Wouldl like to make a benctop varible power supply

audioguru said:

Hi MR Cecil,
What is the current rating of your power supply?
What is its transformer voltage and current rating?

I notice that the power transistor Q1 has a very low max base current of only about 30mA so if its gain is low then the max output current will be less than 1A.
Doesn't R4 get extremely hot when the load current is high?

Click to expand...

Hi back at ya AudioGuru,

I posted the wrong schematic without checking properly; a lack of due diligence. I have edited my original post to Marks256 with the as-built. However, even it is not corrected as built. I added a 3PDT toggle sw. during assembly to change range and DP on the Vo DPM's (3.5 digit) for the 20-24V supply range. Onward to your questions...

The xformers are rated at 24V & 2.4A for the desired 2A output. R4 doesn't get too warm at all with only about 160mw worst case to dissipate (see as-built schematic). Its main function is to relieve Q2 of some of the power burden. If I had a variety of TO-92NL packaged numbers on hand R4 would have been eliminated altogether; trade offs are a fact of life. But I don't stock JIL spec semi's, and a $0.06 resistor and $0.04 xistor are cheaper than a $1.50-$2.00 Japanese xistor.

Regarding the DC current gain of the pass xistor, that would have been a major consideration in 1970 when process control, process tools, material and gas purity, Fab environment induced defects, wafer quality, etc. in virtually all Fabs regardless of manufacturer were far below today's standards. Personally, I haven't seen current gains in the 20's and 30's for the 2955 or its 3055 compliment for a very long time...perhaps I have just been fortunate though.

Merv

No prob. Thanks for the fix.