Help required for Linear Power Supply build using LM317

[MrOmnos]

Hi! I have been trying to build a liner power supply using LM317 voltage regulator but I am stuck.

Here's my schematic. All the values are also leveled on the schematic. Since, I was going to use this power supply to run a small DC fan I made so, I didn't bother to add extra caps. Just basic stuff.

I am using a center tapped transformer with 12v-0-12v/1A rating. I am using the both 12v taps to get the output of total 24 volts Ac. Instead of using 4 diodes as rectifiers I am using a D2SBA60 rectifier. I am using 4700?F 50v as an input cap and a 22?F as an output cap. I also added couple of diodes so when the fan stops, the collapsing field won't destroy the regulator IC. I soldered all the components according to the schematics on a perfboard. Turned on the power, used a DMM to measure the output voltage and it was showing around 30 volts. So, the first thing I did was check all the connections. It was all fine. I measured the rectified voltage, it was around 22-23 volts. I measured the voltage with 4700?F cap and it showed 30v. How?? (shouldn't the caps be charged up to the input voltage i.e 22-23?) This 30V was appearing directly on the output. The input and output of Lm317 seemed to completely shorted. So I did a continuity test and yes, it was shorted. Then I measured the voltage between output and adjust pin and it was way higher than 1.25 volts so, at this point I figured out the IC was fried. So, I plugged in another one. It was fried too. First I thought the ICs were Chinese knock offs because I had checked my circuit 100 times and it was good. So I went out and bought couple of more ICs from a trusted supplier. I tested them on a breadboard with a 5v DC input on the same circuit as in the schematic expect for the input cap and the rectifier and they were working fine. Then I plugged one in to my power supply circuit and checked the output voltage. It was 30v. Adjusting the pot had no effect what so ever. Whatever voltage I was measuring at the input(30v) , same was appearing at the output as well. And I had fried another IC. So, now I started doubting my circuit and my soldering. I connected the same circuit on the breadboad with my last Lm317 and turned on the switch. Measured the voltage at the output, same 30v. The chip was fried. And this time, I also saw a spark in the pot in one of the leads connected to the adj. pin. But the pot is fine. I checked it afterwards. Now, can someone please tell me

is happening?

 

[Luis Costa]

that sounds a lot... 4700uF? wow that would probabily be ok if you weren't using the regulator. try just the 100nF as input and 1uF as output as it sais in the datasheet. have you tried that?

 

[Bluejets]

DS2BA60 does not bring up any results when Googled so anyones guess as to what you have there.
Assume it's a bridge rectifier but who knows and since you are having problems, it's normal not to second guess anything.
Your circuit shows the pot with terminals 1 and 3 connected to the adjust, it should be 1 and 2 or 3 and 2 to get any variation in resistance there.
Your voltage readings are as one would expect.
For further explanation check power supplies theory.
Other than that show us a pic of what you have done, maybe some clue there.

 

[MrOmnos]

DS2BA60 does not bring up any results when Googled so anyones guess as to what you have there.
Assume it's a bridge rectifier but who knows and since you are having problems, it's normal not to second guess anything.
Your circuit shows the pot with terminals 1 and 3 connected to the adjust, it should be 1 and 2 or 3 and 2 to get any variation in resistance there.
Your voltage readings are as one would expect.
For further explanation check power supplies theory.
Other than that show us a pic of what you have done, maybe some clue there.

Sorry it's D2SBA60 . And I have connected the pot correct. It made the mistake when I was making the schematics.

 

[MrOmnos]

that sounds a lot... 4700uF? wow that would probabily be ok if you weren't using the regulator. try just the 100nF as input and 1uF as output as it sais in the datasheet. have you tried that?

I did some research before making this circuit and people generally used higher value caps for power Lm317 based power supply. Most people use 10,000 micro F. Because with smaller caps, when the load is increased the ripples also increase. That is why I chose 4700micro F. I have seen people using 10,000 and the supply works fine. Search one made by jumperonetv on Youtube.

 

[dorke]

1.Check that diode D1 isn't shorted or connected in reverse.
2.R2 value is too big change it to less than 5k ohm
3.Remove diode D2 ,in your circuit it serves no purpose.
4.Better use the values shown in the pic bellow,add input cap close to Vin pin of LM317
5.Take a good photo of your circuit and post it here.

 

[dorke]

BTW,
What is the voltage your "small fan" needs?
Most of all,Why does it have to be regulated???

 

[AnalogKid]

Check the 317 pinout. The adjust pin is in the middoe of the schematic symbol, but not the device. Also, note that on the original National Semiconductor datasheets they frequently showed devices from a bottom-side view. Statistically, the #1 problem with new linear regulator designs is the pinout.

ak

 

[davenn]

I did some research before making this circuit and people generally used higher value caps for power Lm317 based power supply. Most people use 10,000 micro F. Because with smaller caps, when the load is increased the ripples also increase. That is why I chose 4700micro F. I have seen people using 10,000 and the supply works fine. Search one made by jumperonetv on Youtube.

The rule of thumb is 1000uF for every amp drawn ... in your case with a LM317 which is around 1 - 1.5A max when well heat sunk. 1000 to 2200uF is ample

Turned on the power, used a DMM to measure the output voltage and it was showing around 30 volts. So, the first thing I did was check all the connections. It was all fine. I measured the rectified voltage, it was around 22-23 volts. I measured the voltage with 4700?F cap and it showed 30v. How?? (shouldn't the caps be charged up to the input voltage i.e 22-23?)

The output voltage on the bridge rectifier WITHOUT cap or regulator will be around 22-23V with lots of AC ripple voltage

when you add the capacitor, the voltage will increase the voltage by ~ x 1.4
so 22 x 1.4 = 30.8VDC

have a VERY good read of this page .....
http://electronicsclub.info/powersupplies.htm

its essential basics for understanding PSU's

The input and output of Lm317 seemed to completely shorted. So I did a continuity test and yes, it was shorted. Then I measured the voltage between output and adjust pin and it was way higher than 1.25 volts so, at this point I figured out the IC was fried. So, I plugged in another one. It was fried too. First I thought the ICs were Chinese knock offs because I had checked my circuit 100 times and it was good. So I went out and bought couple of more ICs from a trusted supplier. I tested them on a breadboard with a 5v DC input on the same circuit as in the schematic expect for the input cap and the rectifier and they were working fine. Then I plugged one in to my power supply circuit and checked the output voltage. It was 30v. Adjusting the pot had no effect what so ever. Whatever voltage I was measuring at the input(30v) , same was appearing at the output as well.

as others suggested CHECK your wiring !! make sure it isn't reversed
also if you are using that diode across the input and output of the regulator ... make sure it isn't reversed ... if it is, that will give you an instant 30V on the output
regardless of the trimpot voltage setting for the reg

failing that take some sharp and well lit pix of your construction and lets see if any mistakes can be seen


cheers
Dave

 

[Luis Costa]

for me being a person new at this i'd have to follow the standards bacause they work. That may work and maybe it can be explained by one with a more profound knowledge of the LM317 internal circuit...

 

[Luis Costa]

i've got a question to the LM317 subject issues... i don't intent to use much the space of this thread because it was created with it's own specific purpose but i guess there's no need for another thread. In the function Vout=1.25x(1+....
can Iadj.R2 be replaced with something like (Vout/R1+R2).R2 or something. How does one get that Current?

 

[dorke]

No you can't.
The current is found in the datasheet under "adjustment pin current"
It is a maximum of 100uA ,use this value to calculate Vout.

 

[Luis Costa]

thanks dorke.