|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.