Introduction

Relying on a fancy benchtop power supply might limit your possibility unless you have experience in designing of an electronic system. A “general” power supply on a bench is good to have, but your future project may need multiple voltages, various polarity, and even high voltage or high power. In this article, we will quickly go through the methodology for busy students how to prepare the power supply with minimum effort first. Then prototyping of experimental power supply will be described with schematics.

Get at market or dumpster

This section present zero-effort ways to prepare your wanting voltage. If the voltage is upto 30 or 40, following method might be helpful.

AC adaptor

If you are really in hurry or not familiar with electronics and you need typical voltages such as 5V~24V, AC adaptor is the best way to achieve your goal. AC adaptor is a very common component for making a regulated DC voltage out of AC110V. Perhaps you have it in your house came with old laptop or cable modem. Voltage and capable current are written on the label on AC adaptor, so you can immediately use it for your project. If you don’t want to ruin connector, you can search “barrel plug” in Digikey or somewhere else. Measure the inner and outer diameter to get an appropriate one.

The disadvantage of the use of the AC adaptor is it supplies only a single voltage. If you need dual polarity or various voltage, DC-DC conversion must be considered as mentioned below.

Switching regulator

Switching regulator generate a specific voltage by chopping DC voltage, instead of using a transformer. The right figure is a picture of typical switching regulators found in a dumpster. Usage is quite simple, connecting L, N, and ground from a power cable to the block terminal shown in front, and low DC voltage appears in other terminals. Some switching regulator has a potentiometer to adjust an output voltage and dual polarity is also possible. It cost about $10 in eBay and Amazon.

USB + mini-switching regulator

if your circuit does not consume a lot of power, using 5V from USB connector is really handy. As source voltage is small, the switching regulator can be also smaller than the one shown above. A bunch of a small DC to DC converter board is available on Amazon. The circuit convert higher voltage into a small voltage is called “Buck converter” or “Step-down converter”. On the other hand, convert small voltage into higher voltage called “Step-up converter” or “Boost converter”. Let me note that the switching regulator emits a lot of electromagnetic noises as the switching regulator involve impulsive current flow to obtain enough potential at an inductor.

Battery

A battery is the best voltage supply if the noise is matter. There is no switching noise and no ground-loop issue. Also, if you lead-acid battery you could get really high current as well. So keep this in your mind as a backup plan.

Rush work with solder iron (DIY)

The technique to create voltage supply out of discrete components makes you unstoppable. One you prepare stocks such as transistors or passive components, you can basically build any voltage anytime. The right picture is the dual polarity DC voltage supply I made when my budget was super tight.

Transformer + linear regulator.

This is my favorite combination to prepare voltage between -30 to 30V. First, transformer converts 120VAC from power inlet to low voltage. By using transformers with center tap (ex. FD7-56, Triad) or parallel output (ex. 3FD-256, Tamura), you can generate three levels: positive, common, and negative.

It is good idea to buy the transformer whose secondary voltage is exactly what you need. However, In most of the situation, voltage adjustability is very convenient. To achieve this, “Linear regulator” is introduced after the rectifying stage. The linear regulator named “LM317 (for positive) ” and “LM337 (for negative) ” is one of the components I strongly recommend to have in your stock. This IC can generate voltage or current regardless of the input voltage by referring bandgap of Silicon (Wikipedia:LM317). Once you have passive components such as a capacitor or potentiometer, it is really easy to obtain a specific voltage. Please note that power efficiency becomes worse as a voltage difference between Vin and Vout becomes larger, and this results in the heating up of the IC itself. It is okay to build this adjustable power supply for laboratory use, but I would recommend you to think about power efficiency if your project is a serious one.

If you need an adjustable linear regulator for large voltage such as 120V, you can check the components such as (LM317HV,TL783)

* Note for LM317 series

I have googled too many times with keyword like “LM317 pinout” or “LM317 circuit” every time I use it. so I’ve finally made a summary figure for myself to speed up cirucit design and layout with LM317. The space behind LM317 is kept for heat sink.

Zener and Bipolar transistor

Voltage regulation using Zener diode is also a handy method to achieve specific voltage with a minimum number of components. As shown in the figure below (a), very sharp breakdown properties in reverse voltage make Zener diode itself a voltage regulator. Here Rs must be picked to limit the current according to a power rating of Zener diode. This simple circuit will work up to 10mA load, but if large current flow into the load, the voltage easily fluctuates. To avoid this, adding bipolar transistor enable detouring the current path as shown in figure b.