Transistor as a switch help

I am working on a project that consists of two separate circuits. On one side is a DC fan powered by a 9V battery. On the other side is an igniter that is powered by 1.5v.

What I need to happen is for the fan to run only when the igniter is receiving 1.5v.

After some research it appears a Transistor may be the best way to do this. However I'm still unsure what type of Transistor would be best and how the circuit would be wired.

Thanks in advance for your help.

You can use NPN transistor working as a switch and driven by 1.5V from the igniter ..
See attached picture ..
The type of transistor will depend on the current required to drive the fan ..
Connect the fan to a 9V battery and measure the current ..
Then, select a transistor that has I[c] at leaset twice as that current ..
For example, the 2N3904 can conduct up to max200mA, 2N2222 - 800mA, etc. etc...

IanP
:wink:

IanP thanks for the help! The fan draws 380mA so I'd likely need to go with the 2n2222. Figures, I had a 2n3904 already soldered into place

Can you explain the purpose of the 10K resistor that is inline to the base is? I was playing with the Circuit Simulator and it appears to work without this, what is the need for resisting the voltage into the base?

You can leave without 100k resistor that pulls the base down to GND when there is no input signal but you can’t get away without the series 10k resistor between the signal and the base ..
This resistor limits the current that otherwise would flow through the BE junction and if not limited may damage it ..
Keep in mind that the U[be] junction voltage is roughly 0.65V so the 10k resistor will drop 1.5V igniter voltage down to required 0.6-0.7V level ..

IanP
:wink:

cfortner said:

IanP thanks for the help! The fan draws 380mA so I'd likely need to go with the 2n2222. Figures, I had a 2n3904 already soldered into place

Click to expand...

The problem is that you will not be able to get a collector current of 380mA because the base current is too low.
The base current is (1.5v-0.7v)/10k=80µA.

You need a higher base current or a darlington transistor which has high current gain.

Alex

Alex's remark is really very important as you need roughly 4mA base current (assuming transistor's hfe>100) ..
For that the base resistor should be somewhere around (1.5-0.7)/4mA=>200ohms ..

IanP
:wink:

If you are going to use 2N2222 then the minimum current gain for 500ma is 40 according to the datasheet
so it would be better to go with a higher current of about 10mA to be sure that it will work even with the minimum current gain.
Another solution would be to use a transistor with higher gain, for example 2SD882 ( https://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00062867.pdf )
has a current gain of about 150 for 500mA

An alternative solution would be to use a N-Mosfet but with a very low threshold voltage , like https://www.fairchildsemi.com/ds/FD/FDN327N.pdf , the max threshold voltage is 1.5v (typical 0.7v) but you would be able to get 400mA current with a lower voltage too.
The mosfet will not consume current from the 1.5v source once it is turned on (it draws 3ma for a short amount of time while switching on).

An example of the mosfet connection would be


Alex

Thanks again guys for the help! While playing around with the values on the circuit simulator, I noticed that have a 10K resistor for R1 gave a collector current of 6mA. It looks like R1 should be ~150ohm which gives ~350mA.

Thread: Transistor as a switch help

This thread was closed today but the answers to it are WRONG!
The transistor that drives the 380mA fan must saturate. The datasheet for nearly every transistor says that the base current must be 1/10th of the collector current for the transistor to saturate well. hFE should not even be talked about when you want a transistor to saturate.

The hFE for a 2N2222 is listed when it is not saturated because its collector to emitter voltage is 10V and then it is no where near being saturated. The max saturation voltage when the collector current is 500mA is shown as 50mA.

The 2N2222 transistor recommended might not work. Its max saturation voltage will be 1.0V when its collector current is 500mA and you are blasting 50mA into its base so the igniter will get a very low voltage and probably won't work.

The batteries have a high current so they will not last long. The resistor supplying base current to the transistor should be calculated when the "1.5V" igniter battery has dropped to 1.0V.

Sorry for my two cents.

Thread unlocked and answer merged.

Keith.

sherazi said:

for those who come across this thread by search :
Transistor as a Switch

Click to expand...

That article is also WRONG!
It calculates the base resistor using the transistor's hFE. But the hFE is used ONLY when the transistor is a linear amplifier with plenty of collector to emitter voltage, not when it is used as a saturated switch.

I repeat:
The hFE of a 2N2222 is when its collector to emitter voltage is 10V so it is not saturated.
Its base current is shown to be 1/10th the collector current when it is a saturated switch.
hFE has nothing to do with a saturated transistor.

For something as simple as a switch, wouldn't it be easier to take the MOSFET approach?

What alexan_e had posted is what I would have recommended.