HP 97 VERY hot charger

[Dave Shaffer]

I acquired my Dad's HP97 when I cleaned out my parents' duplex a few months ago.

I have since managed to get it to work with the battery (which may even be the almost 30-year old original!) installed after a lengthy charging time (a few days). It even worked, at least briefly, with the charger disconnected.

However, a couple of days ago, I removed the battery to take a look at the battery compartment. After putting it all back together, I plugged in the charger. A day or two later, I tried to turn it on, and nothing happened (no display). I then felt the charger and it was almost too hot to touch. I immediately unplugged it from the wall and the '97 to let it cool off, fearing that it might be fried.

However, if I check the output voltage, I get about 10 V AC, which seems reasonable when the charger (an 82059B) is not loaded. I also checked the battery pack, and get only about 1.35 V DC - clearly not right for a 4-cell pack.

Any ideas what happened? I supposed it is possible the battery pack is now more-or-less short-circuited and putting a heavy current load on the charger, but why that should have happened just when I took it out of the calculator briefly seems a bit odd. I haven't yet tried measuring the charging current. If the battery pack is shorted, I wouldn't expect the battery pack to show ANY voltage. If the battery had just become loose in its holder when I pulled it out, I wouldn't expect any charging current to flow, and the supply should at best be only a bit warm.

Any other thoughts or tests to try, other than checking the current from the charger? Given the age of the battery pack, I suppose in any case it's time to replace the NiCADs!

[Katie Wasserman]

(10-16-2014 01:21 AM)Dave Frederickson Wrote:  The battery pack is bad. It's possible for one of the four sub-C cells in the pack to short, yet the pack will still output some voltage. While the cells are easily replaced, it might be even easier to buy an aftermarket battery off eBay. Just search for HP 97 battery. Looks like $15 is the going price.

I agree. Nicad cells almost always fail by shorting out, the charger is not current limited so it's getting really hot becasue it's looking at almost a dead short (only one of the 4 cells is accepting any current). Mostly likely your 97 and charger will work fine with a new battery pack.

[brouhaha]

(10-16-2014 02:10 AM)Katie Wasserman Wrote:  the charger is not current limited so it's getting really hot becasue it's looking at almost a dead short

Actually the charger IS current limited. UL will not list it otherwise. However, it is current limited by the design of the transformer, not by additional components.

For wall warts, UL examines the design documentation, and tests with a dead short for some number of hours, and they must not exceed a temperature limit. (I no longer recall the specific hours and temperature.) This is achieved either by current limiting inherent in the transformer design, or additional current-limiting components, or an electrical or thermal fuse.

For UL listing, the wall-wart doesn't actually have to be usable for its intended purpose after the testing, so if a fuse opens, that satisfies the test.

Some higher-quality wall warts are designed so that the transformer is adequately current-limited, AND there is a thermal fuse, as a belt-and-suspenders sort of thing, but cheap ones don't do both since that isn't required. I don't know whether any of the HP calculator wall-warts have a thermal fuse, because they don't normally get hot enough with a shorted output to open such a fuse. I think the only way to determine whether a thermal fuse is present would be to disassemble or X-ray the transformer.

[Katie Wasserman]

(10-16-2014 03:30 AM)brouhaha Wrote:  

(10-16-2014 02:10 AM)Katie Wasserman Wrote:  the charger is not current limited so it's getting really hot becasue it's looking at almost a dead short

Actually the charger IS current limited. UL will not list it otherwise. However, it is current limited by the design of the transformer, not by additional components.

[/quote]

You're right from a safety standpoint, of course, but I was trying to explain why it got so hot. I meant that the circuit design does not limit the charging current, so it can/will overcharge nicad cells in a pack if some of the other ones have shorted out. Having 4 good cells in the pack is what prevents the transformer from getting too hot normally.

[robert rozee]

if the transformer in the charger develops a shorted turn, then it may still output the expected voltage when unloaded but also get very hot.

try plugging the charger into the mains without the calculator connected, leave running for half an hour (keeping a close eye on it!) and see if it gets hot.

cheers,
rob :-)