B.J.T transistor

[ronv]

That is not the rule of thumb. The rule of thumb is to divide beta by 10, not the collector current.

I like the theory, but don't think it's right. Using it would give his transistor a gain of 1.5 at 10 amps. I know it is not the hottest transistor in town but it is not that bad. The data sheet states a gain of 5 minimum at 10 amps.
If you look at the 2N2222 which has a little better spec sheet the vbe Sat is speced at Ic=10XIb. And it only has an Hfe of 50 at 10ma.

But to Amir1's concern: You choose the base current for the maximum collector current tou expect to see. In your case 8 amps. If it is saturated at 8 amps it will be saturated at 2 amps.
Mike is right, this transistor is not the best for your use. It will waste a lot of power. You could use a P channel fet for the high side and protect the gate voltage from being to high with a zener. This would also simplify your driver. Last, don't forget clamp diodes for the motors inductive kick.

 

[Mike odom]

the equation Ic=B*Ib does it work in saturation mode ?? as part of my knowledge it only works in active mode am i correct?

correct. once you hit saturation, by it's very definition, you can't go farther than that.

 

[Mike odom]

I like the theory, but don't think it's right. Using it would give his transistor a gain of 1.5 at 10 amps. I know it is not the hottest transistor in town but it is not that bad. The data sheet states a gain of 5 minimum at 10 amps.
If you look at the 2N2222 which has a little better spec sheet the vbe Sat is speced at Ic=10XIb. And it only has an Hfe of 50 at 10ma.

yes,the transistor has different betas at different Ic, so this is why you divide the beta by 10, not the Ic. with a gain of 50, your rule would only pump 1mA into the base, and not guarantee saturation. My rule would pump 2mA into the base to guarantee it saturated.

It may not be your rule, but it's not my fault you weren't taught correctly. It's the way I was taught in the 70s.... particularly since different transistors have different gains at different Vce and Ic.... you up the gain factor by 10 to ensure saturation. Your method would be hit and miss, might work for most combinations of Vce and Ic, but not all.

Also, the correct beta for 10mA = 75 minimum (2n2222 or 2n2222a).
oh, by the way, we're not looking for Vbe saturation... we're looking for Vce saturation... Vce x Ic = power dissipated in the device, and we are looking to make sure Vce is minimum for a specific Ic to net lowest power consumption.

View attachment 69103

 

[ronv]

So just to make sure we are on the same page:
Using Amirs datasheet -
The curve shows a beta of 15 at 10 amps so your method would suggest to get saturation we would need 10 amps divided by 1.5 or 6.6 amps of base drive to get saturation at 10 amps of collector current? And likewise at 4 amps of collector current the gain would be 2-- 20hfe/10 or 2 amps, even though the data sheet specs Vce sat at 3.3 and 0.4 amps?
Your rule wastes a lot of power and mine would let this transistor come a bit out of saturation at 10 amps.

 

[Mike odom]

So just to make sure we are on the same page:
Using Amirs datasheet -
Your rule wastes a lot of power and mine would let this transistor come a bit out of saturation at 10 amps.

If the transistor come out of saturation at 10A, that's where the power would be wasted.