2 Jul
2004
2 Jul
'04
11:25 a.m.
Here's where circuit details caught me: my main
charge circuit for the
NiCads was limited to about 5.1 volts - perfect for a 5-volt RAM card.
However, NiCads should have a "trickle" charge of 0.1-1% of capacity to keep
them healthy: for a 7Ah battery, that's 7mA (drawn from a 12-volt supply via
a resistor). This is important, since the RAM card, idle, draws 0.1mA.
Without the battery to absorb the trickle current, the RAM voltage goes
(way) out of spec. (Fortunately, the Superboard had a completely independent
regulator...)
Greater men than you have fallen into this trap!. The HP2xC and HP3xC
calculators (Woodstock and Spice series with continuous memory) have much
the same problem. The charger gives out about 12V off-load to charge a
2.5V (2 cell) NiCd pack. When the machine is running, this battery drives
a DC-DC converter to produce +6.2V and -12V for the PMOS logic chips in a
Woodstock or +6V for the (NMOS?) chips in a Spice.
There's also a RAM backup supply that comes straight from the battery +ve
terminal via a diode (sometimes!) and resistor to the Vss pin of the RAM.
If the battery pack is open-circuit or missing and you connect the
charger with the machine turned _off_ then the RAM gets about 12V and
promptly expires, sometimes taking other chips -- the ACT CPU and ROM --
with it. If the machine is turned on, the DC-DC converter normally draws
enough current to pull the voltage down, but I wouldn't bet on it. I
normally charge my battery packs in the HP 'reserve power packs'
(standalone chargers), just to protect said custom RAMs.
-tony