Modern power supplies are generally divided into two types, linear supplies and switching supplies. Linear supplies may contain a transformer that outputs the required voltage(s) and a rectifier such as a full-wave diode network to produce the desired dc. Additional circuitry provides whatever filtering and/or regulation that is appropriate.
A simple, minimal power supply is derisively termed a wall wart, in reference to the fact that these devices, ubiquitous in today’s homes and offices, occupy wall receptacles. The proper term for these devices is “external power supply.” Because they always consume electricity through the primary even when the load is off, they are also known as electricity vampires. They are always warm to the touch, and a house full of them will consume a perceptible amount of energy. Nonetheless, they are a necessary part of life, powering cordless phones, laptop battery chargers, external hard drives and numerous other everyday items.
That said, new energy efficiency standards for external power supplies are about to take effect. Termed Level VI standards, they dictate more stringent efficiency levels that will probably accelerate the trend using switching power supply designs for efficiency purposes in external supplies.
When a wall wart quits working, the plug-in adapter is the first thing to check as it is prone to failure. A failed unit may or may not still run warm.
The input and output voltages and maximum load in milliamps are printed on the housing. Within these limitations, substitutions are permissible. Also, for oscilloscope demonstrations, a selection of these transformers is valuable to have on hand. Some plug-in adapters contain rectifiers and have a dc output. Details are printed on the housing.
In contrast, large electronic equipment such as desktop computers and TVs invariably have built-in power supplies, outputting several dc voltages with excellent filtering and regulation. From discarded equipment of this type you can salvage a ready-made and serviceable bench-top power supply. But great caution is in order because the capacitors plus distributed capacitance store potentially lethal high-voltage energy long after the set has been disconnected from the power source. It is necessary to bleed out this energy through a suitable resistance before proceeding. Some technicians simply short out the high capacity electrolytics, using a screwdriver or whatever is handy, but that is a bad method because capacitor, screwdriver and technician can be damaged by the abrupt high-voltage arc.
A computer power supply, repurposed as a bench instrument, makes available high-capacity 3.3 V, 5 V, ±12 V. All computer power supplies these days are switching supplies; they regulate these voltages by cycling on and off the load circuitry. As a safety feature, the power supply will not turn on unless it is connected to a computer motherboard, or is made to think it is so connected. The green power-on wire must be connected to ground, one of the black wires, to activate the power supply.
Additionally, computer power supplies typically must see a load to remain on. If a minimum load is not connected, the output voltages will run wild or the power supply will shut down. The solution is to connect a 10-W resistor across the output terminals. This may trigger the need for a cooling fan. What remains is to identify the 20 or so wires and connect them to convenient output terminals.
A top of the line bench power supply, of course, will out-perform this shop-fabricated device. The best units have digital readouts that indicate the amount of current drawn when the power supply is connected to a load. This is an excellent troubleshooting tool. Moreover, a questionable piece of equipment can be gradually powered up so that there is not a catastrophe.
A makeshift instrument won’t have the valuable features that are included in a manufactured unit such as the Tektronix PWS4305 Programmable Power Supply.
When prototyping a new design, individual dc biases can be substituted to find the optimum safe voltage levels that will provide best performance. Similarly, following a repair consisting of component substitution, supply voltage can be slowly ramped up so as to avoid catastrophe if something isn’t right. This is particularly applicable when there has been a blown fuse and the root cause is unknown.
The Tektronix PWS4305 on its front panel has a readout that displays the present status of the instrument, including voltage and current. There is also provision for shutting off the output, which is a good move while adjustments are being made. It is possible to set a current limit from 0 A to the maximum current output of the instrument. That way, if for any reason the impedance of the load drops, current will not exceed the limit. To do this, simply push I-set. Then, use the numeric keypad, up, down, right or left arrows or the Multipurpose knob followed by Enter to set the limit. In like manner a voltage limit can be set. These values can be saved into and recalled from up to 40 memory locations, using Save, Recall and Enter buttons.
Other features can be readily accessed using controls and ports on the rear panel. It is possible to connect the PWS4305 to a remote computer. To do this, load VISA into the computer, using the LabVIEW SignalExpress CD that comes with the power supply.
The software is also available as a free download. Click on Downloads, Manuals and Datasheets under Services and Support at http://www.tek.com/product-support-hub/pws4000-dc-power-supply. Then, connect instrument and computer using a standard USB cable. The computer can then access and control the power supply.