How can this supply work 2

Someone else missed something. Not you.
The LM7812 isn't supposed to deliver more than 1 A. Maximum. So, the .1R going to it will not produce more than 100 millivolts. Which obviously can't even start to turn the 2N2995 on. If that resistor were 1R, it would produce something like 1 V, maximum. But still not enough to drive the bases hard enough to make Ic even close to 5 A. OK, if the .1R resistor in the emitters were removed. But then you get current hogging, so that would be a bad idea. I think that the resistor should be more like 2.2R or somewhere near.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

The obvious answer is that someone at mis transcribed the schematic. The resistor is more likely to be a 5 or 10 ohm. It needs to be high enough that the majority of the LM7812 input current is coming through the bases of the TIP2955s. You really don't want to be burning current through this resistor and making the LM7812 running at its max current just to burn it in the resistor.

A quick web search shows: which has a much higher valued input resistor, 100R, which is consistent with my initial thought of a 10 ohm resistor.



TTFN (ta ta for now)
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Thank you, guys

Now it makes sense. Let's build it
✔

Sadly, the bad schematic is almost as common as the good schematic on the internet.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list