🚀 go-pugleaf

gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR) writes:

>>    Given an incandescent lamp with a dimmer switch. The lamp is on.
>>...
>>    switch up, no matter how slow, the light always comes on rather
>>    suddenly and much brighter than in the reverse process .

>Yes, but it has to do with the circuit triggering the triac.
...

I disagreed with this but I am a great believer in letting experiment
settle these issues so last night I pulled the dimmer out of our dinning
room wall and wired it up on the kitchen table with a couple of
meters to see what was going on.

Sadly, it turns out I was wrong, at least partly, and that Geoff is
probably right - it is the hysteresis in the triac that is doing it.
(I hate being wrong)

My explanation was that it arises from the non-linearity of the light
bulb resistance with temperature.  Had I been correct then as I
turned the control from zero up to the point where the bulb comes on
I would have seen the current slowly increasing and at the point where
the bulb comes on the current would drop.  Instead the current remained
at the "quiescent" value until the light came on at which point it
suddenly jumped to values shown below.

Similarly going down when the bulb goes out the current should have
risen slighly but it dropped to about 1 Ma.  Interestingly immediately
after turning the control to zero the current drops to about 1 Ma but
then as time goes on (~30 seconds) the current rises to several Ma.
On one occasion the control was turned to zero and I was sitting there
wondering what I would do next when the bulb came on, full power.  I
turned the control just slightly off zero and it went out again.  Would
Geoff have an explanation for what happened there?

Following are the numbers that I got - if anyone's interested.  I have
arranged them in order of decreasing bulb resistance.

			current	bulb volts	calculated bulb
			(Ma)			resistance
	40 Watt bulb

full on			335	115		343
intermediate value	289	60 		207
up from zero - bulb on	237	35		147
down - bulb goes off	~120	~6		50
all the way down	1-4	~0		.03 (measured)

	100 Watt bulb

full on			850	115		135
intermediate		717	60		84
up from zero - bulb on	590	36		61
down - bulb goes off	~280	~5		17
all the way down	1-5	~0		.013 (measure)

The reason I said I was only partly wrong is because the bulb 
resistance is extremely non-linear and I have seen this effect
using a rheostat but that was quite a number of years ago.
Unfortunately I didn't have a rheostat of suitable value but I
am going to try to round one up.  In the meantime perhaps someone
who does have a rheostat of the right value could try it and let
us know what happens.

My expectation is that one would see the effect but it would be
much less pronounced than it is with the triac.

	Jim