"mike" wrote in message
...
On 2/27/2013 12:09 PM, John Fields wrote:
On Wed, 27 Feb 2013 16:17:04 -0000, "Ian Field"
wrote:



"John wrote in message
...

My request was about having you describe the dynamics of a
light-bulb-in-series-with-a-battery current limiter which, no matter
how loudly or how long you squeal excuses about why you can't, you
can't, of course.

The bulb trick isn't that great because of its pronounced PTC
characteristic - as the filament voltage decreases the current doesn't
decrease in due proportion.

---
???

If a you want the current to change linearly with voltage, then a
simple resistor will do the trick so, obviously, you don't understand
how the bulb trick works.
---

I did try to dumb it down for you - I do hope I didn't use too many big
words for you!

---
The words are no problem but it seems, for you, the theory _is_.

Would you like me to explain how the bulb trick works in terms even
you could understand?

I understand how the light bulb works, but I'm very interested in
hearing YOUR explanation.


When the battery terminal voltage is low and the drop across the filament is
high; the bulb does its best job of current limiting - just when high
charging current would be no bad thing. As the battery terminal voltage
approaches or exceeds the required voltage, the inherent PTC characteristic
of the filament means as it cools its resistance falls, its much less
effective as a current limiter - just when the (possibly overcharging)
battery needs it most.

The bulb as a current limiter is less than ideal for battery charging - but
better than nothing if you don't have a float capable charger to hand.

I didn't bother to dumb it down for JF - you can draw pictures for him as a
homework excercise.