[R-390] OT: Pass Transistor Question

[2002tii]

Barry wrote:

>I have a transformer with a 6-amp capacity and I would like the 
>power supply to deliver up to 5 (maybe all 6 amps) if I want.  Is it 
>possible to put another 2N3055 in parallel with the one in the 
>circuit allowing thus allowing the pass transistors to deliver the 
>extra amperage.  Will this work?
>
>The two 2N3055s are paralled and have 0.1-ohm resistors in the 
>emitter lines.  Not sure what these equalizing resistors do, but 
>perhaps that's all I need to add?

Andy said what the resistors are for.  I would typically use more 
than 0.1 ohm for 2N3055s, say 0.22 or 0.33 ohms.

One other consideration -- voltage regulators need a certain amount 
of "overhead" -- the unregulated input voltage needs to be greater 
than the regulated output voltage by a certain amount for the 
regulator to work properly.  This is typically on the order of 2-3 
volts for this type of regulator.  When you start drawing 5 or 6 
amps, the unregulated supply will sag.  You have to be sure it 
remains at least 2-3 volts above the regulated output voltage at the 
highest output current, or the supply will drop out of 
regulation.  Note that there may be a fair amount of AC ripple on the 
unregulated supply (generally a 120 cps sawtooth wave) when you are 
drawing 5-6 amps.  You need the minimum input voltage (not just the 
average input voltage) to be 2-3 volts higher than the regulated 
output voltage.  A dual-trace 'scope synchronized to the AC mains 
supply is very handy to check this.

Also, the pass transistors will dissipate heat according to the 
formula P = EI.  In this case, E is the voltage across the output 
transistors and I is the output current.  With an overhead of 3 volts 
(all but about 1 V of which will be across the transistors -- the 
remaining 1 V or so will be across the equalizing resistors), your 
pass transistors will be dissipating around 10 watts at 5 amps.  You 
will need a non-trivial amount of heat sink to keep them cool.  As 
you can see, the more overhead your power supply has (good for 
regulation), the more power (heat) the pass transistors will 
dissipate (bad for thermal design and potentially for 
reliability).  Welcome to the world of design tradeoffs.

Best regards,

Don