Transistor Switch or Relay?

[wordizlife]

Hey Guys,

I am designing a circuit that will require a PIC16F877A to comunicate with another pic and the pc through serial communication. I want to be able to control a certain portion of the circuit and be able to cut power to it. I am currently using a 2n3904 as a switch, the base is connected to my microcontroller pin through a 10k resistor and there is a pull down resistor of 100k also. All the negative(ground) connections of that portion of the circuit are connected to the collector of the 2N3904 and the emitter to the circuit ground.

For some reason the transistor is not working, it says it can handle up to 200 ma of current and my circuit is putting out less then 150 ma. Should I be using a relay for this and controlling this relay with the transistor switch?



Thanks

this is my code to initialise the port and set it high.

[SherpaDoug]

For hardware, using a 2N3904 with a 10k base resistor is limiting the current you can deliver. A 1k resistor may improve things a lot. You should be able to go as low as 220 Ohm is you need to. Beware that the 2N3904 may get warm if it is handling high currents for a long time.

My choice for this application would likely be a MOSFET like a 2N7002. It costs a few pennies more but will switch on more completely. A relay may also work but it will likely require a driver transistor between the PIC and the relay.

I am surprised your software is working at all. First you don't need to worry about the TRIS register. The CCS compiler will handle that automatically unless you tell it not to.

The statement a5 = 1: will not set pin A5 high. You should use the BIT_SET() and BIT_CLEAR() functions.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[Mike Walne]

[wordizlife]

I reduced both resistors by as close to a factor of ten as possible and still no results and yes 5V appears at the output. This is weird because I used this exact configuration not long ago on a similar project and the microcontroller triggered it perfectly fine...
I am not sure how to check my guaranteed Hfe for my device..?

You can check out a picture of the schematic for the circuit, I posted it in another forum since I cannot attach any files here.
http://forum.allaboutcircuits.com/showthread.php?t=68932

[jeremiah]

Most of us don't want to sign up on another forum just to see a picture. Can you not upload it to a image host, like ImageShack and post the link?

[wordizlife]

here is a link the image on imageshack:
http://imageshack.us/photo/my-images/214/schematics.png/

[Mike Walne]

[Ttelmah]

[Mike Walne]

OK. So I've just checked Fairchild's version of the 2N3904.

Yes maximum current IS quoted as 200mA.

But USEFULL switching current is 100mA!

At 100mA collector current, and 1V Vce, MINIMUM Hfe is only 30!.

At your 150mA load current the gain is falling out of the sky.

It's just possible that you stuffed your device at an early stage when you were not providing enough base current.

Also you've told us very little about the load you're switching.

What is the voltage supplying the load?
Is the load inductive?
Is the load capacitive?
...........

Mike

[asmboy]

have you considered using a P-channel power MOSFET as a high side switch and controlling the gate directly from your pic?

if it is 5V you are switching and the PIC is operating at 5v

it is very EZ and really no resistors are required unless you put a safety pull up resistor of about 20K ohms from the gate to +5v prevent a startup
glitch.
simply connect P-fet source to +5v ,, drain to load side.

pull gate DOWN with pic to turn switch ON
very simple VERY hi current capable

[wordizlife]

The voltage supply is 5V at the resistive load.

I guess I will have no choice to go for a MOSFET. Would you possibly have any schematic examples that you could point out to me? I need to know how to use a mosfet as a switch with a microcontroller.

What P-channel power MOSFET would you recommend for my application? This will be my first time working with MOSFETS.

Here is a link to a clear schematic of what I was attempting to accomplish with the 2n3904.

http://imageshack.us/photo/my-images/801/screenshot20120417at706.png/

EDIT:

I just found this schematic, could i possibly use a 2N7002 N-CHannel mosfet with this circuit?
http://imageshack.us/photo/my-images/10/screenshot20120417at725.png/
What resistiance values would I have to use for Rin and Rgs?

[asmboy]

With a clear schematic you need a LOW side switch and it is just as EZ.


A 2n7000 is good for up to 200 ma, 150 ma steady state.

An MTP3055 is good for 10A.

Your example is fine too.
Use 20k to ground
and 100 ohms in series driving the gate from the pic.


The ground shunt resistor keeps it from glitching to the on state during pic power up.

[wordizlife]

Perfect! Thanks a lot for your help, I will get the components tomorrow and will hope for the best. I'm glad that I don't even have to change my schematics or my PCB at all since the circuit is practically the same.

[bkamen]

[languer]

In addition to the 2N7000 (2N7002), you could also try the SS8050. With a 100-ohm or 200-ohm in series with gate or base.