Variable DC Power Supply Using LM317T With LED Indication Of Voltage Levels

Thread Starter


Joined Apr 15, 2010
Hello Guys ... !

I'm assigned to complete an electronics project named 'Variable DC Power Supply Using LM317T With LED Indication Of Voltage Levels' ...

Max. Output: 15 VDC
Max. Current: 1 Ampere
IC Allowed: LM317T Only
Output Range: 0V to 15V (In steps of 3V ... i.e. 0V, 3V, 6V, ... , 15V)

Using a transformer of 220/24 VAC (1 Ampere), I stepped down the input ac voltages ...

Using 4 diodes (1N4007), I successfully rectified ac input signal ...

What should be the value of filter capacitor, if the maximum voltage required at output is 15 VDC and maximum current allowed is 1 Ampere ... ?

I've used a capacitor of 1000 uF/50 V, it works fine, if I'm using 15 VDC at output and using a maximum of 100 mA current ... Would it work fine if current at output is 1 A ... ?

My LM317T is working fine ... I've regulated voltages required at output ... (Remember my output is: 0V, 3V, 6V, 9V, 12V, 15V ...

Now, my second and last question ...

I've to display the output voltage level using LEDs ... That is, if output from LM317T is 0V, there shouldn't be any glowing LED (indicating 0V) ... If output is 3V, there must be 1 LED glowing so as to represent 3V visually ... Similarly, if output is 6V, 2 LEDs must be glowing ... If output is 9V, 3 LEDs must be glowing ... If it's 12V, 4 LEDs ... And if it's 15V, 5 LEDs must be glowing ... !

In short:

OUTPUT / LEDs Glowing
0V / 0 LEDs
3V / 1 LED
6V / 2 LEDs
9V / 3 LEDs
12V / 4 LEDs
15V / 5 LEDs

I'm unable to make a circuit with 5 LEDs, sensing voltage levels and displaying on LEDs ... Please, tell me how can I make a circuit, that would visually be displaying output voltage ... ?

P.S. Ripple factor allowed is 3% ... I can only use ZENER diodes ... I cannot use any ICs for sensing voltage levels ... (I'm restricted to use simple electronics for indication from University) ... Help me out ... Please ...
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Joined Mar 12, 2010
√2 C Er F = I
1.414 capacitance (p-p ripple voltage) Frequency = Current
1.414 C (3% x 15V) 120 = 1 amp

I was going to suggest zeners for the level indicator, but the assignment told you that. Make the output voltage struggle through progressively larger zeners before it can get to the LEDs.

Enough clues?


Joined Dec 5, 2009
Build a zener ladder that when the output is at or above the zener voltage, the LED lights.

Bychon, Hours later and I didnt see your post.. sorry
But..great minds... ;)
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Joined Dec 5, 2009
I had to edit this because I realized I was giving the answer without helping the OP find his way.

Do you have a specific zener to use? Did your prof. or University give you a kit to choose from, or do you have to choose the zeners on your own?
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Joined Sep 7, 2009
How are you controlling it? If for example you were using a 6 position rotary switch, you could use a dual one and put the LEDs on the input side.

John P

Joined Oct 14, 2008
Can you really get the output of an LM317 down to 0 volts? I thought it would go down to 1.25 but no lower. You can always drop the output by putting 2 (maybe 3) diodes in series with it, though.

Thread Starter


Joined Apr 15, 2010
To Bychon ...

Thanks for the formula, it helped me in evaluation of capacitance ... :) :)
Actually, I'm doin' Mechanical Engineering ... So, 'struggle through progressively larger zeners' ... I didn't get it ... :) :) ... !

To retched ...

Can you, suggest me a circuit ... ?
I'm NOT at all given any specified zener diodes to be used in my power supply ... I've to choose 'em myself ... :) :) ... !

To Markd77 ...

I'm actually using a '6-way sliding switch' for controlling of the voltages at output ... !

To John P ...

Hehehe ... Yeah, Your right ... Output is 1.25 V from LM317T ... I've considered it 0 V, I'm allowed to consider 1.25 V as 0 V, in this supply ... (Since no LED will be glowing at this voltage level, if 5 LEDs are used) ... !


Joined Dec 5, 2009
That is a proven way to guarantee 0v.. From a LM317 or pretty much anything else on the mains..

I can suggest you take a look at VU meter circuits.. These are used to progressively light leds with voltage increases.


Take a look at, and read this page. It tells you how to pick and use zeners to light an led based on voltage. You can use this as a base and add zeners to ladder up each level
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Joined Apr 20, 2010
You could get an led that displays different colors w/ different resistors like you find as smd's on a router or something, and when you select a different voltage a different R is selected, or do the same but with multiple LED's in a row all different color or labeled, but you would not want all 5 lit at the same time. Think about it, when have you ever seen such a simple circuit with a graph style display unless the voltages to the display, line out, LCD, etc. are all drawing off a fixed supply or the line out is regulated/powered separately. I actually don't know too much about electronics per se' but you can't supply 1A with 1A and still have 1A. You need to ditch the 1A limit or limit the 1a after the LED's use their share, if you really want all 5. If that is even possible. Not sure how much power LED's use off the top of my head, but that is what comes to mind now. I just built a small power supply with one this morning and think I might go with a digital readout/selector, and maybe selectable between voltage/current regulating...