I would like to create a dimmable feature for a low-voltage lighting
configuration (several indoor, halogen lights, 20 watt each, run in
series behind a single low-voltage transformer supplying 12v current
at less than 150watts).

Would the best solution be to use a dimmer at the AC level, or a
variable resister at the DC level (behind the transformer).  I would
prefer to use a variable resister at the DC level because it will
yield flexibility by splitting the circuit into multiple runs with a
separate variable resister in front of each run.  For example, say
lights 1-3 are located behind resister "A" ? 4-6 behind resister "B" ?
7-9behind "C", it would be nice to use a potentiometer/rheostat in
front of each of these chains of lights for customized dimming.

If I use a variable resister, can you tell me the best method
(potentiometer or rheostat -- straight resistance or voltage
regulation) to use for wiring the circuit, and which size
potentiometer I should use -- and the best location on the Internet to
buy this type of small, inexpensive electronic accessory?  The local
RadioShack stores have discontinued electronic components and I would
like to find a reliable source for this type of thing, where prices
and shipping are reasonable, and the company is dependable.

Thank you in advance!

---

Clarification of Question by rmckenzie-ga on 05 May 2006 15:14 PDT

You're 100% correct, it is AC power, I apologize.  I had been under
the impression landscape lighting was 12v DC, but that is incorrect as
you suspected.

With this in mind, can inexpensive dimmers like those you mentioned
based on thyristors (from home improvement stores), be inserted on the
12v AC side behind the transformer, or can they only be used on 120v?

Thank you again, I'll mark the question answered following your reply.

Using a potentiometer has the disadvantage that you are going to have
to dissipate a significant amount of power in the resistor. To control
150 watts will require a large and expensive potentiometer. The old
fashioned approach was to use a variable transformer or an
autotransformer. The modern approach is to use AC phase control using
solid state switches called thyristors. That is what is inside one of
the inexpensive dimmers that you buy at the hardware store. These can
control 600 Watts or more in a small box that dissipates very little
power.

You imply that the transformer is followed by a rectifier to produce
DC. My suggestion is to use a wall switch dimmer followed by a
tranformer and then run the lights on low voltage AC. The rectifier
will complicate matters and is not necessary for incandescent lights.

Thank you for the response.  I should have been more specific, I
apologize.  To clarify, yes, I am going from the wall-power AC to a
"Low Voltage Lighting Transformer" that provides 150watts DC.  These
are most often used for garden or "Landscape Lighting" but in this
case it's an indoor lighting application (to see an example of indoor
lighting fixture transformers of this type, see:
http://www.lightingfx.com/cgi-bin/cp-app.cgi?pg=ste_cat&ref=6 ).

These are roughly 20watt 12v DC light fixtures (halogen) that are
already wired with the transformer.  I prefer to keep this, and the
existing light fixtures in place.  Is there any way of "dimming" a
group of fixtures "behind" the transformer on the DC side?

What makes you think that the transformer that you mentioned produces
a DC output? Low Voltage lighting systems typically use 12V AC. If
money is no object, then sure, you can put a specially designed dimmer
on the low voltage side, even if it is DC. Perhaps they even make low
cost dimmers specially designed for 12 VAC lights. Using a variable
resistor is not the right solution, though, unless you can afford the
price, the wasted power, and the space required for it. The only way
that I know you can do it inexpensively is with standard 120 VAC
dimmers and a transformer that is compatible with it.

You could, in priciple, build your own dimmer, but unless someone
makes inexpensive 12 V dimmers, I still think you are better off using
the 120 V dimmers you can buy in the hardware store for about $5.00
each.

I'm not sure I understood relative to the question.  So you are saying
the 120v dimmers will not work on the 12v side of the transformer,
even though it's still AC, is that correct?

Based on what you said, I setup a test by using three low-voltage
lighting transformers (since they each dim separate/relative to each
other, I had to use separate transformers for each to do the dimming
on the 120v side).  I have it connected as follows:

120vac hot/black ... into three (3) x 120v Lutron Dimmers
Each Dimmer ... into a separate 12v Transformer
Each transformer back to 120vac neutral/white

The 12v transformers are electronic, not the large magnetic wound type.

However, this doesn't seem to work.  I've verified the hot/neutral and
everything connected properly, but in a way that I completely do not
understand -- one dimmer will cause another dimmer's lights to go off,
even though that separate dimmer controls that separate transformer. 
Sometimes it works for a few minutes, but then stops, etc.  It acts
very unusual.

Is the answer ... possibly that there is no option to effectively dim
three separate sets of LV lights separate from one another, even if I
do it on the 120v side with separate transformers?

Thanks again for your help.

You are correct, a 120VAC dimmer is not likely to work with a 12VAC circuit.

All bets are off with an electronic transformer. A standard magnetic
transformer should work, but you should check the specifications of
the transformer to verify that it is compatible with a standard
dimmer. The 150W transformer on the web page that you cited above is
specified to work with a standard dimmer.

Hi,
First of all, in order to have dimmer, you need to have a resistive
load. You can not connect a transformer to a dimmer because the result
can be some extreme currents when the triac turns off.

Please take a look at the site bellow to understand how dimmers work:
http://www.epanorama.net/documents/lights/lightdimmer.html

You can make a special dimmer for the transfomer on the high voltage
side, but it is more costly and you should be very carefull making it.

If you want to use a dimmer on your 12V, the schematic is pretty
simple and you can create it with just a few components.

Take a look at that page and I bet you find the answer to your
question. Or else, send me a message and I send you the schematic of a
simple 12V AC dimmer.

Best regards,
/Farhad