Why does this only flow 700ma?

[joecool85]

I was looking at a transistor based fan controller here: https://electronics-diy.com/electronic_schematic.php?id=1036

and it says that it is good for 12v 700ma max. Why? The driving transistor, Q2, is rated for 1.5A, so shouldn't it be good for 1.5A?

**broken link removed**

Q1 BC547 45V 100mA NPN Transistor
Q2 BD140 80V 1.5A PNP Transistor

 

[ronsimpson]

It is not a good idea to run parts at the maximum!
The gain of Q2 is low at 1.5A.

 

[unclejed613]

ABSOLUTE MAXIMUM means exactly that. a 50% safety margin improves the reliability.

 

[audioguru]

The datasheet for the old BD140 shows that with a collector current of 0.5A then the base current must be at least 50mA for it to have a maximum saturation voltage loss of 0.5V.
But R4 in the circuit limits its base current to only 40mA. Then some BD140 transistors will have a max output of only about 400mA and others will be destroyed with a max current higher than 1.5A.

The BD140 is in a very old case that is difficult to cool. So it might melt if its current is too high.

 

[joecool85]

The datasheet for the old BD140 shows that with a collector current of 0.5A then the base current must be at least 50mA for it to have a maximum saturation voltage loss of 0.5V.
But R4 in the circuit limits its base current to only 40mA. Then some BD140 transistors will have a max output of only about 400mA and others will be destroyed with a max current higher than 1.5A.

The BD140 is in a very old case that is difficult to cool. So it might melt if its current is too high.

Good to know and makes more sense now. Also makes sense that people we rating it lower to increase reliability.