|Re: Info on Hp-41 battery charger needed|
Message #3 Posted by Steve on 3 Dec 1999, 7:25 p.m.,
in response to message #2 by Dave
The 3VA rating is not really important. A higher power supply would be acceptable.
There are apparantly several versions of the HP41 battery pack. I have seen the circuit diagram of two versions.
Some (the earlier versions) use the batteries to regulate the voltage (and with no or dead batteries the voltage rises to about 14V).
Later versions do not suffer from this problem.
I have an earlier version and I have fitted a 6V Zener so that future battery problems will not cause calculator death (I assume 14VDC is not good for the calculator).
N.B. VDC is Volts DC, VAC is Volts AC (common abbreviation). Also VA is pretty much equivalent to W in this context - so the HP charger is 3W, or 375mA at 8VAC (this is not the whole story, and because of the way regulated DC supplies tend to draw current from AC supplies, 200mA would be the most you could expect from a DC supply). Note also that 200mA is WAY more than the 16 mA that the HP41 nicad pack needs.
Also note. The charger is INSIDE the nicad pack. The thing that plugs into the wall is just a dumb transformer.
All versions have the feature that the charger is incapable of supplying the current needed for the card reader. The batteries provide this extra current. So if you have bad batteries in the nicad pack, you may not be able to use the card reader.
for the versions of the nicad pack I've seen, a DC source of power is just as acceptable a an AC one (and polarity is not important).
However the DC voltage you'll want to use (without nicads in the pack) will be around 6 to 8 volts. With the later versions of the nicad pack, greater voltages (up to 12 volts or more) could be used.
One version is (without batteries) essentially a 6V regulator in series with a diode, in parrallel with a 580 ohm resistor and two diodes. (this is the old and nasty version that I wouldn't want to put more than 8V DC into without a shunt regulator)
The newer version (there ma be more versions) is a 12V regulator in series with a diode and a 370 ohm resistor, shunted with a 6V zener. This version can safely handle input voltages up to around 30V (from memory) but the power diddipation in the regulator would rise. 12VDC would be fine.
As to the advantages and disadvantages of both.
The latter verion is current limited by virtue of the series resistor. nicads aside, the pack can be short circuited without damage (remember this is WITHOUT NICADS). Powered from DC (wide range of voltages) is OK with ot without nicads, but the current is limited - 14VDC is probably the optimum voltage if you're using DC.
The earlier version is not current limited, but by virtue of the charging circuit, relies on the nicads for voltage regulation. Without an auxillary shunt regulator (or nicads) input voltages above about 8VDC will give you excessive output voltage. The lack of current limiting means that shorting the pack (WITHOUT NICADS) could damage it. The internal regulator is current and thermally limited, but you don't usually want to push these things.
Fitting an earlier version with a shunt regulator (1W 6V Zener) means that you can drive it from A DC source of 10 to 14VDC and be able to supply higher currents for short periods (thermally limited). I would not recommend an input voltage above 10VDC if there are nicads in the pack as the charge current is quite high already. There's no point in making things worse (the HP charger is equivalent to about 14VDC input voltage).
NB even if you do these things to increase the current capability, you're still not going to be able to power the card reader without nicads present. But the wand is a definite possibility...
How can 8VAC be equivalent to 14VDC?
The HP chargers are poorly regulated (i.e. not regulated) and when not under load their voltage rises. The AC is rectified by a bridge rectifier which produced a DC voltage of 8 * 1.414 - 1.2 V. Now that comes out to 10VDC, but I measure mine at 14VDC, indicating the RMS input voltage at being more like 10.7VAC. (I just went and measures it and mine is 12.58VAC unloaded (that's 16.5VDC at the cap, or the equivalent of 17.7VDC at the input socket). Under load this falls fairly quickly, with even the 14mA or so charge current into the nicads dropping it significantly.
WARNING: the old chargers charge the nicads at about 14mA, the newer ones at 16mA (which is also their peak current at 6V) which is WAY TOO MUCH for those tiny nicads. So if you've got nicads in the pack, DONT LEAVE THEM ON CHARGE TOO LONG. 16mA is about a 4 hour charge rate.
Sorry if this is more information than you wanted :-)
p.s. Anyone who can supply me with circuit diagrams of other HP41 nicad chargers, or even rumors of their existance, will receive from me at least 5 minutes of unending thanks :-)