Transistor base current

Hi,

what amount of current should a NPN transistor (low power) have going into its base when the emitter is grounded and the collector is used to switch a relay or somthing.


Ive always used a 1k base resistor driven from a 5V PIC micro output, which is
I = V/R
I = (5Vpic - 0.6vbe)/1000R
= 4.4mA

but was wondering if this is too much current?

The transistors are usually BC337 (500mA collector) or BC547 (100mA collector) depending upon what im switching.

kybert said:

Hi,

what amount of current should a NPN transistor (low power) have going into its base when the emitter is grounded and the collector is used to switch a relay or somthing.


Ive always used a 1k base resistor driven from a 5V PIC micro output, which is
I = V/R
I = (5Vpic - 0.6vbe)/1000R
= 4.4mA

but was wondering if this is too much current?

The transistors are usually BC337 (500mA collector) or BC547 (100mA collector) depending upon what im switching.

Click to expand...

hi kybert,
Look at the transistor datasheet for the Hfe values for the transistor collector current.

eg: if Hfe is say 50 at 100ma, then 4.4mA * 50 = 220mA, so the transistor should saturate.
Its important point is that when switching high current is that the switching transistor is driven into saturation.

How do i tell if its saturated?

Is it when hfe * Ib is greater than the current that is to be switched (Ic) ?

If so, then Ic / hfe gives me the base current, correct?

kybert said:

How do i tell if its saturated?

Is it when hfe * Ib is greater than the current that is to be switched (Ic) ?

If so, then Ic / hfe gives me the base current, correct?

Click to expand...

hi,
A visual way to check for collector saturation would be to measure the Vce voltage while the transistor is conducting.
Again, Vce sat, is given in the datasheet, for a regular transistor it will be the order of 0.1V thru 0.2V for the collector currents you are using.

If you use Ic/Hfe you will most likely not drive the transistor hard enough to fully saturate. I would have at least 2 to 3 times the calculated base current in order to switch the transistor 'fast' into saturation.

Does this explain it OK?

OK, so i need to ensure the base has 2x to 3x Ic/hfe as a rule of thumb, and i can check that the device is saturated by measuring Vce, and comparing it with Vce sat in the datasheet.

Thanks you your help.

Im going to do a few simulations on LTspice (the free sim) to prove to myself what we have discused.

hi,
Had a look on the web, this maybe helpful.

http://www.kpsec.freeuk.com/trancirc.htm

Low power transistors have their max saturation voltage spec'd with a base current that is 1/10th the collector current, even if the current gain is very high.

A 2N3055 power transistor has a minimum current gain of 5 when it has a 10A collector current. But its max saturation voltage is a lousy 3V when it has a massive 3.3A base current and a 10A collector current.