Versatile power supply for breadboards

Need a breadboard power supply? Most modules are limited to positive output voltages, lower than the input voltage. This project is different.

Breadboard power supplies are useful when no desktop lab power supply is available. Most modules on the market are limited to positive output voltages, lower than the input voltage. The project we present here is different. Using two ICs from Maxim, a 5V micro-USB input adapter provides up to four adjustable output voltages, three positive ranging from 0.6V to 20V and one negative from -1, 8V to -11V.

breadboard power supply

A breadboard is not only a handy tool for experienced electronics engineers who want to test something quickly, but also for beginners, makers, or students taking their first steps into the world of electronics. No soldering is required to build a circuit and changing components or wiring is very easy. Because every circuit needs to be powered in some way, we designed a power supply that can easily be used on a breadboard.

Although a benchtop power supply is the preferred power source when experimenting with electronics or prototyping, this type of lab equipment is not always available. On the other hand, 5 V power adapters with micro-USB sockets are everywhere. We all probably have more than enough to power or charge the devices we own, such as cell phones and other electronic equipment and gadgets. Most of these adapters are short-circuit proof, and when combined with the project we feature here, you have the perfect, affordable solution for powering circuits on breadboards. Power is limited, but like all of us should be aware: breadboards are not designed for, and certainly not suitable for, high-power circuits.

There are many breadboard power supply PCBs on the market, but unlike this one, most are limited to lower output voltages than the input, and negative or balanced outputs are even rarer, if not non-existent. The circuit presented here is a novelty in this respect. It is designed to allow you to quickly create a negative or balanced (eg +9V/-9V) supply from your standard 5V power adapter for, for example, an op-amp circuit. The positive output voltage can even reach 20 V! But you can also make a low 3.3V for your microcontroller. Everything is adjustable. The breadboard power board contains only two ICs and is designed to provide four output voltages: one negative and three positive.

Feeding diagram

Figure 1 shows the schematic of the breadboard power supply.

Figure 1: Block diagram of the breadboard power supply.

The heart of the circuit is the Maxim MAX8614B.[1] It is equipped with a boost converter which can generate a voltage (VTSB) higher than the input voltage and an inverting buck-boost converter that can generate a negative voltage (VINV).

The P1 potentiometer is in negative feedback from the positive boost converter, allowing the output voltage to be adjusted between:

The inverting buck-boost converter also has a potentiometer (P2) in its feedback circuit that adjusts its output voltage between:

The maximum output power that the converters can provide is around 2W for the boost converter and 1W for the buck-boost converter. This means that for higher output voltages, the maximum output current will be lower. Remember that ripple voltage also increases with higher loads. It is therefore recommended to place an electrolytic capacitor of a few hundred micro-Farads on the supply rails of the breadboard. Pay attention to the polarity of this capacitor and check its voltage rating!

The second IC in this schematic is the MAX38903 (also from Maxim, see [2]), which is an adjustable linear voltage regulator (LDO) with P3 at an output voltage between:

With an LDO, the output voltage cannot be greater than the input voltage. Maximum output voltage will depend on AC adapter voltage (5V here), cable/connector losses, and MAX38903 dropout voltage. In practice, this maximum is around 4.5 V. The maximum current this LDO can handle is 1 A. Most adapters can provide this, but at lower output voltages the power dissipation in the chip will increase considerably:

This dissipation should not exceed 2 W, so 1 A can only be supplied at output voltages above 3 V. Fortunately, the MAX38903 is equipped with various protection circuits, so we don’t have to worry that our power supply goes up in smoke because the power limits are exceeded. The circuit on the model itself is of course at your own risk.

Assembly

Gerber files for ordering the PCB are shown in Figure 2.

Circuit board layout
Figure 2: PCB layout, design and Gerber files can be downloaded.

DesignSpark design files are available for download at [3].

