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I made a power supply from a diy site, the schematics are above. The site showed the project as 2amps output but I'm not getting even close to that. I'm getting around .14A or less. Does anyone know what the problem might be. Also I didn't include the diode that is shown above the lm317, but I don't think that has anything to do with it. Lm 317 says its rated for 1.5 A. The Transformer I'm using is here


The Heat Sink I have is here


I have it connected to a usb charger for my phone, which should use max 5v and draw 1 amp. I'm using a multimeter to measure the current. It is dynascan 2807 digital multimeter.

Update: When the charger is connected the voltage drops to .4 volts?

Update 2: Never going to copy schematics from Makezine website again. Or do any projects from there. Secondly thanks for all the useful feedback Im learning a lot. What would be the best way to do a variable power supply then or is it just not feasible. Should I just make a power supply with 3.3 5 9 and 12 options or is my problem the transformer output is too high so after rectification its way to high? I really only wanted a variable power supply from 3.3 to 24 yet it is much higher than that because of rectification. I'm just gonna blame makezine again.(Although I could have done some research on how bridge's increase voltage

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    \$\begingroup\$ What kind of heatsink do you have on the LM317? \$\endgroup\$ – Dave Tweed Jul 16 '14 at 21:39
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    \$\begingroup\$ Current needs to be pulled. What load do you have connected? The LM317 is an adjustable regulator. What is your output voltage and desired use? Just because a supply can deliver 2A doesn't mean it will. That means the most it can deliver before its regulated voltage starts to droop is 2A. \$\endgroup\$ – ACD Jul 16 '14 at 21:39
  • \$\begingroup\$ What do you mean when you say you can only get 0.33A or less? What kind of load are you using. Are you measuring the output voltage and, if yes, with what type of meter? \$\endgroup\$ – Barry Jul 16 '14 at 21:40
  • \$\begingroup\$ "I have it connected to a usb charger for my phone, which should use max 5v and draw 1 amp." --- The phone should only draw high current from the charger after verifying that it is not hooked up to an ordinary USB host. Do you have this circuit set up to convince your phone that it attached to its special charger and not an ordinary USB host? \$\endgroup\$ – The Photon Jul 16 '14 at 22:19
  • \$\begingroup\$ Also, is the output voltage dropping below 5 V when you have the load connected? \$\endgroup\$ – The Photon Jul 16 '14 at 22:20

Holy cow, talk about an undersized heat sink.

Your 24 VAC transformer will produce about 32 volts at the main filter caps. Producing a 5 volt output from the LM317 means that you are dropping about 27 volts across it. A current of .14 amps says the LM317 is dissipating about 3.8 watts. The data sheet for your heatsink says that the temperature rise AT THE HEATSINK will be about 80 C, for a heatsink temperature of ~105 C. This is consistent with your measurement. If you look up the datasheet for the LM317, it specifies a thermal resistance (chip to case) of about 5 degrees per watt, so 4 watts on the chip will give it a 20 C rise over the case. Since the case is tied to the heatsink, your chip temperature is right about 125 C, which is the absolute maximum allowed. Dave Tweed is right about the regulator shutting down in order to keep from self-destructing.

If you could pull 1 amp, you would be dissipating 25 watts, and your heatsink temperature would be (roughly) 500 C. Plus another 125 C rise to the chip itself, and there is simply no way that's going to happen.


Oh, I think @David Tweed hit it. That heatsink might be too small. (wet your finger and touch the heat sink... what happens?)

24C/W: at 24V in and 5V @.33A out, how many watts are you dissipating in the LM317?

  • \$\begingroup\$ Not much besides sizzle, current stayed the same. I got the non contact thermometer out and it shows 215ish F on the heat sink. Btw its also pulling only .14A I wrote the post of memory on that part, my mistake. \$\endgroup\$ – Walrath21 Jul 16 '14 at 22:11
  • \$\begingroup\$ Heatsinks should not be "sizzling hot"! Your poor regulator is shutting down in an attempt to protect itself. \$\endgroup\$ – Dave Tweed Jul 16 '14 at 22:25
  • \$\begingroup\$ @Dave Tweed I don't know why but the way you said that was really funny. And that makes a lot of sense. Could that be why the voltage drops so drastically when I connect the charger. It drops to .4 v to be specific. \$\endgroup\$ – Walrath21 Jul 16 '14 at 22:29
  • \$\begingroup\$ Dave's right, you have 24V AC in, more like 34VDC (minus some drop/sag) once it's rectified. If you want 1A at 5V out, you need to dissipate 29W in the regulator. That's not too practical, your regulator is going into thermal shutdown to protect itself. \$\endgroup\$ – John D Jul 16 '14 at 22:38
  • \$\begingroup\$ Chuckle, Sizzle is bad. A 'scope on the output would most likely show the output turning on and off. Amazing beasts those lm317's. (I don't suppose someone would give me a thumbs up so I could comment elsewhere?) \$\endgroup\$ – George Herold Jul 17 '14 at 1:12

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