Linear Variable Voltage Regulator 1-20 V

A linear voltage regulator maintains a constant voltage at output if the input voltage is greater than output while dissipating the difference in the voltage times the current watts of power as heat.

You can even make a crude voltage regulator using a Zener diode, 78xx series regulators and some other complimentary components, but that won't be able to supply high currents like 2-3A.

The overall efficiency of linear regulators is very less compared to switch mode supplies, buck, boost converters since it dissipates the unused energy as heat and has to be removed constantly other wise the regulator seizes.

This power supply design is totally worth it if you don't have any power efficiency issues or if you are not powering a portable circuit from a battery.

The whole circuit is made of three blocks,

1. Main variable regulator(1.9 - 20 V)

2. secondary regulator

3. Comparator, fan motor driver(MOSFET)

A LM317 is a great voltage regulator for beginners when used properly. It only requires one voltage divider given to its adjust pin to get a variable voltage at output.
Output voltage depends on the voltage at adjust pin, generally kept at 1.25 V.

output and adjust pin voltage are related as, Vout = 1.25(R2/R1+1)

The current on the load remains almost as same as i/p current at any voltage set. Lets assume If the load at O/p draws current of 2A at 10V, the remaining Voltage of 10V with remaining current of 1A is converted in the form of heat of 10W!!!!!!

So its a good idea to attach a heat sink to it.........why not a FAN!!!!??????

I had this mini fan laying around a while, but the problem was it can take only 12V for maximum rpm but I/p voltage is 20V , so i had to make a separate regulator(using LM317 itself) for the fan, but if i keep the fan on all the time thats just wastage of power, so added a comparator to turn on the fan only when the temperature of the main regulator heat sink reaches a preset value.

Lets get it started!!!

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Step 1: Gathering the Components

We need,

1. LM317 (2)

2. Heat sinks (2)

3. some resistors(check the schematics for the values)

4. electrolytic capacitors(check schmatics for the values)

5. perf Board (project PCB)

6. MOSFET IRF540n

7. FAN

8. some connectors

9. Potentiometers(10k)

10. Thermistor

Step 2: Bringing All Together

Choose the size of the PCB board you are comfortable with.

I kinda made it compact 6cm by 6cm, if you are good in soldering you can go with even smaller size ;)

keeping the Vin connector on left and Vout on the right, comparator IC in center and the regulators on the top with the fan at the top most makes it easy to handle and to use.

Just follow the schematics, keep checking the continuity check now and then for short circuits and proper connections.

Step 3: Placing the Thermistor Feedback

Place the thermistor in contact with the heat sink, i kept it in the ridges of the heat sink.

since the thermistor is in series with another 10K resistor its a voltage divider of exact 10 to 10V ,

when the temperature rises the resistance of the thermistor reduces but the voltage keeps rising towards 20V.

This voltage is given to non_inverting terminal of opamp 741 and inverting terminal is kept at 11V, so when the thermistor voltage goes beyond 11V the opamp outputs HIGH at pin6.

Step 4: It Should Look Something Like This...

Lets test it!!!

giving 20V input from my transformer via FOOOLLBRIDGE RECIFIER!! and adjusting the O/p to around 15V , i connected a 5W 22ohm resistor at O/p which was drawing around 2.5A.

The heat sink started heating and went near 56C, the thermistor voltage raised beyond 11V so the comparator detected that and turned the Mosfet on in saturation region inturn turning the FAN on to cool the heat sink.

Annnd thats it!!! you just made an variable voltage regulator which you can use it as LAB bench power supply, to charge batteries, for supplying voltage to prototype circuits and the list goes on...

if you have any project related questions feel free to ask!!!

seeya!

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