The LM723 datasheet has some great examples of adding pass
transistors to linear voltage regulators for greater output
current.? I use an LM723 based linear power supply and while it's
inefficient compared to switchers it's quite easy to get clean
power out of them.? Linear or foldback current limiting is also
easily available with the 723.
On 10/19/2017 03:44 PM, Dexter N Muir
wrote:
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For more current
output, try this:
Feed the LM regulator via 1 ohm. Feed emitter of 2N2955 (PNP!)
also. Take PNP Base to other side of the 1E resistor (regulator
input) and PNP Emitter to regulator output.
Now, as load increases the reg. develops voltage across the 1E
resistor, and at around 0.6V (0.6A through the regulator), PNP
turns on. By the time the regulator is taking 0.7A it is
developing 0.7V across the 1E resistor (which is dissipating I^2R
= 0.49W), and PNP is turned fully on and well and truly taking the
rest of the load!
2 components: 2N2955 (and heatsink of course - water-cooled,
anyone?) and 1E 1W resistor.
Drop-out voltage = that of the regulator + Vbe(PNP).
The 2N2955 is rated to 10A, and you can add many of them by adding
a (1A rated) diode in series with the regulator and 1E0
(compensates for the Vbe of the 2955/s), and 0E1 resistors in the
emitters. The 2955s are then sharing the load to 10 * Ireg each,
well within rating, and all protected by the regulator.
Boost the Vout? add 1k regulator out to 'ground' (lifted) and
resistors / pot to suit then to ground.
This works for *any* LM regulator, just adjust the 'grounding'
resistance.
From the datasheet, LM317 is 1.2V out so, given 1k across that,
the grounding resistor will be about (desired Vout - 1.2) / (1.2 /
1k).
So, e.g., given regulator LM7812 and 1k across its output, wanting
13.8V out, grounding resistance will be about (13.8-12) / (12 /
1k) = 150E.
Have fun, folks!
73 de Dex ZL2DEX
