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What limits the amount of power you can put through an LM317?

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Beamin:
I built a power supply with a Velman Power supply kit (takes 15VAC and has variable 2-15VDC output) that uses an LM317 where you feed it 15VAC. I put a large heat sink on it then upgraded the cap to much larger value with the attempt to make it more robust. What limit how much current I can pull? Heat of the LM or the fact it has 1/4 watt resistor/parts?

I don't have the exact specs but you can use less voltage to get the same output if you feed it AC vs DC. Why is that? Why would the circuit care about ac or dc, since its has just your usual 4 diode (standard black with silver band size of 1/4 resistor types) regulator before the LM? If I keep the circuit under liquid nitrogen with a huge cap will I be able to pull huge spikes of current and then sustain huge loads?

I'll see if I can find a schematic but its probably close to what a LM317 datasheet  provides as an example way to hook it up.

While we are on it, I still don't understand dave's video where he makes a both voltage AND current regulating circuit with two LM317s. How does the part know what it supposed to regulate if its just 3 pins? That blew my mind.

amyk:

--- Quote from: Beamin on September 27, 2019, 10:08:24 pm ---While we are on it, I still don't understand dave's video where he makes a both voltage AND current regulating circuit with two LM317s. How does the part know what it supposed to regulate if its just 3 pins? That blew my mind.
--- End quote ---
The two-word answer to that is: Ohm's law. A circuit that maintains a constant voltage can maintain a constant current if the current is made to pass through a resistor which creates a voltage that can be sensed.

schmitt trigger:
The OnSemi datasheet indicates that for Ta =25C and Pd < Pmax

Imax = 2.2A typ for Vi-Vo < 15V

Imax = 0.3A typ for Vi-Vo < 40V

Although these are only two points in a SOA, it can be clearly seen that minimizing the input to output voltage is the key to maximum output current.

I have not checked other manufacturers datasheets, they may include additional SOA plots vs temperature.

ledtester:

--- Quote from: Beamin on September 27, 2019, 10:08:24 pm ---I don't have the exact specs but you can use less voltage to get the same output if you feed it AC vs DC. Why is that?

--- End quote ---

A linear regulator needs an input voltage that is greater than its output voltage (by like a couple volts) in order to achieve regulation.

15 VAC means the peak voltage is 15*1.414 = 21 V which allows you to achieve a regulated DC output that is greater than 15 VDC.

If you begin with an input of 15 VDC, however, you can only achieve a regulated output of, like, 13 VDC.

ledtester:

--- Quote from: Beamin on September 27, 2019, 10:08:24 pm ---While we are on it, I still don't understand dave's video where he makes a both voltage AND current regulating circuit with two LM317s. How does the part know what it supposed to regulate if its just 3 pins? That blew my mind.

--- End quote ---

The LM317 maintains a constant voltage between its output pin and its adjust pin. Usually this property is used to make a constant voltage regulator. However, you can also use this feature of the LM317 to make a constant current regulator.

By placing an LM317 configured for constant-current regulation in front of that circuit in front of an LM317 configured for voltage regulation you can make an adjustable current-limiting power supply.

Here's a more in-depth explanation:

https://electronics.stackexchange.com/a/100689/95488

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