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LM317AHVT and Heatsink

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mike_mike:

--- Quote from: mike_mike on March 03, 2019, 07:51:05 pm ---Hello,
I am working at a new project. The schematic is attached in this post. The project uses an LM317T and a PNP pass transistor (TIP36C). I marked on the schematic the pins for LM317 and TIP36. LM317 and TIP36 will be connected to the board using wires.
I made the layout but I am not sure if it is correct.
I read a few documents about layout guidelines and I tried to respect some rules in this layout: I divided the schematic into low current and high current zones, I added copper planes for GND, I put the unpolarised capacitors near LM358, I put the bridge rectifier and 0.1R resistor in the lower part of the layout - because of the heat generated, I tried to make the traces as short as I could.

The DRC does not return any error, but I want to know if the layout guidelines are respected ?
If the layout guidelines are not ok, could you help me to correct the problems ?

--- End quote ---
I am attaching updated version of the layout.

MrAl:
Hello,

33v at 0.7 amps is more than 23 watts which is a lot.  You need a good heat sink.
But that's a lot of power too.  The efficiency is going to be quite bad.

Consider using a buck circuit which will not waste as much power.

Dont know if you need it, but sometimes a small transistor is added for current limit too.

mike_mike:
Thank you for the replies.
I built the circuit and I found that the minimum output current set by the 50k pot is about 0.45A.
The designer of this circuit said that the minimum output current will be 0.15A.
1. Which is the correct value ?
2. How the operational amplifier commands the NPN transistor ?
3. This transistor works in the linear region ?

MrAl:


--- Quote from: mike_mike on March 04, 2019, 09:18:19 pm ---Thank you for the replies.
I built the circuit and I found that the minimum output current set by the 50k pot is about 0.45A.
The designer of this circuit said that the minimum output current will be 0.15A.
1. Which is the correct value ?
2. How the operational amplifier commands the NPN transistor ?
3. This transistor works in the linear region ?

--- End quote ---

Hello,

I get around 0.12 to 0.14 amps minimum, however that is theoretical.  When you use real components that could be off.
However, if you are getting 0.45 amps then that could be because of several reasons.
For one, what is your load?  Remember that there will be a minimum output from the LM317 regardless of the state of the NPN transistor unless you also provide a negative voltage to the emitter so it becomes capable of reducing the output to an absolute zero.
Another reason could be because of oscillation that makes a DC measurement look higher.  You could check for that with a scope.

Another issue though is that the LM358 probably is not fast enough for a good power supply.  You could look for a faster version op amp unless you could put up with the slower speed.  Slower speed means it takes longer to react to an overload.

If the circuit is working properly the NPN works in the linear mode sometimes and saturated for larger current overloads.
The op amp output drives the NPN, so when the current gets high enough and the op amp gain is such that it puts out roughly 0.62 volts, the transistor starts to turn on and enter the linear mode.  With higher current it will turn on fully and so it will be saturated.
The slew rate of the LM358 is a little slow though so it may not be fast enough for a good power supply.

mike_mike:
Hello,
The load that I used was a 1R resistor at the beginning of the tests. I also used as load a 40W/24V light bulb. In both cases the output current was 0.45A.

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