Power supplies for projects

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Cheaper alternatives for supplying power to projects

Let’s face it, EI and toroidal transformers break the budget. Going back a few decades I always recall my own projects being run from batteries. Family would give me their faulty gear to strip and if I was lucky I could get away with a transformer or two. Maybe this is the case for many of you but the fact is, the supply can be a wallet killer.

Always amazed at the amount of money enthusiasts sink into their audiophile equipment it’s no wonder that R3 000.00 for a toroidal doesn’t raise an eyebrow.

I have a well known brand amplifier which uses a 500VA toroidal. I bought the amplifier for about R1800.00 second hand, the transformer alone costs about that in the shops. Why are these items so freakin’ expensive?

Project supplies can be sourced from:

cell phone chargers
UPS transformers
computer supplies
discarded equipment too expensive to repair (if the transformer is still OK)
knowing someone in the repair business

The Computer power supply

Of the five listed above the computer supply ranks top. Modern supplies emphasise higher currents from the 12V rail (+rail that is) and not the +5V rail of the older supplies. One can get +3.3/+5V and +12V rails plus some negative rails as well if you are lucky albeit lower current.

Computer supplies can often be fixed easily or found for under R200.00.

Students of electronics often modify these power supplies for high supply voltages, some even to feed tube plates (anodes). I have done this but be aware that switched mode power supplies in computers are very sophisticated, glitches on trigger pulses will blow switching transistors plus driving circuits.

Cell phone chargers

Cell phone chargers can be ideal for Arduino circuits but be aware of current limitations. A better bet is for the supplies to external hard drive enclosures, some of them capable of feeding +12V at 2A.

Often owners of UPS replacing the entire unit instead of just dud batteries. These transformers are anything from 7V-0-7V 400VA and up. Companies often write off UPS on their assets register when faulty or expired.

CNC machines and 3D printers

On a more expensive note but still cheaper than many alternatives are the switching supplies used in CNC and 3D machines. These supplies more often than not are manufactured in China and their prices have dropped substantially. 500VA supplies can be picked up for about R500.00, steep bust still cheaper than an EI or toroid.

Series connecting

Users of computer supplies often remove ground off one or more supplies and series them to get multiples of +12V at 20A. This is not an advisable practice unless you are 100% sure of what you are doing. At least one case must be grounded. It’s been done but electrical engineers do not advise this practice.

Parallel connecting

A trick to get more power out of a supply is to parallel them. This can be inadvisable if the supplies are not designed to work together, as in a redundant setup but can be done if loading is done through a Schottky barrier diode which has a small voltage drop or in some cases, a small value resistance.

Buck, flyback/boost

Another great workaround for variable supplies is to purchase a boost, buck or buck-boost regulator, often up to 150W for under R100.00. Be careful of the power limitations though – read the instructions. A popular and reliable boost circuit can be found at DIY Electronics in Durban, a 150W SM booster. These deliver about 4A at +35V out. A better alternative for more power is the +10 to +60V supply from Banggood below.

+10V to +60V 600W boosting circuit. — +10V to +80V 600W boosting circuit obtainable at BangGood. Input voltage +10 to +60V.

All of the boost circuits have power out set by “Effective power P = input voltage V * 10A” so if you require 50V out from a car battery (12V) and max input current is 15A (for this device) you are limited to 180W out or 3.6A. Two car batteries in series will therefore allow up to 7A draw. Therefore current output is restricted by your input voltage.

Of course if you have your own computer supply which can deliver +12V at 15A the above two switching boosters can realise a lot more probabilities without having to modify the supply.

The buck/boost circuit configurations shown above are common and using a low voltage supply such as that from the +12V out of a computer supply is a great testing ground. The boost or flyback variety is extremely common but in both cases above limits power output to about 150W. Thereafter one needs to experiment with half-bridge or full-bridge configurations for higher outputs.

Sticking to Linear or using the 317 or 78xx/79xx regulators

The typical linear type power supply plus regulator is still very much in use and is usually the experimenter’s first set up. Regulators from the 78xx (plus supply regulation) or the 79xx (negative supply regulation) is very much in demand. A variable regulator such as the 317 is another very popular choice. These regulators can in turn be used to drive high current series components for higher power outputs.

Typical spec sheet from Texas Instruments: LM340/LM78xx

CRT TV or Monitors

The intent of this article is not to cover what has already been covered but to allow the experimenter be creative in where he or she can get cheaper alternatives to supply power to their projects. For the more advanced, a now very cheap source is from old television receivers or CRT monitors which use SMPS or “chopper supply” to power all the circuits including the EHT tripler stage. As the CRT uses supply rails akin to that of our vacuum tube audio circuits they make a very practical and cheaper alternative to EI transformers.

In CRT devices often the set is discarded when the tube runs low in emission. The newer ranges almost all have easily accessible parts and use one transistor to switch the transformer. The supply board can be cut out from the circuitry one does not need. (note opto-coupler feedback circuit – see under further reading).

Microwave ovens

Often found to be used in spot welders or high current low voltage supplies, the HV secondary now becoming the primary to mains. My own experience has been it’s an absolute bastard to strip out the laminations and in the only two I got right looked rather primitive when rewound. A working transformer will have about 20V on the “new” secondary. (the secondary is usually about 2000V 0.5A). If not too familiar with microwave ovens leave them alone. Absolutely lethal in the wrong hands!

Further Reading:

Electronics Tutorials: Switched mode power supply

SMPS US – Topologies

Repair Guide – Opto-isolators