🚀 go-pugleaf

usenet@crd.ge.com (Required for NNTP) writes:

>>In article <1020006@hpdmd48.boi.hp.com> rrr@hpdmd48.boi.hp.com (Rudi Rynders) 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 .

>This is not a physics problem, it is an electrical engineering problem.
>If you were the incandescent lamp with a Variac, it would turn off and on
>at the same point, smoothly and without hysteresis. The reason it behaves
>as described by Rudi is that residential dimmers do not use variable
>transformers, but do use a semiconductor switch called a Triac to apply
>the 60Hz power to the lamp for variable portions of the cycle. That is, at
>half power, full voltage is applied for half of each 60Hz cycle. The
>triggering circuit for this Triac has considerable hysteresis, which
>accounts for the behavior documented above.

I beg to differ.  The reason that you don't observe the effect with
a variac is that a variac is a low impedance (constant voltage) source
and the bulb can draw as much current as it needs when the filament is
cold (and has a low resistance).

The triac on the other hand is a high impedance source and with a 
variable resistance, as represented by a lightbulb at different
temperatures, will create a bistable system. I can state with some
confidence that it has nothing to do with the hysteresis of the
triac for two reasons;

1) One observes the same effect if the dimmer is a rheostat.

2) the triac hysteresis is repeated 60 times a second but the bulb
   can stay in one of its two states essentially forever.

	Jim

>In article <1020006@hpdmd48.boi.hp.com> rrr@hpdmd48.boi.hp.com (Rudi Rynders) 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 .
>
>>This is not a physics problem, it is an electrical engineering problem.

Yes, but it has to do with the circuit triggering the triac.
The manufacturer was cheap.  Very simplified, the triac is
triggered from a `diac', a bidirectional 5-layer breakdown diode,
a variable resistor, and a capacitor.  The R-C circuit charges from
the line, and when the voltage exceeds the breakdown voltage of the
diac, the capacitor discharges through the gate of the triac and
the triac turns on.

If the diac has not triggered recently, the capacitor holds a
residual charge from the previous half cycle, opposite in polarity
to the voltage necessary to trigger.  To overcome this charge
(which is refilled every half cycle) it is necessary to advance
the control (decrease the resistance) such that the current cycle
can charge the capacitor enough to trigger the diac->triac pair.

However, once the diac has fired, there is much less residual
charge on the capacitor, so now the capacitor can charge to the
trigger voltage much sooner in each cycle, so the lamp is now
on brightly.

The cheapest cure is to use an asymmetrical trigger diac.  This
costs a few cents more than the symmetrical ones.

See a triac manufacturer's application notes for more info.

I hope this is not too confusing.
	geoff steckel (gwes@wjh12.harvard.EDU)
			(...!husc6!wjh12!omnivore!gws)
Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line.
This posting is entirely the author's responsibility.

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