|Question and musings (and maybe a warning)|
Message #8 Posted by Steve on 16 July 1999, 10:40 a.m.,
in response to message #7 by Barrie Fletcher
That's interesting, but I can't quite figure out how it's all connected. It seems to suggest a charge current of 60 mA, which is way too high (so I've obviously got something wrong)
Or is the regulated output delivered directly across the nicads? (Horror!)
Can you sketch out the diagram as a bitmap or something and email it to me?
My unit also has a 78L06A but the output appears to be only a little above 4.1v. I'm not sure if it's related to the dead nicads or if the regulator is fried.
I'm going to replace the nicads with a 1K resistor to provide a suitable load and measure the output again.
The unknown capacitor is marked K1M 103, which seems to suggest 0.01 uF
Also interesting is that two if the diodes in the bridge rectifier in mine are different from the others, and appear to have a much higher forward voltage drop. (Interesting but not useful)
OH! I just discovered a fault in the design of this charger. If you partially insert the charger, then the regulated output is connected directly to the batteries. This could result in a rather rapid charge. I'm not sure if the 7806 is current limited (although in common with the 78xx series regulators it probably has a thermal shutdown).
Hmmm some information about the 78L06 and its application in this circuit.
Input voltage 15v, output 6v (so 9 volts dropped across regulator.
Max junction temp 125C (thermal shutdown)
Junction to ambient thermal resistance for TO92 package 230C/W
Assuming 25C ambient then the device can dissipate about 0.43 of a watt continuously without going into shutdown. (and practically a whole lot less because the temperature in the charger will not remain at 25C for long.
That means that it can deliver 47mA. And I'd derate that by 50%. So the charger can deliver about 25mA.
That's probably OK if you're not pulling cards through the card reader continuously :-)
Curiouser and curiouser. I get an 9v output! (and that's with an 9mA load)
Aha! Without the nicads the charger circuit raises the output voltage. Nasty. The regulator was actually putting out 6.3 volts.
This means that this model nicad pack could deliver voltages to the calculator that are dangerously (?) high if the nicads deteriorate to the point that one goes open circuit (or high resistance). So it may not be a good idea to use this model as a battery eliminator if it contains dead nicads.
Yes, with the (dead) batteries replaced the output voltage is about 8.7v I'm glad I didn't use this in my HP41 :-)
I'm not sure where the reading of 4.1v I measured earlier came from. It may have something to do with the regulator being open circuit???
Gee, this is long winded isn't it? Back to real work...