BEAM/Wilf-20041218

To: <beam@yahoogroups.com>                                                      
Date: Sat, 18 Dec 2004 19:33:00 -0800                                           
From: "Wilf Rigter" <wrigter@...>                                               
Subject: Re: [beam] Re: Most efficient photovore?                               
                                                                                
                                                                                
Darlingtons are not efficient? There are two sides to the answer.               
                                                                                
For a motor driver the MPSA14 has some impressive specs, which is the           
reason it is used, for example, in the Chloroplast SE                           
                                                                                
The voltage drop of this 1A Darlington small package is quite tolerable         
if the supply voltage is 5V or more.                                            
                                                                                
The MPSA14 Vce drop is about 1V at 500mA which is less than 20% loss in         
efficiency if the supply is more than 5V and the base current is 1mA.           
                                                                                
With a 12V supply, the Vce loss is slightly more than 8%.                       
                                                                                
Compare that to a 2N2222 commonly used as a motor driver which at Ic=500m       
has a Vce drop of about 400mV (8% loss) and requires 50mA of base current       
(further 10% loss).
                                                                       
Motor current usually fluctuates and could range from 500mA at start up         
and stall to 100mA low load. At such lower load currents, the 50mA base         
current of a 2N2222 is very inefficient, while the Darlington Vce drops         
at lower current to a modest 700mV, increasing efficiency at lower              
current.                                                                        
                                                                                
For low voltage supply applications (3V-5V consider using a logic level         
power mosfet.                                                                   
                                                                                
Around 3V, a powersmart motor driver, using cheap transistors, works well.      
                                                                                
For supply voltage between 1V and 2V, the low saturation type transistors       
(e.g. ZETEX) really shine.                                                      
                                                                                
For supply voltage less than 600mV you could use a Ge transistor if you can     
find one.                                                                       
                                                                                
wilf