# How to Design a Variable Power Supply

Written by j.t. barett
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A variable DC power supply can be more useful than a fixed supply, as you can use it to power a wider variety of projects. An adjustable voltage regulator such as the LM317 simplifies designing a variable supply. The supply follows a standard linear DC design, incorporating a step-down transformer, rectifier, capacitors and regulator. In this case, the regulator will have a control for voltage. Since the regulator does all the hard work, a variable supply design will be only slightly more involved than that for a fixed supply.

Skill level:
Moderate

### Things you need

• Voltage and current requirements
• LM317 voltage regulator data sheet
• Pencil and paper
• Calculator
• Electronics distributor catalogue

## Instructions

1. 1

Estimate the maximum voltage and current you'll need from the power supply, based on the kinds of projects you'll be powering from the supply. Write these figures down.

2. 2

Select a step-down transformer rated for slightly more voltage and current than you determined in the last step. Add 3 volts to your voltage figure and 30 per cent to the current figure. Find the section for step-down transformers in the supplier's catalogue and look for a transformer that meets these specifications. If you cannot find an exact match, try a slightly higher voltage and current. Write down the manufacturer and catalogue stock number.

3. 3

Find the catalogue section for full-wave bridge rectifiers and look for one rated to handle the maximum current you need. Write down its manufacturer and stock number.

4. 4

Find the catalogue section for capacitors and select a 1,000 microfarad and a 1 microfarad part, both rated for 60 volts or better. Write down these stock numbers.

5. 5

Find a 240-ohm,1/4-watt resistor in the catalogue. Note its stock number.

6. 6

Calculate the ohm value for the variable resistor using the following formula:

Rv = 240 x ((Vs/1.25) --1), where Rv is the resistance of the variable resistor and Vs is the supply's maximum voltage.

7. 7

Find the catalogue section for variable resistors and look for one having the value you calculated in the previous step. Write down the manufacturer and stock number.

8. 8

Draw a schematic for the power supply, following the examples in the LM 317 data sheet. Label the transformer with the voltage and current values you chose for it. Likewise, label the rectifier and variable resistor with the respective values for each.

9. 9

Label the regulator's fixed feedback resistor with the value of 240 ohms. Label the large filter capacitor connected to the rectifier as 1,000 microfarads. Label the capacitor connected to the regulator's output as 1 microfarad.

#### Tips and warnings

• Note that the LM317 is rated for a maximum of 37 volts and 1.5 amps. If you need more voltage or current, you'll need a more elaborate circuit or one based on a different regulator.
• Regulators such as the LM317 need a voltage input higher than they are expected to provide.
• If you do not find an exact catalogue match for the variable resistor value you calculated, the next highest value in the catalogue will also work.

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