Dimmer switch repeat failures...need advice

. I have a restaurant customer with 3 50 watt, 12vdc transformer lights... two cables stretched across the ceiling with the low voltage side of the transformer wires clamped to
each.
I opened the transformer box, it had about a 6" dia toroidal iron core wound transformer, and also a good sized aray of electronics, some chips, resistors, and a fuse etc.. about 15 components...seems to be a cross between an electronic and a wound transformer. No amperage or voltage ratings on the unit lable.
The old dimmer switches were simply wired into the 110 feeding the transformer at a wall box... line 1 and out with a ground to the conduit, no neutral.
Those had been lasting about two years, The last two I installed lasted two weeks, one got ultra hot before sticking to full on (a 1000 watt jobbie), the other just stuck to full on (a 600 watt rated unit straight dimmer no mention of transformer applications)... the load measures 3.1 amps at full load...and down to half an amp at full dim.
(with only 3 50 watt bulbs I would have expected 2 amps at the most at full brightness)
So today I tried a dimmer made specifically for an electronic transformer...rated at 300 watts..at full bright and 3.1 amps it heats the mounting plate to maybe 130 degees or so... so I locked it to 2 amps.. the face plate runs maybe 110 degrees now... This unit called for a neutral connection with a separate ground.
the restaurant wiring however (built in 1989, a nice place) has only line feeds into this wall switch box, no ground wire, no neutrals...but its in conduit... I connected the neutral to the conduit, same with the ground wire... there is no current flow in either the neutral or the ground.
(a 600 watt dimmer for an electronic transformer was not available locally).
can someone clue me in on whats up?
Phil Scott
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First, I hate low voltage lighting. It is always more than a pain in the ass considering what you can do with line voltage.
Me thinks that the dimmer and transformer should be matched. It is possible that power is fed to the transformer and the dimmer acts as an regulator for the lights. Which means you need to replace like for like. You need to find out the manufacture of the dimmer you removed.
It is possible that the transformer/electronics are toast.
Why in gods name would you connect an neutral to an piece of conduit?
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ceiling
clamped to

toroidal
15
a
unit
feeding
ground
sticking
full
at
electronic
amps
so I

degrees
place)
wire,
neutral to

current
pain in the ass

It is possible

an regulator for

You need to find

conduit?
Since neutral is grounded at the transformer, and the conduit is bonded to service box and that to ground...and the diagram called for a connected neutral...and all I had was the conduit... I connected to that..(which I realize is illegal) but its tempory until I figure out what the hell is going on.
I think your advice is correct.. the components need be matched...Im am not the original guy on this job, the previous dimmers were home depot stocked items, those lasted a year or two each by reports,,, the ones I bought lasted only weeks however.
I should call the mfgr.
One other question.. the bulbs in the three low voltage lights are rated for 50 watts... total thats 150 watts... so with say 90% minimum transformer efficiency.. the system should only draw 175 watts or so... yet it draws 3.1 amps at 120 volts.... about 360 watts....
that seems like it would point to a bad transformer / electronics package ..no? And one more questiong if you dont mind. Why to they need electronics on the lighting transformer ...why not just a straight wound coil by itself?
Phil Scott

9/6/2004
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sounds to me like it is a regulated DC supply. the benefit here would be that if the load decreases (i.e. some lamps burn out) the remaining lamps do not get brighter and consequently shorten their lives.
some regulated DC supplies have something called a "crowbar circuit". this senses over voltage conditions and clamps the output. i would not expect to find one in a lighting circuit but i never expected to vote for a republican either. often the crowbar is a SCR that works once or twice then shorts internally. its supposed to blow a fuse.
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aray of

about
and
the
I
to
of
amps at

amps
3.1
so...
with a

ground
not
than a

matched.
as
like.
toast.
piece of

conduit
diagram
illegal)
on.
previous
or
lights
with
dont
itself?
here would be

remaining lamps do

circuit". this

would not expect to

for a republican

shorts internally.

