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boosting current in a lm317t with a darlington transistor

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20v down to 5v was just an example, but you've given some very good info here. Is it possible to create a variable voltage switched power supply?
 
First, the power transistor has ~ the same voltage across it as the 317, except that it has 4A flowing through it, so (20-5)*4 = 60W.

If your heat sink's thermal resistance is 3.5 degC/W, then with 75W, the temperature of the heat sink would be 25degC (ambient temperature) + 3.5*75 = 25 + 262degC = 287degC, which is way, way too hot.

The max temperature you want at the heatsink is ~85degC (still hot enough to burn your hand). Starting from a 25degC ambient, that would mean that the temperature rise of your sink above ambient would have to be 85-25 = 60degC. Since you are dissipating 75W, the thermal resistance of the sink would be 60degC/75W or 0.8degC/W, which you will find will require like 4000cm². Are you convinced that this is dead end, yet?

This is not the way you should be doing this! To get 5V * 5A = 25W to the load, you are dissipating 60+15=75W in the regulator. This is where you should be using a buck switch-mode regulator. Alternatively, start with a different transformer so that the unregulated voltage is much closer to 5V.

I find it pretty amusing how many diagrams are posted on the net, saying 35->1.2v 3A (or whatever) variable voltage power supply , without them actually knowing how much frigging heat you need to dissipate if you actually supply like 1.2v.

You're loosing WATTS / INPUT_VOLTAGE amps on the input side right? 60w / 24v is like 2A you are just literally pumping into thin air?
 
This is very strange.This is the 2de complain. looks like i can only open it while i'm log on to the forum. I'm still new to this forum so how do i report it. Do i just send a PM to the moderator?
 
This is very strange.This is the 2de complain. looks like i can only open it while i'm log on to the forum. I'm still new to this forum so how do i report it. Do i just send a PM to the moderator?

it says my side to pm the mod if it doesn't work, so yea :)
 
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