The PCB has been kept as compact as possible, the best thing is to put the SMD components first, starting with the ICs. Soldering these tiny parts into their TDFN packages is quite a challenge and the build is certainly not recommended for inexperienced builders. There will be solder helpers who can solder the pads to the sides of the packages with a small iron, but soldering the exposed pads underneath requires a hot air station or reflow oven. Solder paste is preferred, although the designer managed to build his prototypes by tinning the pads with normal solder wire, applying additional flux, and using a hot air station for reflow. A magnifying glass, digital microscope, or even a stereo-microscope will be a great help in inspecting the solder joints before moving on to other components. Soldering the micro-USB connector may seem less difficult, but will be a good one to come third. Then the surrounding coils, diodes, resistors and capacitors can be mounted and finally the through-hole components, such as switch, LEDs and pin connectors, which can easily be handled using a normal soldering iron . For JP1, two outer pins of one of the two rows of a 3 x 2 pin header are cut, making it a triangular shaped 4 pin jumper configuration.

Use

The power supply can be plugged into the breadboard as shown in Figure 3.

Power on an MB102 breadboard
Figure 3: The power supply on an MB102 breadboard with additional electrolytic capacitors.

Connectors J1 and J2 are plugged into the breadboard’s horizontal power rails. The top rail is then connected to V+, the bottom rail to V-, and the two inner power rails to GND (all power supplies on this board share a common GND).

The PCB can be supported on the left side of the lab bench using two spacers or M3 12mm bolts, two mounting holes are available for this purpose. This improves the mechanical stability of the connection between the power supply and the breadboard and prevents the board from tipping over when adjusting output voltages.

Switch S1 can be used to turn on and off the entire power supply. When powering on, LED1 lights up green as a power-on indicator.

Since there are three positive supply voltages on the PCB, the user must select which one is connected to the top supply rail of the breadboard (V+); this can be done with jumper JP1. Place jumper in position 1-2 for adjustable low voltage (VYES), in position 1-3 for adjustable high voltage (VTSB) and in position 1-4 for fixed +5 V. Jumper positions are also marked in the PCB silkscreen. The remaining two voltages can still be connected to the circuit on the breadboard using male to female jumper wires as shown in Figure 4.

the jumper wires connect VLDO and +5V to the breadboard.
Figure 4: Positive rail connected to VTSBjumper wires connect VYES and +5V to the breadboard.

In this photo, the VTSB is bridged to the positive rail, while the orange and blue jumper wires connect the +5 V and VYES on the breadboard. The negative output voltage VINV is directly connected to the lower power rail (V-).

The voltages are adjusted by means of multi-turn potentiometers P1 to P3. The serigraphy on the PCB indicates which potentiometer is associated with which output. There are test points for a multimeter to measure V+ (TP3) and V- (TP1) during tuning, TP2 and TP4 are connected to GND.

There are two red LEDs to indicate an overload or short circuit of one of the power supplies: LED2 (also marked with FLT1 on the PCB) for VTSB and/or VINV and LED3 (FLT2) for VYES. The boost converter and LDO limit the output current when overloaded, and the outputs will turn off if the on-chip thermal limits are exceeded. In both cases, the LEDs will be on. If the buck-boost converter is overloaded, both outputs of IC1 turn off and LED2 lights up. Restarting the supply with S1 resets this fault condition, the output voltages will be restored and the red LED will turn off again, provided there is no short circuit, of course. However, there is no overload indication for the fixed +5V. In this case, we hope that the power supply is resistant to short circuits.

Although this project is designed and intended for use on breadboards, it can of course also be useful as a simple low-power power supply for other applications by connecting wires between the output pins and other electronic circuits ( prototyping). Finally, you can put that superfluous 5V micro-USB adapter to good use on the lab bench!


This project can also be found and followed on the Elektor Labs website: www.elektormagazine.com/labs/breadboard-power-supply


Power Project Information
Breadboard power supply specifications

Alan A. Seibert