can you run the HP 55 on AC adaptor without batteries?

[Steve Simpkin]

(03-01-2021 01:18 AM)teenix Wrote:  I think the 97 can be used on charger only but the calculator will not function properly and the printer, card reader will not work.

The owners manual mentions to have the batteries in with the charger, but only for improper display etc.

The service manual states that the 97 won't be damaged on charger only, but won't work properly.

The 97 has a voltage regulator setup on the main circuit board. However, it only regulates over voltage from the charger and stops this voltage exceeding 6V. The charger itself is only capable of supplying a reasonably small current (150mA or so I think) and the output voltage will drop as more current is dragged from it. This is the principle that the voltage regulator relies on which basicly shorts the recharger with a 47 ohm resistor. An external supply should not exceed 5.5V or so if it can supply a large current or this resistor will turn to smoke.

The other question is that the AC adaptor is just that, it outputs only AC and the main circuit does not have much in the way of a smoothing capacitor so the power supply will be fed with a DC ripple voltage from the full wave rectifiers. The battery probably substitutes for a suitable filter capacitor.

Either way, I'd prefer a regulated 5V external power supply without batteries, or keep the battery in whilst in use.

cheers

Tony

Hi Tony, There is a nice thread with good information about trying to use the HP-97 on AC power without batteries installed. Someone mentioned they thought doing that and trying to use the printer had damaged a nylon gear. In any event the HP-97 does not seem to operate reliably without batteries installed. I can't quite remember who started the thread but the name sounds vaguely familiar...
https://www.hpmuseum.org/forum/thread-14...tteries%22