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I used a 220- resistor because that is easily found. The actual current owing through the LED/resistor and sunk by the PIC microcontroller is then I V/R 4.3 V/220 19.5 mA
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Therefore, when the LED is on, the total current passing through the PIC microcontroller is 1.8 mA typical intrinsic current plus 19.5 mA of LED current. For the power supply for this circuit, I probably would derate this by 50 percent in real life, meaning that I would have to provide a 30-mA, 5-V power supply for this circuit. I use this value for the total current used by the application when the LED is on. I realize that I am not including the current through the momentary on switch, but this will be 10 A (according to Ohm s law) and really doesn t change the total current required by the application in any appreciable manner. With the application designed and the total current estimated, it is now time to do an empirical check on what the PIC microcontroller actually requires. To do this experiment, I used the same circuit as the previous experiment, except that I broke the ground connection between the PIC microcontroller and ground and wired in my digital multimeter set on the milliamp reading, as shown in Fig. 20.3 (the bill of materials is shown in Table 20.3). With this circuit, I can now check the actual current drawn by the ledon circuit. When I rst turned on the power to this circuit (LED off), I found that the current passing from the PIC microcontroller to ground was 1.4 mA. This is a bit lower than the typical value quoted by Microchip but only off by 400 A. This may have been a
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4 MHz
Figure 20.3 Checking the current draw in the simple LED application by measuring the current from Vss to ground.
SOME BASIC FUNCTIONS
TABLE 20.3 CIRCUIT PART
BILL OF MATERIALS FOR CURRENT-CHECK
DESCRIPTION
16F85 LED 1N914 4 MHz 0.1 10-k 220 Momentary on Misc.
PIC16F84 04/P Red LED Diode to replace LED (see text) 4-MHz ceramic resonator with internal capacitors 0.1 Tantalum 10 k , 1/4 W 220 , 1/4 W
Momentary on pushbutton switch Breadboard, wiring, +5-V supply
part variation, but it is still within the minimum derating tolerance I set for PIC microcontroller circuits (which is 25 percent) and at a level that is dif cult to measure with most low-cost handheld digital multimeters. When I turned on the LED, I found that the current jumped to 14.1 mA which is 28 percent lower than what I calculated above. This was the rst time that I had really checked the current drawn by an LED connected to a PIC microcontroller, and I was surprised at the difference which was much larger than what I expected. Looking at the operation of the circuit, I found that one of my basic assumptions was incorrect. When I assumed that the LED had a 0.7-V drop, I was wrong. The actual voltage drop across the LED was 2.12 V, and the actual drop across the resistor was 2.6 V, which changes the current calculations dramatically. Going back to Ohm s law for the resistor, the current owing through the resistor/LED combination is actually 11.8 mA. Adding 11.8 mA to the observed intrinsic current of 1.4 mA, the total current expected is really 13.2 mA a difference of only 6 percent from the observed current of 14.1 mA. When I replaced the LED with a 1N914 silicon diode, I found that the actual current from the PIC microcontroller through ground jumped to 18.7 mA, which is very close to my original calculation. You may be asking yourself why do I still want to derate this value by up to 100 percent The answer lies in the ef ciency and stability of most power-supply circuits. As the load approaches the rated maximum current supplied by a power supply, the power supply may sag or have extra ripple that will affect the performance of the application (i.e., it will be intermittent). As I have said elsewhere, power-supply problems are probably the most dif cult to nd and x. Thus, by derating the circuit s current requirements, I have added a safety margin for the power supply that will help it to work properly, even when the application is drawing its full load.
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