All,
Here's what REALLY helps lower the dissipation of a 7805 - cut down on the
head room. If you feed it 12V, that's a head room of 7V. The chip winds up
dissipating E*I, or the head room (7V ) times whatever current is going through it.
The 7805 only needs 3 volts of headroom. That is, it can operate properly off a power
supply of 8V. 12V can be handily dropped to 8V with a series string of forward-biased
rectifier diodes. Six or Seven diodes should work fine. A forward biased diode acts like a voltage regulator, so 7 of them produce a reasonably stable voltage drop that doesn't change too much with current.
A 7805 dropping 3 volts suffers less than half the dissipation of one dropping 7V. Of
course, the diodes make heat too, but it spreads the pain.
- Jerry KF6VB
toggle quoted message
Show quoted text
