I need a small, cheap 277vac to 12vdc LED driver

[jgeesen]

I need to find an inexpensive way to run some 12vdc LED work lighting on 480v 3ph equipment. Problem is - all the small transformers/drivers take from 80ish to 240ish vac. I am only looking for about 1w. My idea is for the lights to come on with the machine - wired into one of the 3ph legs.
Is it kosher to just throw a resistor inline to the driver to knock the voltage down withing the range of the driver? Any drawbacks with doing it this way? What resistor would I need? Any better ideas to make this happen under $10/machine? I'll probably use SMD 5050 or 3528 strip lights for the LEDs. I need to get rid of some "shadowy" areas on ,mostly, old presses. Most of the machines do not have any other transformers for control voltage on them, so tapping off one of those is not an option.
Thanx for any and all help with this project.
John

 

[Resqueline]

Many (Chinese) LED bulbs simply employ a capacitor as a lossless ballast/resistor driving one (or two) 160V series-string of LED's.
In your case a 20mA 48V (16 LED) string would be sufficient to get 1W. What's the mains frequency in your place?
But this scheme of course puts the whole LED string at mains potential and could potentially be unsafe for your application.

I would not dare use just a resistor in series with a normal driver (even if it might work).
Perhaps some of the available drivers could be candidates for a modification though.

 

[(*steve*)]

When you say 480V 3 phase, do you mean something like the fairly standard 3 phase with around 240 volts between phase and neutral, but with around 440V between phases?

If so, the voltage between a phase and neutral will probably be low enough to deploy a LED bulb rated for 240V (as long as they allow excursions to around 270V)

 

[jgeesen]

I get about 278v to ground. The highest rated, inexpensive, transformer I could find was about 265v in.

 

[jgeesen]

It is 60Hz. I always thought resistors were okay to use on a constant load.

 

[(*steve*)]

If you imagine that you have a string of LEDs operating from 12V that require 1W of power, then you will need a resistor that will dissipate in excess of 20 watts.

*THAT* is why a resistor is not a great option.

A 265V transformer would be fine (even a lower voltage like 240V should me OK) but your output voltage will be a little higher than it is nominally rated at.

Using a transformer also has the benefit of meaning your 12V LED lights are not operating at mains potential and thus wiring to them need not take into account the risk of death should you come into contact with them.

 

[Resqueline]

Is that 265V transformer iron or electronic?
Dropping 13V doesn't sound like much and provided the load really is constant (at every instant) it could be done, but there are several considerations.
Is the 480V (277V) a nominal standard? If so the 277V could go as high as 305V and as low as 252V. This variation needs to be included in the calculations.
Specifying a resistor for an electronic LED driver is not easy due to its constant power draw (and hence its negative "resistance").
Specifying a resistor for an iron transformer is not easy due to its inductive/reactive nature.
An iron transformer made for 240V 50Hz can be used at 288V 60Hz. Of course its output will then be 20% higher than spec also.

 

[jgeesen]

I just got 2 of these for $10.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220698644179&ssPageName=ADME:L:COSI:US:1123
These are what I had in mind to power the LEDs and be fed off one leg of the 3ph 460v 60Hz power already at the machine. I thought it might be best to scrub off a few volts to get within range of the transformer. If 265v is close enough to 277v for input, I doubt a mild overdrive on the LEDs would harm them.

 

[Resqueline]

Those are electronic transformers/ LED drivers and will not overdrive the LED's under any circumstances but they will go bang with an overvoltage input.
How much of an overvoltage reserve they might have I can't say, you'll just have to try and see, some may survive and some may not.
(Doing some math it seems the input voltage spec' is actually 100V - 15% to 230V + 15%.)
I can't even say (with any certainty) how the input current will follow the input voltage and actual LED current/voltage.
It seems to be a constant current driver with an approximate 12V design target but I can't say how high the output voltage will go if underloaded.

 

[(*steve*)]

My recommendation would be to now get a small transformer (a *real* transformer) and wire it up as an autotransformer so that you can tap off a lower voltage.

A transformer with a 240V primary and a 40V or higher secondary would be fine.

Essentially you then place the windings in series (they have to be phased correctly) and take your voltage to these devices from across the 240V winding.

This article tells you how to do it.

When I said transformer -- I meant a real transformer. Not a switch-mode LED driver. I realise it is confusing that something that is not a transformer can be described as a "transformer"

 

[jgeesen]

This project may not be as simple as I was expecting. I am out of my depth with designing a circuit to pull this off. I may just wire the driver up and see what happens. I just need to figure out how many LEDs to keep in the strip lights.