need a quick and simple answer for the question i found posted on another website

'Why a dimmer preferred to be used on primary side of transformer?'

Maybe just because it is easier to find 120v dimmers? I know I have one feeding a transformer for 12v string of lights and it seems to work fine.

Just a wild guess ... but think for a moment ... how does a dimmer work? Usually by letting only part of the sine wave through. Where does the rest go? Back into the grid.

Would the unused part of the wave cause additional heating, or other problems, inside the transformer?

IMO, dimmers and transformers are a problem looking for a place to happen.
If you're controlling a resistance type load - incandescent lights and heaters - then perhaps an autotransformer is what you really want.
If you're controlling a motor, then a VFD is called for.

If it's a DC load, then control it AFTER the DC is created. Just my thoughts.

Actually a dimmer simply chops the sine wave at a certain point. The unused part of the wave is simply unused. It doesn't go anywhere any more than the power hitting an open switch goes anywhere,
Chopping up DC is harder than AC and you certainly don't want resistive control if wasting energy is a concern to you.

I haven't seen any issues with having a dimmer in front of a Malibu transformer

Just a FYI, a solid state dimmer cops the sine wave. A reostat is jusr a glorified adjustible risistor. Using a solid state dimmer may cause adnormal operation.

Maybe just because it is easier to find 120v dimmers? I know I have one feeding

a transformer for 12v string of lights and it seems to work fine.



ok, i like this answer, but im looking for more technical, amperagevoltage related info.any thoughts -----
preferably in laymans terms....

Why? A transformer will simply smooth out the spikes and give you an average voltage out. There is nothing uglier than the output of the chopper in a switching power supply and they feed that into a transformer.
I guess I could take a scope to the one I am using and see what the waves really look like in and out. You might get some strange results going into an unregulated supply feeding a radio or audio amp but if this is just going to feed LV incandescent lights I don't see the issue.

Here's my guess: A dimmer is an electronic device that has a voltage drop of maybe 2 volts. On a 12 volt circuit, the max voltage left for the bulbs is 10 volts, or about 90 percent. The same 2 volt drop on a 120 volt circuit leaves 118 volts for the bulbs, or about 98 percent. On a 12 volt circuit the lights would be dimmer.

These 12V lights are typically 50W. If you have only 3 lights you have 150W. On the primary side at 120V, the dimmer has to deal with just over 1A (including the transformer). On the secondary side at 12V, I would guess there's going to be more current than the dimmer can handle.

Andy

Andy and TW may be on to something. Although the current itself might not be a problem (a 50a device is not that more expensive than a 5a one) the power dissipation will be.
The output triac will drop ~.7v (silicon device). Since power is amps times volts and the .7 volt dropped in the triac is fixed, doing this on the 12v side will mean the device has to shed 10 times what it would on the 120v side for the same light output.
I am still curious about what these wave forms look like. When the mob of kids and grandkids go home I will take a look.

A good explanation of how a phase angle control works (with waveforms) is available here:

http://ubasics.com/adam/electronics/doc/phasecon.shtml

The steep wavefronts generated by chopping the sinewave like that generate a LOT of RF noise, which needs to be filtered out to prevent radio interference.

Every incandescent dimmer I have looked at had specific warnings NOT to use it to control inductive loads, such as the primary of a transformer or ballasted HID/fluorescent lighting. Presumably, the fast switching of the current through the load would generate high voltage spikes that will damage the TRIAC. There are similar types of controllers intended for use with motors (fan speed controls) which might be a better choice if you want to control a LV lighting transformer.

The best choice for varying the output of a transformer is to use a variable autotransformer or "Variac" on the input. These produce a variable voltage, clean sine wave output like a rheostat would, but without the terrible efficiency/heat dissipation problems of a resistive control. They are somewhat large and expensive, though.

I usually use Lutron Diva dimmers for ballasted fixtures. I find I'm trying to lay my hands on the $80 one used exclusively on electronic ballasts more often than the model used in magnetic ballasts. I'm guessing the main difference between the two is the frequency they are tuned for.

I have had a customer hire me to remove a pair of Lutron Smart dimmers (incandescent) because the RF they generated interfered with their habit of listening to AM radio in the kitchen.

Okey Dokey, I was wrong
The transformer does reflect the input to the output almost exactly. I guess I should have known that because of the way we couple audio. I am not sure why I thought it would smooth out the signal. ... sorry.

http://esteroriverheights.com/electrical/dimmer_on_transformer.jpg

I guess the next question is, does that really matter if the load is a light bulb? I can't imagine it hurts the transformer in any way.

Oops, I was wrong about the frequency thing too. And I should have known better because I had read this page a while back.

http://www.lutron.com/technical_info/pdf/LutronDimmingBasics.pdf

A good reason not to use an ordinary light dimmer in series with a power transformer is that if the Triac is not conducting symmetrically on both positive and negative swings of the AC sine wave, then a DC component will be created.
As a transformer primary usually has a low resistance, a significant DC current will flow causing heating of the winding as well as core saturation. This in turn reduces inductance of the winding.
It's this principle that is behind saturable reactors used for power control.
The best way to deal with the situation is to use a Variac, or a light dimmer designed for transformer loads, which will be designed not to cause a DC component, as well as incorporating the correct RC network to deal with inductive spikes created.

Where is the DC going to come from ... unless you consider zero volts a DC level? When current is flowing in the triac it is AC although not a pretty sine wave.

Assuming a Triac is not switching symmetrically then the area under the curve will not be the same on both sides of the zero axis. This is what creates a DC component. As an example, say the triac is being triggered at 90 degrees on the positive cycle and 85 degrees on the negative cycle. More current will flow into the load on the negative cycle than on the positive cycle. So, there will be a slight negative DC offset to the AC waveform.
You can prove this by observing such a waveform on an oscilloscope with the input set to DC coupling.

Cheap/poor QC dimmers may be unintentionally providing some rectification to the output signal if the triacs are not providing equal gating for + and - current, just like if you clipped the waveform with a diode, only not as severe. So, instead of getting, say, equivilent to +/-60V, you get +65 & -55V: superimposed 60VAC+5VDC.