Newsgroups: sci.electronics.design,sci.electronics.components,sci.electronics.misc
Subject: Re: Touch dimmer circuit
References:  <[email protected]>   
> John Woodgate  writes:
> > Please explain why the dimmer must handle an inductive load. The
> > resistance of the lamp(s), reflected through the transformer must
> > dominate the impedance seen by the dimmer. Until all the lamps have
> > failed, anyway!
>
Tomi Holger Engdahl  writes:
> You are right that the resistance of the lamp(s), reflected through the
> transformer dominate the impedance seen by the dimmer. But in practical
> non-ideal transformers there is always some part of the inductance
> of the transformer itself seen as addition to the lamp(s) resistance.
> And the cheap transformers used in many lighting applications are
> anythign but ideal.

Here is some extra information to my previous article to support
my view of the inductance of the halogen lamp loads.

I just did some measurements on the 12V halogen lamp systems to
figure out how much inductance is then the load when there are
lamps on the secondary side of the transformer connected. I did
the measurements so that I used an RLC meter to measure the inductance
seen at the primaty side when the secondary side is in near short circuit
(cold 12V halogen lamp in it). This measurement closely resembles
the transformer leakage inductance measurement methods where
secondary of transformer is shorted for measurements.

I did this measurement with two 12V halogen lighting systems. 
First one was 3 * 20W halogen lamp system 
which included traditional 230V AC to 12V AC 60W transformer
(Chinese cheap model). Other one was a small 20W halogen lamp
bought from Ikea with small 20W wall transformer. For comparision
I also measeured the inductance seen from line side on PAR 36
pinspot light with brand name "Eurolyte"
(it uses 6V 30W halogen lamp and has built-in transformer).

Measurement results were the following for the case that light
bulb is in secondary and the inductance is measured in primary
using RLC meter:

3x20W halogen ligt system transformer: L = 0.18 H
20W halogen lamp:                      L = 0.26 H
PAR36 pinspot:                         L = 39 mH

For comaprision I did some measurements of inductances without
the load on the transformer secondary. I did this measurement
with the same RLC meter. For both halogen transformers the
inductance were in order of many henries. The primary
resistance of 60W transfomer was around 40 ohms and 
for 20W transformer aroun 80 ohms.


The conclusion is that those transformers used with halogen lamps
have very high inductance when there is no load on output (was expected).
And they still have a considerable inductance left when the
outputs are loaded with the lamps. I would say that around
a load 0.2 H inductance seem quite inductive for me. 

Anyway this was a quick test and may not be the most accurate,
but should give a figure of the inductance of halogen lamp
with transformers. 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/