Hey all, I am new here to the bit-tech forums . Anyway, I have been following this guide to build a LM317 linear fan controller. I have tried a PWM fan controller, but unfortunately it caused my panaflo 120 U1A's to growl, even after blunting the PWM signals... The linear regulator works fine, except that the max voltage it puts out is around 9.1V (I used a multimeter to test this, placing the leads between Vout and ground). The guide says it will put out up to 11.5, which is considerably higher than 9.1. I used all of the same parts, except a 2.2 uF cap instead of the 1 uF one (yes, I used an electrolytic polarized one). I honestly doubt the cap makes a difference, though. Does anyone know what could be going wrong? Can I get a higher Vout? My PSU puts out right around 12V on the 12V rail, so that isn't the problem... I can live with 9.1V max I suppose, but I would want to make it so this LED voltage monitor lights up all bars when at 9.1V http://casemods.pointofnoreturn.org/voltmon/tutorial-full.html. At the moment, it only lights up about 8 at full scale... I am still rather new to electronics, FYI, so concise guidance is greatly appreciated . Thanks a ton.
Welcome to Bit, darkenreaper57! You'll never get 11.5V out from 12V in with a 317T, a loss of around 1.5V-2V is typical. But 9.5V is a bit low, a possible reason is the resistor combination you've used. Pots can have +/-20% tolerance, resistors maybe +/-10%, which can mess calculations up if the tolerance is all the wrong way. If you've used 1k with a 10k pot as at the linked site, try reducing the 1k to 820R. Better still, see here. For the bargraph you need to make the range adjustable, so swap R2 to a 1k preset potentiometer, one end to ground, other end to R1, and wiper to the capacitor & input pin 5, then tweak to suit.
Thanks for the help. I will try those things. Oh, one more thing: I DID try the suggestion at the end of the bargraph tutorial about lowering the high value, but I couldn't get it to work. In the end, it actually made fewer LED's light up when the pot wiper had no resistance. Here is the quote of what to do: One last note - what if I want to use a different high value than 12V - say, 7V or 5V? In this case, get a second resistor and place it next to R1 - connect R1 to it in series, and connect this new one (call it R4) to ground. Disconnect pin 8 from ground and connect it to the middle between R1 and R4. Then, the top LED will be equal to 1.25 * (1 + R4 / R1) - it's identical to a LM317." Now, the high is 12V. I want to get it down to 9V or so, so the LED's all light up at full scale. I have followed the part at the bottom of the guide, but it didn't work. I did the math and I figured that a resistor around 7.5k will do the job. I did the following: 1. Disconnect R1 from ground. Place new resistor R4 in series with R1, and connect R4 to ground. 2. Disconnect pin 8 from ground (and 4 for that matter) and connect it in between R1 and R4 (thus it is a voltage divider). 3. Keep pin 4 connected to ground.
Ok, well I guess I was REALLY tired last night because I measured the potential difference between the legs of the pot instead of Vout and ground to get 9.1V . Anyway, I did remeasure (correctly) and it is still sorta low at 10V or so. Oh well, I am happy with that, since I will never run my U1A's at 12V...they are too loud. Ok, I also got all LED's to light up on the 10-element brick by following your advice, except instead of tweaking a 1k pot (something I don't have) I experimented with resistor values in parallel, since I don't have many small ones. Using a 5.6k and 1k resistor in parallel (yielding about 850 ohms) worked just fine in place of the 680 ohm resistor. This should work fine without damaging the circuit (i.e. supplying too much current to the LEDs) correct? Thanks for the help.
Resistor R1 (or the resistance between pins 7 & 8) sets the LED current to 12.5/R1, so 1.2k gives 12.5/1200 A, 10.4mA per LED. The LM3914 datasheet gives all the info.
Ok, that is what I thought, but I wasn't quite sure. Once gain, I am rather new to electronics and grasping all of the data from datasheets is still a bit of a challenge for me... Thanks for the help .
