i have bd 139 transistor whose h[fe] is given as min-40 and max-250. I need to calculate values to drive dc motor. Which value should i take into consideration minimum or maximum.
Is h[fe] and current gain (beta) same thing?
Further i read somewhere that the gain of transistor cannot be greater than 100 i.e. the collector current can be maximum 100 mA when the base current is 1 mA. Is that true? how is it not true in case of bd139?
Me neither. In the pdf file that you linked is not enough information.
Try another datasheet.
Click on the blue button "Download Datasheet", Intelligent Power and Sensing Technologies | onsemi
In Figure 1, you see the current gain at a certain collector current.
For example at 500mA, the gain is 80.
Neither is appropriate - you need to know what base current causes saturation (for a particular collector
current). For many old transistors like this the base current for saturation is 10% of collector current
or more even.