This Hackspace project aims to create simple, low cost, robust, and safe variable voltage power supplies for experimental use in projects. While we have good bench power supplies in the Hackspace it is often useful to have something personal and portable for you projects. It is hoped that the Personal Bench Power Supply will develop over a few iterations as we learn.
Personal Bench Power Supply Mk.1
I have gone for a very basic specification for the Mk1 version to achieve an availability during October 2012.
- Low cost £5 to £10
- No exposed mains voltage to builder or user
- Voltage range < 3 Volts to > 12 Volts variable
- Current up to 2A (not variable limited) Fuse protection
- 4mm screw binding post output
- Visual readout of voltage
- Laser cut or simple self build case
- Pocket size
Power Source To achieve the voltage range, current, and safety objectives; a laptop power supply is used. Usually these provide between 16 Volts and 19 Volts and adequate current). These are power supplies are usually very low cost or free.
The plan is to plug the laptop power supply into the variable regulator module. Panel mounted power jack sockets seem common in 2.1mm and 2.5mm pin, but not common in 1.5mm, so a laptop power supply with a 2.1 or 2.5 plug is preferred (need to look into long vs. short plugs)
From the voltage range, current, and size perspective a switching regulator is most appropriate for the variable regulator. The disadvantage of switching regulators is their higher ripple voltage on the output especially at lower currents.
This is the switching regulator module I chose. The module is low cost at £1 each, and avoids having to make a PCB.
The boards come in a few different pcb layout but generally have the following circuit using the LM2596 regulator:
The datasheet for the LM2596 is here: LM2596
The TI/National WebBench power supply design too also supports the LM2596. Analysis of the above circuit with the design tool shows that the LM2596 will operate in discontinous mode at the currents we will supply, so ripple on the output may be an issue in some applications.
The voltmeter is based on a ready built module costing around £1.70 each. It is a 3 digit red LED meter reading from 0 to 99.9V:
The schematic shows the voltmeter is based on an STM8 flash micro and is hackable in itself:
Note: the devices S1 and S2 are not switches, but 4 off 220R resistor array to limit LED current. The digit drive M4 is not used because it is only a 3 digit display.