On Mon, 14 Jul 2008 22:20:21 -0400, Dave M
wrote:

On Mon, 14 Jul 2008 19:44:02 -0400, Fred McKenzie wrote:

In article ,
cliff wright wrote:

If a search tells you that they are Darlington devices get
rid of them right off!!!
Darlingtons have a minimum collector/emitter voltage of ~1.5 volts or
more so they will get very hot indeed at 30 amps or so.

Cliff-

Are you sure that applies to Astron-type power supplies? The ones I'm
familiar with do not operate with the pass transistors near saturation.

Are you sure about that ? Have you looked at the capacitor voltage
with an oscilloscope with full nominal current and minimum mains
voltage ? The goal should be that the capacitor voltage just before
the rectifier starts to conduct is only slightly above the output
voltage.


Another point that I would like to mention is that of a transistor's SOA or Safe
Operating Area. There are two graphs in the datasheet for the transistors. The
SOA graph shows the safe combinations of collector current and collector-emitter
voltage AT A CASE TEMPERATURE OF 25 DEG C. The power derating graph shows how
much power the transistor can safely dissipate at a given temperature.

The SOA is important especially in the short circuit current limiting
case, when both the current and voltage across the transistor is high.
A fold-back current limiting solves this problem.

The SOA derating usually starts above 5 V, so in a properly designed
power supply the input-output voltage difference should be less than
that, so in normal operation, the SOA is not very critical.

Paul OH3LWR