Genicom 3600 - Manual (Page 159)

Appendix
A-5
A
Power Fail Detection
The power fail detection circuitry consists of IC U31, diodes D7, D8, D9, and
D11 and various resistors and capacitors. Diodes D7 and D8 rectify the
incoming 12 VAC, and R101 and R107 act as a divider. D9 is a zener diode used
to prevent the input on U31 from exceeding +5V. D11 is a 1.235V reference.
When the voltage at pin 3 of U31 exceeds 1.235V, the output is pulled up by a
10K resistor, R183. R104 adds some hysterisis to help filter out noise or
brownouts. C44 and C49 are also for filtering. C46 and R107 set up a time
constant of 1.5 60 Hz cycles to filter out brownouts. The hold time of the +5V
varies with line voltage and the state of the printer. The printer exceeds 20 ms
at low line and when not printing.
Battery
The battery is used to back up the static RAM for font, format, and UDC
storage. R34 limits the reverse current into the battery, a safety requirement for
lithium batteries. The battery itself is a 3.6V lithium battery with 1.75 Ah. Diode
D4 is a low leakage schottky diode used to isolate the battery voltage from
VCC of the circuit card when the power is off, but it has minimal voltage drop
to RAMVCC when power is on. Transistor Q2 and IC U5 cause the RAM chip
select to be held in a high (inactive) state when /RESET is low to prevent
inadvertent writes to RAM during power up or power down.
Battery Life Calculations
Battery capacity = 1.75 Ah
RAM Data Retention current = 20
µ
A maximum, 1
µ
A typical
With printer off all the time and worse case scenario:
Battery life = 1.75 Ah / 20
µ
A = 87,500 hours = 10 years
With printer off all the time and typical case:
Battery life = 1.75 Ah / 1
µ
A = 1,750,000 hours = 200 years
Reset
/RESET is generated by MKC34064 under voltage sensing circuit, U29. The
output of U29 is low for any supply voltage less than 4.6V.