Simple NPN transistor issue. Please help.

[jwilson]

I am new on here, searched google for an electronic design forum and found this one. I will probably ask a few questions here and there, but mainly will read and learn. I will try to give my advice where it is needed.

I have a simple NPN transistor that I am using as a switch. The original design had a BC857C which drove a 50mA load. The load has now changed to 250mA, therefore I changed the transistor to something more powerful. I chose a MMBT3702. The original design worked fine. The load is another circuit, I cannot see the internals of this load, therefor I take it as a black box for now. The load is a driver for a laser diode module.

On to my problem...

I have the transistor connected as follows:

Base - connected to microcontroller output pin through a 1K resistor. Also have a 100K pullup to 5V

Emitter - connected to 5V

Collector - connected to my load

With no load connected I get 5V on the collector when the transistor is fully saturated. I measure practically 0V for Vce with the transistor saturated and I measure 0.7V for Vbe.

With the load connected I still have 0.7V for Vbe, but I have a voltage for Vce. I now have a 2.3V drop across the collector-emitter of this transistor.

Can someone shed some light for me? I am not the best at analog design. Hope someone on here can help me out. Like I said the original design worked with the smaller load and the BC857C.

Thanks

-John

 

[Audioguru]

A transistor works with currents, not voltages. You did not say what is your load current.

With a series base resistor of 1k then the base current from the microcontroller is about 4mA so the load current should not be more than only 40mA.

 

[FvM]

Using a small PMOSFET, e.g. Si2301/TSM2301 with about 0.1 ohm Rdson would give effectively no voltage drop respectively higher current handling.

 

[dick_freebird]

If you want to saturate the device (|Vce| < 0.5V) then you
probably need to be at a forced beta of 10 or less. At 250mA
that means you want 25mA or mor out of the uC and through
the limiting resistor. That's not going to play well, I bet. Either
go with a logic level PMOS or go Darlington (if you need the
same logical sense as now) or a NPN first stage and use the
NPN collector current to light the base of the PNP with maybe
a 100 ohm series resistor there to establish ~35mA base
current.

 

[Audioguru]

A PIC micro-controller has a max allowed output current of 25mA. when it has a resistor in series with a transistor's base then 150ohms is 27mA.

 

[jwilson]

Thanks for the advice guys. I am going to try a P-channel MOSFET and see if that will help things.

I am stuck to using a single sot23 package component.

 

[FvM]

The suggested type is SOT23 package.

 

[jwilson]

Yeah I seen that after I posted. Thanks

 

[nandhu015]

Emitter - connected to 5V

I think Emitter should be connected to Ground

Nandhu

 

[jwilson]

Emitter - connected to 5V

I think Emitter should be connected to Ground

Nandhu

Not in this application. Its a PNP, not an NPN...

 

[Audioguru]

The title of this thread is wrong.