The 2W resistors R5, R7 and R9 will produce enough voltage to bias the pass transistors at low currents. But they exhibit some output stability issues which can be mitigated by adding capacitors at the output.
PNP transistors are used as pass elements in low dropout and ultra-low dropout regulators. But the reason I chose PNP ones is because they do not alter the output voltage as NPN transistors would do (output will be +0.7V higher when NPN is used). You could use TIP3055 or 2N3055 as pass element on the negative side or the output side. Here I've used TIP2955 power transistor as pass element on the positive side. But in this case, with large difference in input and output voltages, LM317 will have to dissipate the excess power as heat so much heat. LM317 can provide output current in excess of 1.5A. U2 will output a constant 12V, U3 will output a variable voltage and U1 will produce an auxiliary 12V for other 5V and 3.3 regulators so as to reduce the heat dissipated by them. Each of those LM317s will reduce the 36V input to programmed voltages.
There are three LM317s and three TIP2955 PNP pass transistors for each. You have to select proper packages if you want to create a PCB layout. So I didn't care about the component packages. But it was not designed to generate a PCB file as I usually perfboard for my one off designs. If you like this post probably you might like my next projects, so please support me by subscribing my blog and like my Facebook Page.The schematic will give you an insight on my plan. Thanks for reading, you can contact me by leaving a comment. And as you decrease voltage to the base of a NPN transistor, the transistor turns on less and less, until the voltage is so low, the transistor no longer conducts across emitter to collector, and shuts off.When you increase voltage to the base of a NPN transistor, the transistor is turned on more and more until it conducts fully from emitter to collector.
A NPN transistor conducts from emitter to collector.And as you decrease voltage to the base of a PNP transistor, the transistor turns on more and more, until the transistor fully conducts from collector to emitter.When you increase voltage to the base of a PNP transistor, the transistor is turned off more and more until it no longer conducts and completely shuts off.A PNP transistor conducts from collector to emitter.The difference between a PNP transistor and a NPN transistor is their polarity and their actions are reversed… PNP Transistor (you should see my project after reading this post) I’ve decided to write this post to support a bit of the theory behind my new project that identifies the PNP and NPN transistors easily.
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