Nuts and Volts - November 2013

regulator as it did not require a heatsink. It puts out less heat and it's more efficient for a wide voltage input range. I ran the board for six months in varying temperature conditions (hot and cold) and not once did the component overheat, oscillate, or behave erratically. The switcher is also a drop-in equivalent to the LM78XX series in the TO220 case, so I did not have to change any components in my PCB program.

2. The two resistors in parallal were done for two reasons. The first is heat dissipation due to the voltage drop from 24 VDC to power the LEDs ( 2 VDC at 20 mA). This enables a complete parts list using only 1/4W resistors. Also note that the input voltage is 24 VDC — fairly common for industrial applications. Hobbyist applications typically run at 12 VDC. By removing one of the two resistors where they are paralleled and changing the output relay coil to a 12 VDC model, the board can now run on 12 VDC without any other modifications. So, the extra components add some design flexibility.

3. I have had a unit powered down for over three months and I just recently powered it back up. There was no loss in data and the unit picked right back up where it left off. This is a result of two things: the low current consumption of the time keeper chip and the storage capacity of the super capacitor. Based on some quick math, the data in the time keeper chip should be good for a year of storage. This is based on a RC time constant factoring in the chip voltage and current consumption combined with the 1.5 F capacitor.