Anyone have a line on batteries for the NLS

Tony Duell ard at
Mon Feb 9 14:34:33 CST 2009

> 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!.


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