supply voltage=6v relay is 6v 100ohms transistor=2n3019 Ic=1A max hfe=100 min Vbe=1.1 my question. is this the correct way to work out the base resistor. load=6/100=0.06A=60ma Ib=Ic/hfe=60/100=0.6ma Rb=Vin-Vbe/Ib=6-1.1/.6=8.1K question 2. if i wanted to use two seperate voltages a smaller voltage to the base.and a larger voltage to the relay .how would i work out the base resistor for this setup.THANKS FOR ANY HELP.
You want the transistor to saturate so you must look at the max saturation voltage on the datasheet. It is spec'd when the base current is 1/10th the collector current. A few transistors in a bucket full of transistors can use less base current.
The hFE is spec'd when the transistor is a linear amplifier with 10V from its collector to its emitter so it is far from being saturated.
You must also look at its base voltage at a collector current of 60mA, not when its collector current is 150mA. The max base voltage is probably 1.0V when the transistor is saturated with a collector current of 60mA and a base current of 6mA. So the base resistor is (6V - 1V)/6mA= 833 ohms. Use 820 ohms.
If a smaller voltage is used to drive the base resistor, like 2V then the calculation is the same except substitute 2V instad of 6V: R= (2V - 1V)/6mA= 167 ohms. Use 160 ohms.
thanks audio for that info.i found this on line that to find the base current needed to saturate the transistor use Ib=Ic/hfe and to find base resistor use Rb=Vin-Vbe/Ib.but this doesent make sense like audio pointed out the Ic for the 2n3019 is based on 150ma and Ib=15ma.same with the 2n3904 is based on Ic=50ma with Ib=5ma .so is this formula no use ive seen this on a few sites.
Most internet idiots who know nothing about electronics use hFE for everything. hFE is DC current gain. hfe is AC current gain. Both are when the transistor IS NOT saturated.
transistor base resistor help
Posted - Mar 07 2009 : 10:00:16 AM
