Build Your Own Line Conditioning Unit
by Dennis Feucht
In the design or repair of line-operated equipment, it is often useful
to have a single unit that does line conditioning. This conditioning consists
mainly of two functions, line isolation and line voltage 
adjustment. The first function
is accomplished by a line isolation transformer, usually designed for 50
Hz or 60 Hz with a 1:1 turns-ratio. The other function of providing variable
line voltage is accomplished with a Variac: a variable autotransformer with
a movable tap operated by a knob. The autotransformer is a single-tapped
winding with the tap as output and the input applied across the winding.
A simple line-conditioning unit (LCU) is diagrammed in Figure 1.
This
is useful in testing line-operated circuits for both low- and high-line
ends of the voltage range. Protection from over-current is provided by a
fuse in series with the front-panel power switch. I did not use the over-voltage
tap on the isolation transformer because it is so under-powered, to maximize
output current at the full-scale voltage. The unit, now decades old, is
pictured in Figure 2.
The
underpowered component in this LCU is the isolation transformer. It was
obtained in-house at Tektronix and is good for only up to 27 W. Its only
advantage is that it fits in a small enclosure that can be placed on a workbench.
Its dimensions are shown in Figure
3, to provide some intuition for the size of a 25-W, 60-Hz transformer
for those unfamiliar with power components.
The variac is a 200-W component. The sizing of the design is hardly uniform but illustrates what can happen when a simple project must be thrown together in short order. The unit has served me well for 30 years, though it is only good for repairing or testing low-power loads. Yet many loads have been adequately powered by it. And it is certainly a low-cost LCU.
250-W Line Conditioning Unit
The
next LCU is similar but capable of 250 W. The circuit diagram is shown in
Figure 4.
It is very similar in design, though it has a digital panel meter (DPM) with its own little line-powered supply, input line filtering, a varistor to prevent damage to the digital panel meter from line surges, a somewhat more substantial isolation transformer, and provision for unregulated dc voltage output. (The ac output, not indicated, is taken across the bridge rectifier input terminals. The Variac has an additional tap near the top end (pin 5) to which the input is applied. This allows the tapped output to be moved beyond the input (up to the end of the winding terminated at pin 4) to a somewhat higher voltage than that of the line. The transformer primary windings also have an additional tap for low-line systems, but the ends of the windings were wired instead for 120 Vac.
On the secondary side, the DPDT switch provides additional versatility. In one position it places the secondary windings in series for 120 Vac out across both of them. The primary windings are wired in series, requiring that the secondary windings also be placed in series for a 1:1 turns ratio overall. The DPST switch allows the secondary windings to be paralleled for twice the output current at up to half the maximum Variac voltage.
The versatility of the LCU is again increased by adding a bridge rectifier and large electrolytic capacitance for an unregulated dc output, suitable for supplying motor drives or regulating converters under development or repair. The dc output voltage reading on the front panel reduces bench clutter by one DMM and provides a convenient readout near the Variac knob. The NTC thermistor is an inrush current limiter. For higher-power rectifier-capacitor ac-to-dc converters, at power-on, the inrush current through the rectifiers into the uncharged capacitors can be substantial because the larger-power components have less parasitic resistance. The power-on current surge can even destroy components, most likely the diode bridge. The inrush limiter has a relatively large resistance at ambient temperature. As it conducts current, it self-heats, reducing its resistance to a low equilibrium value.
1000-W Line Conditioning Unit
For
real power projects, I built a 1000-W LCU, pictured in Figure 5, also with an unregulated dc output. It uses
a circuit breaker for the over-current protection device which doubles as
a front-panel switch. The entire unit was mounted on a piece of plywood,
painted blue, and placed onto the "enclosure": a Rubbermaid three-shelf
cart.
The cart is not only a convenient way to move around a large and heavy piece of equipment, but its additional shelves provide space for auxiliary equipment such as a 1000-W load (also mounted on a wood board) made from 10 100-W incandescent lamps. With breaker-switch and Variac mounted in front, the cart can be rolled up to the main workbench at right angles, and its controls reached handily on the middle shelf of the cart.
The components were obtained from Rick Daniels' store(s), Electronics
Surplus in Cleveland, who receives a fair amount of surplus industrial equipment
and removes LCU-type components. It is unlikely that anyone within driving
distance of downtown Cleveland, Ohio, USA would regret a visit to Electronics
Surplus, but be sure to visit the warehouse down by the river. It is loaded
with surplus test equipment at very reasonable prices -- and also LCU components,
which are at the downtown store too.
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