How can this supply work
How can this supply work
(OP)
Here's a schematic:
https://encrypted-tbn0.gstatic.com/images?q=tbn:AN...
It's a very popular power supply
But I wonder: how can a PNP transistor conduct if the voltage at the emitter is the same as the base
One could argue that the resistor between the capacitors cause a voltage drop on the base, but the same resistor can be found on the emitters.
The regulator only lowers the voltage on the output... So... Answer not here, I supoose
What am I missing
RE: How can this supply work
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
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: How can this supply work
A quick web search shows: https://www.eeweb.com/blog/circuit_projects/lm7812... which has a much higher valued input resistor, 100R, which is consistent with my initial thought of a 10 ohm resistor.
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RE: How can this supply work
Now it makes sense. Let's build it
✔
RE: How can this supply work
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm