9 Feb
2009
9 Feb
'09
12:34 p.m.
I can't speak to the NLS scope specifically, but
it may be worth noting: in
some equipment with rechargeable batteries the battery is an integral part of
the power supply, providing both filtering and voltage limiting. Charging
circuits don't need filter caps when the battery is fulfilling that
function, so filter caps get left out of the design. An absent or open battery
and the equipment gets pulsating DC.
Worse than that, as you mention in passing, in some designs the battery
acts as a shunt regulator and stabilises the votlage on one of the main
supply lines. A number of classic computers used a 4-cell NiCd pack as
the supply _and regulator_ for the nominal 5V line. The Epson HX20
certainly does, I think the PX8 does too. There _may_ be a protection
circuit to limit the voltage if the battery goes open-circuit, but it's
best not to depend on this.
Chargers also may run at a much higher voltage than the battery. The battery
electrochemistry together with series R losses in the charger pulls V down to target.
HP calculators from the 70's are an example of this. The display goes
This depends -- a lot -- on the HP calculator. The 'classic series'
_without card readers *35, 45, 55, 70, 80) have a mains adapter which is
a spearate 4.2V voltage-regualted supply to run the calculator and a 55mA
constant-current supply to charge the NiCd pack, These machines work fine
-- and safely -- on the adapter with no batterty fitted.
The 65 and 67 (with card readers) run the card reader chip off the
battery directly. With these, plugging the adapter in without a battery
to clamp the voltage can daamge the card reader sense amplidier chip.
This is more of a problem with the 65 -- IIRC the chip was redesigned for
the 67 and is a lot more tolerant of overvoltage.
The Woodstock (20 series) and Spice (30 seires) use a 2-cell NiCd pack
and a simple charger. Tghe battery here acts as a smoothing and votlage
limiting deviee./ Without it the calculator diesn't work. Worse than
that, on -C (continuous memory) models, certainly Woodstocks, the voltage
from the charger with the machine turned off (and thus
drawing little
current) is still supplied to the RAM chip and is high enough to
either
damage the latter (STO/RCL/program storage doesn't work) or damage the
ACT (processor chip)
The Topcats (90 series) have a circuit in them to load the charger if the
battery terminal voltage tries to exceed the Vss (logic supply) rail.
This generally allows the machine to run from the charger with no battery
installed, but the printer and card reader (97) won't work. Note that
this cicuit will try to pull the battery voltage down to 0 if the power
converter circuit fails (e.g. if the oscillator transistor goes
open-circuit)m and gets hot and bothered doing so. One to watch for.
Since this circuit is after the powr switch, I suspect the (very rare)
95C has the same problem with damage to the RAM chips if powered from the
adapter with no battery fitted. I don't have a 95C, and when I borrowed
one to invetigate I certain;y didn't try this!.
-tony