# How to use a NPN transistor correctly

#### FelixB

Joined Dec 26, 2022
31
Hello,
I want to replace my big L298N motor driver bridge with a lightweight transistor. I never used them but i heared that you need to put a small current on the Base-Pin and after that the current can flow from the Collector to the Emitter.
Here is my schematic:

When i wire this schematic up the pump/motor doesn't spin, but rather the transistor gets really hot, so I know I make something wrong.
The pump/motor runs on 4.5V to 6V and draws 50mA to 200mA.
The power source are batterys.

Felix

Joined Jul 18, 2013
27,659
Use a FET, 2n7000.

#### FelixB

Joined Dec 26, 2022
31
Ok, but for now I only have these 2n2222a laying around. Is there a way to make this circuit working with these transistors?

#### Ian0

Joined Aug 7, 2020
8,419
Did you look at the 2N2222 datasheet?
You need the transistor saturated for efficient switching. So look at the saturation data, and you will see that you need to supply the base with about a tenth of the current you need to switch i.e 10mA for 100mA load
Your circuit supplies about 2.3mA, so the transistor will not be fully switched on, so it will get hot.

Do as @MaxHeadRoom says and use a FET, but make sure you get enough VOLTAGE on the gate to make sure it is fully switched on (3V might not be enough) and don't forget the diode across the motor to deal with the flyback voltage when you switch if off.

#### crutschow

Joined Mar 14, 2008
32,932
A BJT, such as the 2N2222, typically is biased with a base current equal to 1/10th of the maximum collector current to fully turn it on.
For 200mA, that would be a base current of 20mA.
For the 3V base voltage source, this means a base resistor of (3V-0.7) / 20mA = 115Ω (the typical base-emitter voltage is about 0.7V).

So your transistor was getting how because the base current was only about 1/10th of what is needed, and the transistor was not saturated.

You don't need a separate source for the base voltage.
You could use the 6V with a base resistor of 240Ω.

Last edited:

#### Papabravo

Joined Feb 24, 2006
20,400
Hello,
I want to replace my big L298N motor driver bridge with a lightweight transistor. I never used them but i heared that you need to put a small current on the Base-Pin and after that the current can flow from the Collector to the Emitter.
Here is my schematic:
View attachment 295717
When i wire this schematic up the pump/motor doesn't spin, but rather the transistor gets really hot, so I know I make something wrong.
The pump/motor runs on 4.5V to 6V and draws 50mA to 200mA.
The power source are batterys.

Felix
Here is the problem. Even the best NPN transistors will have some voltage drop across the Collector-Emitter junction. You cannot under any circumstances get this value closer to zero than say 0.2-0.3 Volts depending on the particular device. You certainly do not have enough base current to turn the transistor fully on (2.32 mA is my estimate) and it would be handy to know how much current the motor takes. The 2N2222, according to the attached datasheet, has a maximum continuous collector current of 800 mA, and a total maximum device dissipation at 25 °C of 500 mW. Knowing the collector current and the voltage drop across the transistor we can calculate how much power is being dissipated. This a primary reason why people have switched to N-channel MOSFETs for this purpose.

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#### FelixB

Joined Dec 26, 2022
31
it would be handy to know how much current the motor takes
The pump/motor runs on 4.5V to 6V and draws 50mA to 200mA.
So if I use a 100Ω resistor in combination with the 3V power source it would work?

#### crutschow

Joined Mar 14, 2008
32,932
You cannot under any circumstances get this value closer to zero than say 0.2-0.3 Volts depending on the particular device.
The TS states the maximum motor current is 200mA, so a 0.3V drop would give only 60mW dissipation in the transistor, which would cause only a small increase in its temperature above ambient.

Joined Jul 18, 2013
27,659
2N700 spec.

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#### FelixB

Joined Dec 26, 2022
31
I just testet it with a 100Ω resistor and it works. Thanks everyone. Later I will definitly checkout the 2n700, but for now I'm happy with my testing.

#### Papabravo

Joined Feb 24, 2006
20,400
The TS states the maximum motor current is 200mA, so a 0.3V drop would give only 60mW dissipation in the transistor, which would cause only a small increase in its temperature above ambient.
The real problem was that he was nowhere near this value because the transistor was not in saturation, so the Vce drop was considerably larger, I missed the specification of the motor current in the original post. Sorry.

#### Ian0

Joined Aug 7, 2020
8,419
I just testet it with a 100Ω resistor and it works. Thanks everyone. Later I will definitly checkout the 2n700, but for now I'm happy with my testing.
Did you remember to add the diode across the motor? If not, it won't work for long.

#### MrChips

Joined Oct 2, 2009
29,251
1) You don't need the 3V supply. Connect the base resistor directly to 6V.

2) Why do you need the transistor in the first place? Just connect the motor to the 6V supply.

Joined Jul 18, 2013
27,659
Using the 2n7000, you would only need the 6v supply.

#### WBahn

Joined Mar 31, 2012
29,175
MOD NOTE: Moved to General Electronics Chat.

#### BobTPH

Joined Jun 5, 2013
7,561
Using the 2n7000, you would only need the 6v supply.
Neither does the 2N2222.

#### sghioto

Joined Dec 31, 2017
4,920
You don't need the 3V supply. Connect the base resistor directly to 6V.
I believe the TS is going to control the motor with a 3 volt output of a micro from what I derived in post #1.

#### MrChips

Joined Oct 2, 2009
29,251
I believe the TS is going to control the motor with a 3 volt output of a micro from what I derived in post #1.
One would assume that to be the case but TS did not state that in post #1.

#### sghioto

Joined Dec 31, 2017
4,920
Not directly.
He stated replacing a L298 motor driver.
Since they are controlled with digital logic or PWM it would be easy to assume that is the application.

#### Ian0

Joined Aug 7, 2020
8,419
I believe the TS is going to control the motor with a 3 volt output of a micro from what I derived in post #1.
If so, the micro is going to struggle to provide the 20mA drive. I would therefore suggest (in the absence of a suitable low-threshold FET) an emitter follower to drive the output transistor - with its collector connected to 3V, rather than a darlington which would have a higher voltage drop.