|Re: Like Dead HP 41C Calculators - Series Diode|
Message #4 Posted by Andres Rodriguez (Argentina) on 13 May 2006, 3:37 p.m.,
in response to message #3 by jbssm
If I recall correctly (from memory), the HP41C CPU board has a diode connected in series with the batteries, so reversing the batteries would have no effect (neither good nor bad).
I suppose such a diode is also present in halfnut models, but am not sure about it. I also think (not 100% sure) that this same device protects the VBat connection on the I/O ports.
A series diode causes a voltage drop (and, in fact, a little power loss). Such drop is dependent on the semiconductor element used. Most electronic devices made in the last decades are silicon (Si) based. Si has a forward voltage drop of about 0.7 volt.
However, the particular diode in the abovementioned circuit is a germanium (Ge) diode, which were more common in the 50s and the 60s. Germanium has only a 0.3 Volt forward drop, so HP showed once more a "thinking outside the current fad" attitude, protecting the calculator from misplaced batteries but also losing as little battery power as possible.
Germanium is far from obsolete, but you will not usually find this kind of detail and care in a consumer device.