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Low Power, Low Cost Display Power Sources—In small

line operated systems using TTL/MSI and LED or incan-

descent displays, a significant portion of the total dc power

is consumed to drive the displays. Since it is irrelevant

whether displays are driven from unfiltered dc or pulsed dc

(at fast rates), a dual power system can be used that

makes better utilization of transformer rms ratings. The

system utilizes a full wave rectified but unsmoothed dc volt-

age to provide the displays with 120 Hz pulsed power while

the reset of the system is driven by a conventional dc

power circuit. The frequency of 120 Hz is high enough to

avoid display flicker problems. The main advantages of this

system are:

• Reduced transformer rating

• Much smaller smoothing capacitor

• Increased LED light output due to pulsed operation

With the standard capacitor filter circuit, the rms current

(full wave) loading of the transformer is approximately

twice the dc output. Most commercial transformer manu-

facturers rate transformers with capacitive input filters as

follows:

Full Wave Bridge Rectifier Circuit

Transformer rms current

= 1.8 x dc current required

Full Wave Center Tapped Rectifier Circuit

Transformer rms current

= 1.2 x dc current required

Therefore, the removal of a large portion of the filtered dc

current requirement (display power) substantially reduces

the transformer loading.

There are two basic approaches. First (Figure 3) is the

direct full wave rectified unregulated supply to power the

displays. The '74 decoder driver constant current feature

maintains the specified segment current after the LED

diode drop and 0.5V saturation voltage has been reached

(

≅2.2V). Care must be exercised not to exceed the '74

power ratings and the maximum voltage that the decoder

driver sees in both the “on” and “off” modes.

The second approach (Figure 4) uses a 3-terminal voltage

regulator such as the 7805 to provide dc pulsed power to

the display with the peak dc voltage limited to

+5.0V. This

approach allows easier system thermal management by

heat sinking the regulator rather than the display or display

drivers. When this power source is used with an intensity

modulation scheme or with a multiplexed display system,

the frequencies must be chosen such that they do not beat

with the 120 Hz full wave rectified power frequency.

FIGURE 3. Direct Unregulated Display Supply

FIGURE 4. Pulsed Regulated Display Supply