Hi all,

I am using a 2n2222 transistor for switching a 5V 185mA 27ohms relay. I calculated that the base resistor should be 270ohms. Why am I getting approximately 3.2V at the relay terminal and not a value close to 5V? 3.2V are not enough for switching my relay.
Also, the 2n2222 transistor is getting quite hot.

Any help or suggestion would be highly appreciated.

 

Got schematic?

 

What is the base current or voltage ahead of the base 270ohm.

 

With the transistor saturated, you should have a voltage at the collector of ~0.3V, meaning that the relay should get 5V-0.3V= 4.7V.

An logic-level NFET would be a better choice for a relay driver.

 

Hi all,

I am using a 2n2222 transistor for switching a 5V 185mA 27ohms relay. I calculated that the base resistor should be 270ohms. Why am I getting approximately 3.2V at the relay terminal and not a value close to 5V? 3.2V are not enough for switching my relay.
Also, the 2n2222 transistor is getting quite hot.

Is the transistor connected as an emitter follower or a saturated switch?

ak

 

Got schematic?

Hi, the 2n2222 have a base resistor of 270ohms which is connected to a micro. The collector is connected to the relay and the emitter is connected to ground.

What is the base current or voltage ahead of the base 270ohm.

5V from a microcontroller.

 

5V from a microcontroller.

(5-.6)/270=.0163 A=16.3 mA

Assuming Vcc=5 volts.
(5-.4)/27=0.1704 A=170.4 mA

Ok, looks about right, Ib*10 is about equal to Ic.

 

Except the microprocessor cannot source 16 mA. It can probably only put the transistor into the linear range and that is why it is not turned on. What voltage is the processor putting out ahead of the 270 ohm base resistor?

 

Thanks for the replies. The PIC18f I/O can supply up to 25mA.

 

Note that when I changed the transistors to TIP122, the circuit worked (using same base resistor etc.). The problem is that I have a PCB design with the 2N2222 package so i cannot fit a TIP122.

 

Thanks for the replies. The PIC18f I/O can supply up to 25mA.

But not at 5V. When sourcing 25mA, the PIC pin wont even make it to 4V. Look at the data sheet.

 

TIP122 is an NPN darlington transistor. There are some darlington NPN in TO92 foot print eg. BC517. Just do a google search and see which one is easily available to you.

Did you try MOSFET? I think that should be the right choice for your job.

Allen

 

Thanks for the replies. The PIC18f I/O can supply up to 25mA.

Guess again. While it may be rated to put out such a current without damage it does not mean the output will stay at 5V when doing so.

Measure the PIC pin when doing this. Actually drawing the schematic and noting the voltage at every node is the best way to go.

 

Hi,



5V from a microcontroller.

Measured?

Then 2222 mis-connected or bad.

 

Thanks for the replies. The PIC18f I/O can supply up to 25mA.

It is easier for a pin to sink current than to source current. Are you sure it can source the required current and maintain the output at or near Vcc?

 

It is easier for a pin to sink current than to source current. Are you sure it can source the required current and maintain the output at or near Vcc?

We don't actually know what Vcc is. OP never said what it is.

 

Hi,



5V from a microcontroller.

..................................................................................................................

 

Thanks for the replies. The PIC18f I/O can supply up to 25mA.

An logic-level NFET would be a better choice for a relay driver.

+1
After all the previous posts here on the subject, I am surprised why some still opt for a bi-polar for Pic or μp outputs?
Max.

 

After all the previous posts here on the subject, I am surprised why some still opt for a bi-polar for Pic or μp outputs?
Max.

My guess, based on my own education, is that we just follow our education. My curriculum spent a lot of time on BJT in lectures, homework and lab experiments. I had some instruction in FETs, but not anywhere as much as BJT and I don't remember doing any lab experiments with FETs.

And inexperienced people like me don't really know the pitfalls of uC. We read on paper that it will do: "5 volts and 25 mA" and we take it for complete truth. Many times I look at datasheets and I know that I don't know what most of the stuff in the tables actually mean.

 

My guess, based on my own education, is that we just follow our education..

It probably influenced mine .
Having gone through the tube era, which is very close in concept to the FET I sort of had an affinity to it and the transconductance type of principle of a Mosfet.
Max.