HP Forum Archive 07

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HP-97 charger Message #1 Posted by Achim Buerger on 24 Nov 2001, 5:27 a.m. Recently I received a HP-97 without charger and with batteries completely empty. I would be able to build a charger if anyone could tell me which type of power (DC/AC) and which voltage the charger supplies to the calculator. If it supplies DC, I also need the polarity of the connector pins. Achim

Re: HP-97 charger Message #2 Posted by Todd G. on 24 Nov 2001, 8:27 a.m., in response to message #1 by Achim Buerger Building one seems like an awful lot of work, in addition to taking a chance on using the incorrect voltage and frying your machine. Why not just buy a charger for it? They're easy to find, and usually cost less than $10. Todd

Re: HP-97 charger Message #3 Posted by David Smith on 25 Nov 2001, 3:57 p.m., in response to message #2 by Todd G. When rebuilding a HP97 pack I always use the highest capacity cells that I can find (usually 2200maH). The HP97 dumps a pretty good slug of current into the battery pack and it gets rather warm. I have found that the high capacity cells do not get nearly as warm as the original (1100-1400 maH cells) and last for a much longer time... particularly if you use the machine on a desktop and keep it plugged in all the time.

Re: HP-97 charger Message #4 Posted by David Smith on 24 Nov 2001, 4:15 p.m., in response to message #1 by Achim Buerger The 82059 series chargers put out 8 volts AC. Just a simple transformer will do. Building a suitible connector would be the biggest problem... but a couple of Molex style pins and some heat shrink will do in a pinch.

Re: HP-97 charger Message #5 Posted by Tony Duell (UK) on 24 Nov 2001, 8:45 p.m., in response to message #1 by Achim Buerger It's AC (there's a bridge rectifier inside the HP97). The external charger is just a transformer, but it's one with deliberately high internal resistance. The voltage is normally claimed to be 8V on load, but the open-circuit voltage is around 12V from a transformer with an internal resistance of about 11 Ohms (I've measured the I-V characteristics of this charger). And yes, the high internal resistance is critical -- it's partically controls the charging current. The official HP charger shouldn't be too hard to find though -- it's used with a lot of different machines and peripherals (HP91, 92, 95C (!), 97, 97S, 10A, 19C, 41 rechargeable pack, 71, 75C and D, 82143 printer, most HPIL interfaces and peripherals, and so on).

Re: HP-97 charger Message #6 Posted by Achim Buerger on 25 Nov 2001, 12:34 p.m., in response to message #5 by Tony Duell (UK) Thanks for the infos. Maybe I found an answer by myself: the HP-97 service manual states: "The ac adapter/charger is a transformer that drops the line voltage to 12.8 Vac at the input terminals of the calculator." I tried a simple 12V transformer and it charged the batteries without problems. Also the calculator itself can be operated by this source. Achim

Re: HP-97 charger Message #7 Posted by Ion Abraham (New Mexico USA) on 25 Nov 2001, 12:34 p.m., in response to message #1 by Achim Buerger Hello, You will need to get a battery for the 97, which is pretty easy to find, because the sub-C NiCd batteries are still being made. Also, it's easy to find a dead battery pack, because it is used on the 41 printers and peripherals as well, and then you can have it rebuilt easily. I use a 82059D charger with the 97, that is the same charger used for the 41 series, and it fits fine. Remember that you should not run the 97 on the charger WITHOUT the battery pack in place. The battery serves to regulate the voltage. If you really can't find a battery pack or charger, contact me. I might have a spare. Take care, Ion Abraham Albuquerque, New Mexico

Re: HP-97 charger Message #8 Posted by Tony Duell (UK) on 25 Nov 2001, 2:22 p.m., in response to message #7 by Ion Abraham (New Mexico USA) Actually, the Topcats (HP9x series) are designed not to be damaged if the charger is connected without a battery pack in place. There's a circuit (a low value resistor, a power transistor, a small signal transistor, and a few other R's) that detects if the input voltage is too high (defined as more than the Vss line should be) and if so, it loads the power supply to reduce the voltage. The machine will just about run like this (although the printer and card reader don't work). For this to work correctly, though, the transformer must have a fairly high internal resistance, like the original HP adapter does. A random transformer might not be able to be pulled low enough by this circuit. And I don't like trusting this circuit to work -- I feel it's there for an emergency if the battery pack goes open-circuit, but I don't make a habit of connecting chargers to Topcats with no battery in place. I don't know about the 95C either. Maybe the standby supply to the RAM is not properly limited, so connecting a charger without a battery will damage the RAM, like on a 25C/29C, etc. I don't have a 95C, of course, and if I did, I _certainly_ wouldn't risk it. What is safe, though, is to run the machine off a 5V bench supply connected to the battery contacts (no battery in place) -- the one on the card reader side is -ve I think. That's how I normally run Topcats when I am repairing them.

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