Submitted by: Argiris Theopoulos (firstname.lastname@example.org).
Dear Elektor team,
I am writing to you in order to share my application about a conversion of an old PC (ATX type ) power supply to a low cost laboratory power supply with power management functionalities. The basic idea was to build a cheap power supply for my home laboratory which could provide me some first information about current consumption and also some short circuit or overcurrent protection for my test prototype applications.
In a few words from the various voltage outputs of an old PC power supply I create 3 controlled output channels for my supply Channel 1: stable 3.3V Channel 2: stable 5.2V Channel 3: regulated output +-1.25 ~ +-10V In each channel I designed an auto-scaled ammeter (approx. 0.1mA~4A for Channels 1,2 and 0.1~1.5A for Channel3).
I used a simple Rsense technique with instrumentation amplifier with a group of resistors (for Rsense) in order to manage large currents e.g 3-4A witch the PC ATX supply could provide me.
All outputs are controlled from relays. The user may set a maximum current limit each output can let pass, a quite usefull option when you test prototype circuits with sensitive parts. Also there is a short circuit event functionality in all channels simultaneously despite which channel appears on the screen.
Some other functions of load detection and active-channel auto display option are provided as well.
The measurement results compared with my Range low cost multimeter are quite decent. The accuracy limits of the ammeters according to ADC 10-bit resolution of ATmega168 is from
+-0.1mA for Channel1 (1st Scale 0~10mA) to about +-50mA for Channel3
(7th Scale 2A~4A). The basic inaccuracy comes from Rsenses variation values due to temperature and also from the cancellation technique of large common mode voltage (witch in addition varies in Channel3). The software provides a max 100ms output cuttoff when a shortcircuit or over-current event occurs. Despite above issues I found the accuracy of device full-feeling my expectation and very usefull for my embedeed projects. My designed pcb for power management and the the old commercial atx power supply circuit where encased in a plastic box as you can see in the attached photo.
So 2 years after the first design searching for some other parts in Texas Instruments I found some instrumentation amplifiers capable to manage larger common mode voltages with was the basic issue I faced up in my design. So I thought maybe it is worth to improve the old design (perhaps make it simpler) if you find it interesting too and want to share it with your readers.
In this first contact I tried to give you some basic information of the project, any other details (detailed circuit, pcb,more photos) are available if you want more information. Please feel free to contact me and discuss anything about this project or a future version of it.
Thanks in advance
Physicist, MSc Electronics Engineering