This low voltage transformer lighting thing is a lot more complex than I had assumed.
thanks for your reply.
Phil Scott

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Many LV lighting fixtures today use a switch mode electronic x-formers instead of a regular transformer. They do so because the SMPS is more compact than an ordinary transformer and, more importantly, silicon is cheaper than copper. These electronic transformers output an alternating current that's in the 100KHz range and bares no resemblance to a sine wave. The are sort of like a SMPS minus the hi-speed rectifier diodes and PWM regulator (there are no SCR crowbar circuits in these things either). This being the case, your clamp-on ammeter - which was designed to measure sinusoidal current - will not give you an accurate reading.
Paul
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12vdc
the
wires
dia
etc..
electronic
on
110
with a

two
before
stuck
mention
2
and
110
nice
the
no
was
more
acts
going
be
year
weeks
voltage
should
120
you
x-formers
compact than an

cheaper than copper.

that's in the

of like a SMPS

are no SCR

ammeter - which

an accurate

I have been around for a long time, but had no clue about all these complexities with transoformer lighting... I still cant fathom why a simple 24 vac transformer at 60hz wouldnt work to run a string of parallel lights...its a constant 24 volts unless you add too many lights, then the transformer would get overloaded and a fuze would be fit to blow.
Obviously there are other issues.... but I sure as hell do not understand what they can possibly be.
As it happens I have another customer, billion dollar house over looking the ocean...and wants a lot of landscape lighting...untill this conversation developed I was just going buy a large 240v/24vac transformer and run light strings out from that on relays etc and put it all on a photo electric cell to start the lights and timer to shut them off at midnight or whatever.
So maybe there flaws in that plan... I have no clue at all whats up with the need for voltage and frequency control in this application.... I am looking for more insight on the over all scene with transformer lighting.
Thanks
Phil Scott

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yes as long as the load is constant. all transformers have internal resistance (R int)
then the

and the voltage drop across the internal resistance has increased and the output voltage has decreased

use ohms law and standard wire gage tables.

low voltage (i e quartz lamps) tend to cost more then standard incadesents.
and frequency control in

use theater type dimmer packs to control banks of low voltage transformers. companys like this http://www.nsicorp.com/ offer quality long lasting products.
I am looking for more insight on the

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I
internal
to
increased and the

not
house
going
out
all
standard incadesents.

voltage transformers.

Thanks...thats what I needed...I was reading a book on these issues...dimming the bulbs makes them last a lot longer, and I want dim light anyway..so I will buy higher wattage lamps and run them at 50% or greater dimming for a reliable system.
Im also going to solder the landscape lighting splices.
Phil Scott

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some theatre lighting systems have a "dim" or "preheat" setting. once you power them on they turn on just enough for a faint glow. the theory is that when you run the show there is not a shock to a cold filament thereby extending bulb life.
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these
and I

and
setting. once you

theory is that

filament thereby

another good idea...thanks... I could wire that in with a timer and resistors for start up. for my landscape lighting job.
Phil Scott

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Phil Scott wrote:

Are you sure this has a d.c. output? I have something that sounds quite similar (with a toroidal xmfr), but it runs the lamps with 12 vac.

This might be a dimmer circuit, or a protection circuit for shorts across the 12 lines (usually bare with these installations).

Its possible that the non-linear characteristics of the transformer plus electronics module is interfering with the operation of your dimmer module.

By this, you mean that there is a hot feeding in and a switched hot feeding back out through the same conduit, right? If so, this is OK for a plain old switch, but not correct for a dimmer (or other device) which requires a neutral.

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Leno
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And put the transformers and dimmers where a buzzing noise won't annoy anyone.
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understands
customer
commercial
the
will
to
socket
operate
is
be
could
I
won't annoy

Good idea... what about running 12vac wire just stapled to the woodwork in a restuarant for instance... it seems you could toast the wire easily enough, short it across and cause a fire... what is accepted practice on running that wire...do you know?
Phil Scott

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You'll have to get Code comments from others. If I ever staple any kind of wire I use the staples that have the little stop mechanisms (tabs) halfway up their legs. The staples can only be driven in about half way. They will not pinch the wire. You may want more staples (for looks) this way as the wire will droop in between staples. A surface mount wiring system (Wiremold) may be usable but probably costs about 10000 times as much as a couple staples. The kinds of wire I have ever used staples for are, e.g. coax cable for television etc., not for any power application.
Make sure you don't do any work that requires an electrician and/or permit if you don't have an electrician and/or permit.
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Phil Scott wrote:

There are a few articles in the NEC that describe what is allowed for low voltage circuits, energy limited or signaling circuits and low voltage wiring specific to low voltage lighting.
Keep in mind that the two things that will 'burn down' a building are excessive conductor temperature in contact with combustible materials and arcing/overheating caused by a fault on the circuit. Appropriate overcurrent protection and sizing wire to the loads will minimize these problems.
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