| T O P I C R E V I E W |
| cookster |
Posted - Mar 07 2009 : 10:00:16 AM
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. |
| 3 L A T E S T R E P L I E S (Newest First) |
| audioguru |
Posted - Mar 07 2009 : 8:31:13 PM
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. |
| cookster |
Posted - Mar 07 2009 : 4:46:59 PM
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. |
| audioguru |
Posted - Mar 07 2009 : 12:04:08 PM
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. |
