EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Muhammad Nasar on March 01, 2011, 02:50:23 pm
-
I had made this circuit to make a powersupply which can supply voltage upto 21 volts, i have made the circuit but there is an unusual problem with it i.e. i get the desired voltages on the output but as soon as i connect a 80mm computer fan to it the voltage dropsdown drastically and fan doesn't run, i set it to 12 volts but when i connect the fan the voltage got droped down to 3 volts and even by adjusting the POT's Nobe to increase voltage , nothing happen the voltage remained the same. But when i disconnected the 2200uf Cap the circuit ran fine but still there was the voltage problem i.e. i set the voltage to 12volts but when i connected the fan it droped to 6 volts. I then connected the 1000uf cap instead of 2200uf after bridge rectifier and the same condition repeated that i was getting voltages at the output but as soon as i connected fan to it it droped and fan didn't run.
Plz help me as i am very new to electronics and have started my Electrical Eng. at the university. here is the schematic
http://img192.imageshack.us/i/image0004.png/ (http://img192.imageshack.us/i/image0004.png/)
-
What is the voltage going in to the LM317? Maybe the transformer is less than 24VCT.
-
I have checked it with the mutimeter it's 24 volts DC
-
Can the transformer supply enough current?
For powering a fan, you can use an LM7812 in place of the LM317 and get rid of every other component apart from a small 1uF output capacitor and a 470uF input capacitor.
-
Does it work without the Fan?
Can you control the voltage wit a lighter load?
What is the load of the fan. Depending on the model the LM317 can supply 100, 500 or 1500 mA.
Maybe the LM317 is in current limiting mode.
The 100uF looks a bit large, is it for soft start?
Remove this for test.
-
How much current dfoes the fan use?
-
Sounds like the fan draws enough current, and you do not have a heatsink, and it is going into thermal shutdown.
paul
-
Sometimes computer fans develop a dead stage in their internal electronics, they still spin but draw large currents to start.
-
You don't show the fan on your schematic. It should not be on the same circuit as whatever you are supplying with the regulated supply. I'd suggest powering it from the other (unused) transformer secondary. Either the fan is drawing too much current from the LM317, or the LM317 is drawing too much current from the transformer. What is the VA rating of the transformer? What is the current draw of the fan? Do you have heatsinking on the LM317?
-
maybe you could post a photo of your circuit + fan, might help us see what the problem is.
-
Im no ee so take everything I say with a grain of salt. I just read here and there and try and pickup stuff.
Your circuit would need that variable resistor set to aprox 1050ohm to make 12v, check that.
Youve also got a 10uF cap in there, I assume your doing this to reduce ripple ? something you really shouldnt need for a 12v fan. never the less, you dont have a protection diode between adj and Vout like the datasheet recommends to prevent the LM317 from being damaged by that cap discharging through the adj pin. Id pull that cap for now just to test. Do you have another LM317 you could test incase that one your using is toast ?