Quote:
Originally Posted by Mike Dabrowski 2000
MR Mik,
I can see by your questions that you need some lessons in basic electronics, and this is not the purpose of this thread. Get a good basic electronics book, and experiment with what you learn, and eventually it will all become clear.
It all gets down to ohms law.
Fully understand that formula and how op amps work, and you will understand how the value of the 4.7 M resistor will effect things.
No shortcut to understanding.
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I did buy a book today! And a bunch of capacitors and other assorted bits and pieces.
Have not read the book yet, but played around with massive hysteresis some more and found it very useful indeed!
Adding a 1000uF electrolytic capacitor between the pos and neg outputs of the 6V DC helped against the on-off oscillations to a degree. But not enough to prevent continuous on-off oscillations with only a 4.7MOhm resistor for hysteresis.
Adding a 100uF electrolytic cap across the PTC made it all a lot worse! Changing the resistance of the simulated PTC strips (a 25 turn 500 Ohm trimpot and another resistor) was just like a smooth dimmer for the LED! I don't understand it, will need to read that book! Maybe I misinterpreted what "across the PTC" means?
Similarly, a 100uF electrolytic cap on the setpoint potentiometer centre tap made things worse, but the "dimmer" function was nowhere near as smooth as with the cap across the PTC strips.
I did not bother trying a cap on the centre tap for the "Warning" setpoint. I like it that it blinks for a while before going solid.
So it was back to trying out lower value resistors for more hysteresis.
I ended up putting a 560KOhm resistor in parallel with the 4M7 resistor, and it works BEAUTIFULLY!
The effect of this massive hysteresis is such that I can still get the shut-down to happen within 10 Ohms PTC resistance (or less if I wanted) of the warning light coming on solidly. About 15 Ohm from the warning light beginning to blink.
And, once the LM324 switches with this amount of hysteresis, it stays switched and that is very good for this particular charger. It should always stay turned off for a minute or so after it has been turned off, to avoid the risk of excessive voltage across the motor-run capacitor. If the sine wave was at one extreme during shut-down, and is at the other extreme during turn-on, then there could be almost 600V potential difference upon turning back on. The resistor across the motor-run cap serves to discharge the cap to avoid this. But it takes time!
And, the best part, the massive hysteresis causes the shut-down to persist past the warning level. Once the charge current has been turned off, the continuing cooling impeller action cools the battery down. The massive hysteresis causes the shut-down part of the circuit to hang on to the shut down state for about -10 Ohm past the warning point.
So the shut-down lamp stays on, and the charge current off, until the temperature is again lower than the warning temperature level set.
Only then will it begin charging again. I think this is very handy and good for the batteries.
With little hysteresis the charger would always turn on again as soon as the temperature has fallen just a little bit, effectively charging the battery as closely as possible to it's set "too hot" point. Large hysteresis gives the battery a breather when it has become too hot and only commences charging again if the temperature falls low enough so that the conditions are favourable for EQ charging.
Here is the schematic:
The problem stopping the charger altogether (yesterday) turned out to be a snapped-off PC-stake, caused by all the wiggling about during installation of the comparator circuit into the somewhat crowded box. It's all working now!
The next step will be to try it out on a real NHW10 battery again, there might be more surprises and challenges ahead...
