Re: HP67 AC adapter logic Message #2 Posted by Bernhard on 11 Mar 2006, 1:11 p.m., in response to message #1 by Hal
The "bigger" voltage comes from a current source to charge the battery. You may want to measure its current to ground (not the voltage). Expect something around 50 mA, but in any case, less than 100 mA. "Ground" is the middle pin.
The smaller voltage runs the calculator. Beware that many of those old chargers have a bad electrolytic capacitor which may cause excess ripple and calculator malfunction. You need an oscilloscope to check that. Ripple under full load should not exceed a few 100 millivolts. Full load can be simulated with a 25 Ohms / 0.5 Watts resistor. Voltage at 144 mA load current should be around 3.6 volts.
Beware that the higher voltage from the current source might damage the calculator if no battery is inserted (or the contacts are bad). There was/is a dispute among HP users on the risk level. My own experiments indicate that somehow the HP67 Card Reader IC (which is supposed to get damaged) clamps the voltage to a harmless level but gets quite warm, so prolonged plug-in without battery might cause damage. The HP65 card reader IC didn't have that clamp and the manual explicitely warns about operating it on the charger without battery inserted. If, however, you switch the calculator ON and then insert the charger into the wall outlet, the current source is too weak to develop dangerous voltages (if it is intact). This may be a safe way to operate HP67 without battery from a charger - but never switch it off, just pull the charger from the wall outlet. If you switch it off with the charger still in the outlet, the potentially dangerous higher voltage will appear on the card reader IC.
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