HP-97 power supply


Hi all,

A very simple question: can the HP-97 be run like a classic model (as the HP-67) without the battery pack in place?

Thanks in advance.

Bo Kristoffersen



Not according to the HP-97 manual which has this warning:

"Attempting to operate the HP-97 from the ac line with the battery pack removed may result in wrong or improper displays."

I also dimly recall some earlier postings talking about the voltage regulation role of the battery.

The batteries are pretty easily available because they are used in the HP-41 printers as well, and they can be easily rebuilt with sub-C size NiCd's.


Ion Abraham
Albuquerque, New Mexico


The answer is no; the HP-97 will malfunction, although it won't be damaged. You'll just see a flickering display and the keys won't respond properly.

However, the HP-67 WILL potentially be damaged if you're running it without a battery pack. That is because the card reader circuitry receives the UNFILTERED charger voltage, which (when there's no battery pack in place, hence no load present) can be quite excessive; if memory serves me right, it's 16-18 volts, far in excess the rated supply voltage of the sense amplifier chip. (It is true that you can run all the OTHER classics - HP-35, 45, 70, 80 - from the charger with no battery pack in place, but these have no card reader circuitry that is connected directly to the battery pack/charger line.)



If I recall correctly, the card reader circuitry is more than enough cause to avoid operation from AC adapter without a battery pack in the HP 65, 67 and 97, at least.

Similar considerations against use with AC adapter and without a rechargeable battery pack apply to Woodstock and Spice series (HP 22, 25, 27, 25C, 29C, 31E, 32E, 33E, 33C, 34C, 38C); since the battery has a **critical** role as voltage regulator.

This is NOT to imply models not detailed here are safe to run from AC with batteries, it is always preferable to have a good battery pack and with clean contacts while using a AC adapter.


As far as I know, in the HP-97, the card reader circuitry is NOT directly connected to an unregulated power source, so it's less likely to suffer any harm if you connect a charger without a battery pack present. Indeed, in the HP-97 manual, HP actually states that trying to do this will cause no harm, but the calculator won't operate correctly. (That said, I agree with your recommendation; it's best not to risk damaging irreplaceable chips.)

In HP Classics, the wall adapter actually provides a _regulated_ voltage source in addition to, and separate from, the regulated current source used to charge the batteries. This is why the adapter's connector is a 3-pronged connector. This is why HP classics can be operated without a battery pack present. The HP-65 and 67 are the exceptions, because in these machines the extra circuitry (sense amplifier) is running off not the regulated source but the battery terminals which, when no battery pack is present, are at the full idle voltage of the current-regulated battery charging output.



NEVER connect the AC adapter to a woodstock (HP2x) or spice (HP3xE, HP3xC) model unless there's a good battery pack in place. This particularly applies to the continuous memory versions (Hx25C, HP29C, HP33C, etc). Connecting an AC adapter to one of the latter without a battery pack will damage chips.
Connecting the AC adapter to an Topcat (HP9x) or HPIL printer/tape drive won't do any damage, but the unit will not work unless there's the battery in place.
It is safe (and the machine works correctly) to connect the AC adapter to the classics _without card readers_ (HP35,45,55,70,80). The AC adapter for these models
is in 2 sections -- a constant voltage supply for the logic and a constant current supply to charge the batteries. When you plug in the charger, you disconnect the battery from the logic, and contect the 2 sections of the charger to the appropiate parts. The logic runs off the constant voltage supply independantly of whether the battery is being charged or not.
But the HP65 and HP67 connect the card reader sense amplifier chip to the battery terminals (so as to be able to get enough current to run the card reader motor). It is important to have a good battery in placewhen connected the adapter to one of these models, or the sense amplifier chip gets the ful 18V (or so) output of the charger section. This can cause damage.
The HP71 can be run of its adapter without batteries in place. As can the HP75 -- it will work fine off the AC adapter with no battery pack (in fact this is documented by HP somewhere).

Possibly Related Threads...
Thread Author Replies Views Last Post
  HP Prime Save Power Turn Off Not Working Timothy Roche 12 3,402 10-27-2013, 01:41 PM
Last Post: Michael de Estrada
  Low power warning for HP-15C LE and batteries Nick_S 1 1,135 09-16-2013, 09:34 AM
Last Post: Hans Brueggemann
  HP 9845 option 200 power supply Dave F 2 1,309 05-30-2013, 08:10 AM
Last Post: Dave F
  HP-87 power supply repair philippe_elie 0 769 11-18-2012, 11:12 AM
Last Post: philippe_elie
  HP-48: extract coefficient of power series Thomas Klemm 3 1,245 10-21-2012, 11:42 PM
Last Post: Han
  HP-41CV power problem Don Davis 2 1,031 10-11-2012, 09:13 PM
Last Post: Don Davis
  What power adaptor does an HP-65 use? Robert Morton 3 1,153 09-30-2012, 02:32 PM
Last Post: Kees Bouw
  HP 15c LE low power anomalies Nick_S 21 4,026 03-31-2012, 09:32 AM
Last Post: uhmgawa
  strange classic power supply (pics) Alberto Fenini 4 1,715 01-16-2012, 02:06 PM
Last Post: Ethan Conner
  Very relevant to the HP-15C LE power usage Seth Morabito 1 807 01-06-2012, 10:38 PM
Last Post: htom trites jr

Forum Jump: