuit for a low-side or common emitter switch.

Try this and report back.

Note: if it is a 12 V fan then you should be using a 12 V power supply - not 18 V.