HP-55 charger issues

[per]

I recently mannaged to clean out the HP-55 I recently got, and I have done some testing.

It works perfectly off a regular +4.5V 3LR12 alkaline battery, but the AC power adapter has issues. When running the calculator off the AC adapter, it glitches all the time, allthough it seems to work briefly at random spots of the screen at random times. This would suggest either insufficient current or insuficcient voltage.

This is most problably a charger issue as it works just fine with the battery. I'll eventually rebuild the battery pack with a new rechargable battery, but I'm a little unsure what recent batteries which are safe to use with the old charger. Any suggestions?

 

[g4ugm]

I don't think these were designed to work off the charger on its own. The batteries stabalize the load, although it could be a break in the charger leads.

If the HP-55 is the same as the HP-45 I have I am pretty sure there is no cut-out in the charger. It just trickle charges. Most moderen cells are NiMh. Looking on the web the safe trickle charge rate for NiMH is 0.03C whereas for the NiCads originally fitted its 0.1C. So as long as you fit cells that have three times the capacity of originals you will be in the safe charge area of Nimh.

however:-

http://www.national.com/appinfo/power/files/f7.pdf

has more info. For authenticity I would try and find tagged Nicads although on checking the guy I normally get stuff from:-

http://www.strikalite.co.uk/prodcat_type/2/ALL/0/NiMH_Tagged_Rechargeable_Batteries.html

only appears to have NiMh with tags...

Dave

 

[per]

I'm pretty sure it's much the same as the HP-45. The HP-55 is in itself just a HP-45 with limited programming abilities and a timer. Hardware-wise, that's three added RAM chips, one added ROM chip, and a quartz crystall for accurate timing.

Looking at the charger (82010A), it provides +4V DC @ 150mA on one line (Expected load is therefore 27 Ohm), and +4.2V DC @ 50mA on the other line (Load = 84 Ohm). I haven't meassured which of them charges the batteries, but I guess it is the +4V one.

When I get access to a multimeter, I will set up some resistors matching the loads and meassure the voltages.