# Resistance calculation for transistor base

I read this question's answer on calculating the base resistor value for a 2N3904 transistor. I'm in a similar situation however I'm using a TIP3055 to switch a $V_{CE}$ of 12V and a $I_{CE}$ of 250mA, with a base voltage of 5V.

I'm having trouble following the above answers calculations mainly because the datasheet for the TIP3055 doesn't have any values listed for $I_{CE}$ currents of 1A or less.

Can I just use the smallest values in that datasheet to calculate the resistor's value, or is there some other way to calculate the base resistor value when the listed datasheet values are way out of range?

EDIT: I just followed this post that suggested working it out by taking the base current as one tenth the value of the collector current. By following this I get the following values:

$$I_{CE} = 250\text{ mA}$$ $$V_{CE} = 12\text{ V}$$ $$I_{BE} = 250 / 10 = 25\text{ mA}$$ $$R_b = \frac{5V - V_{BE}}{I_{BE}} \\ = \frac{5 - 1.8}{0.025} \\ = \frac{3.2}{0.025} \\ = 128\text{ Ohm}$$

128 Ohms seems like a very small value. I could try it but I only have one TIP3055 on me at the moment and would rather not damage it.

• Plugging the numbers into this transistor base resistor calculator gave a value of 94 Ω, not far off from what you calculated. – tcrosley Oct 16 '14 at 9:08
• @tcrosley I swear I went through all 40000 pages of Google looking for one of those... thanks! – jduncanator Oct 16 '14 at 10:08