On Mon, 13 Sep 2004 12:36:04 GMT, "G.Beat"
wrote:

Paul and Eric and provided a good background on how to adopt the ARRL
design.
You can substitute the 2N3771 transistor for the common 2N3055 pass
transistor (this is what Astron has done on many of their latest models).

This suggestion makes sense for low voltage (5-12 V) high current
power supplies, since the worst case current gain (15) has been
specified for Ic=15 A and in general, the typical current gains are
better at very large currents than with 2N3055. Also the Vbe and
Vce(sat) voltages are lower at similar collector currents, thus, the
capacitor voltage can be slightly lower. From the current handling
point of view, the number of devices could be reduced. However, the
power dissipation would have to be checked, although the junction to
case thermal resistance is smaller, this does not help the heatsink
very much.

However, the ARRL 24 V supply uses only 2.5 A/transistor, so the
current gain and Vbe and Vce(sat) are not really an issue, but the
power dissipation issues are much worse on 24 V than on 12 V. The
power dissipation may still require that ten 2N3771 are used, which
are usually a bit more expensive than ten 2N2055 transistors :-).

Please note that the maximum Vce for 2N3771 is only 40 V and since the
capacitor voltage can exceed 40 V and in case of a short circuit, the
emitter is close to 0 V, the Vce could be exceeded. At least use the
2N3772 for the 24 V supply, since the maximum Vce is 60 V. The 2N3055
is also specified for Vce=60 V.

Paul OH3LWR