What limits the amount of power you can put through an LM317? - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: September 27, 2019, 10:08:24 pm
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: September 28, 2019, 02:49:40 am
content: Quote from: Beamin on September 27, 2019, 10:08:24 pmThe 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 28, 2019, 03:11:10 am
content: The OnSemi datasheet indicates that for Ta =25C and Pd < PmaxImax = 2.2A typ for Vi-Vo < 15VImax = 0.3A typ for Vi-Vo < 40VAlthough 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.

author: ledtester
author_rank: Super Contributor
timestamp: September 28, 2019, 03:18:13 am
content: Quote from: Beamin on September 27, 2019, 10:08:24 pmA 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.

author: ledtester
author_rank: Super Contributor
timestamp: September 28, 2019, 03:24:48 am
content: Quote from: Beamin on September 27, 2019, 10:08:24 pmThe 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

author: MyHeadHz
author_rank: Regular Contributor
timestamp: September 28, 2019, 04:12:07 am
content: Here is a PDF I keep handy for thermal calculations of heatsinks-http://www.ti.com/lit/an/slva462/slva462.pdfLinear regulators dissipate the extra wasted energy as heat, so the bigger the voltage difference, the more heat. To maximize the thermal stability, use a good thermal paste and an appropriate heat sink. That being said, they vary significantly, especially with cheap knock-off versions. If you need a given efficiency, check out the data sheets and pick one that is acceptable from a reputable brand. If you are just curious, then the PDF above may be a more general answer so you can plug in the dimensions and values from any brand LM317 datasheet that properly lists it.

author: Jwillis
author_rank: Super Contributor
timestamp: September 28, 2019, 07:59:23 am
content: Liquid nitrogen? You better check this out .https://www.thenakedscientists.com/get-naked/experiments/freezing-electronics-liquid-nitrogen

author: MosherIV
author_rank: Super Contributor
timestamp: September 28, 2019, 09:56:04 am
content: QuoteSo many questions.......Q1A1 simple answer is the weaks part in the current path. If the part is in the path that outputs current, it must be able to continuosly sustain that current. Some parts may overheat.If you push parts beyond their specifications.Some parts like LM317 will simply shutdown when they get too hot.Q2A2 There is a missconceptiin here. The bridge rectifier in the psu will drop the same voltage whether it is AC or DC.The problem may be with the AC voltage measurment. DMM will display the rms or avaerage but if you look with a scope the peak is higher than the dmm value.Q3A3 The diodes are a rectifier circuit. It converts negative voltage into positive (for a 4 diode rectifier). So you can feed the circuit with AC or DC. AC will also vary the voltage so it will need a capacitor to hold the voltage up inbetween the voltage drops of the AC waveform. The cap is not needed if fed with DC.Q4A4 No. The psu circuit will still be limited by the weakest part in the current path of the psu. The liquid nitrogen will just allow higher continuous power/current because you have better cooling. It will not allow high pulsses of currents, that will always be limited by the components.Q5A5 ledtester explained how the 3 pin LM317 regulates voltage or current.It is not easy to make a psu with both V regulation and current limit using the LM317.I would not advised it.

author: Kilrah
author_rank: Unknown
timestamp: September 28, 2019, 10:49:55 am
content: The LM317 has an internal overcurrent protection, so whatever you do you won't be able to get past that (well, unless you add external buffering).