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

 

[tcmtech]

One really simple and cheap option is to use a common multi volt lamp ballast as a voltage matching transformer or to simply rewind the secondary to work at a lower output voltage.

By physical design they are very easy to modify for use in other applications and come in many different VA ratings too.

 

[jgeesen]

Sounds easy - if you know what u r doing. Which I don't when it come to customizing transformers. Conceptually I kinda know how they work, but have never tweaked one.

 

[colin55]

The simplest is to use a capacitor and current-limiting resistor.

 

[Diver300]

The simplest is to use a capacitor and current-limiting resistor.

View attachment 52327

That circuit will make the lights flicker. If you are working on rotating machinery that can make it look stationary.

If you add a bridge rectifier, and run the LEDs as one series string from the output of the rectifier, with a 100 uF capacitor in parallel with the output of the rectifier, there will be minimal flicker.

A 1 Meg resistor in parallel with the 0.22 uF is a good idea to discharge the live and neutral when the circuit is unplugged.

Alternatively, find a 415V input transformer. It will work fine on 277V but it will just have a proportionally lower output voltage.

 

[Diver300]

BLOCK|STEU63/24|TRANSFORMER, 63VA, 230/400V, 2 | Farnell United Kingdom

That transformer has a 400 V input option. Feed that with 277 V and you will get about 9 V out.

 

[colin55]

You don't need a transformenr, a capacitor is all you need. He said he needs a small, cheap, solution.

 

[jgeesen]

I was planing on 12vdc at the LEDs. My assumption with this is that the LEDs will not flicker. I was expecting to put the resistor between the 277v line and the input to the LED driver - not wire the LED up to the AC source.
I just got 2 of these for $10. I assume I will need to know the exact LED load to size a resistor. The led strip lights can be cut along their length in 3 LED increments.
**broken link removed**

 

[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.

 

[carbonzit]

Don't give up yet!

I believe this will work for you:

**broken link removed**

(I haven't assigned any values: that's left as an ... exercise for the reader.)

The rectifier is needed since I've added a filter capacitor to eliminate any possible flicker if this is used around moving machinery. I don't know how many LEDs or what type you're using, so you need to figure out how to wire them up (series/parallel), then calculate the total voltage and current for the bunch of them.

 

[colin55]

The circuit above will not work.

Strip lights have series/parallel LEDs. You really need series LEDs.

This is basically what you need:

**broken link removed**

 

[carbonzit]

The circuit above will not work.

Strip lights have series/parallel LEDs. You really need series LEDs.

This is basically what you need:

**broken link removed**

Why won't my circuit work? Mine uses a single rectifier diode; yours uses a bridge rectifier. BFD.

I left the question of how the multiple LEDs were to be wired open intentionally, you'll notice. Why do you think this won't work with some combination of series and parallel LEDs? No reason it can't.

 

[colin55]

What is discharging the capacitor????

As you get more LEDs in parallel, the capacitor has to be increased in size. 12v strip lights have only 3 or 4 LEDs in series.

 

[Diver300]

Don't give up yet!

I believe this will work for you:

**broken link removed**

(I haven't assigned any values: that's left as an ... exercise for the reader.)

The rectifier is needed since I've added a filter capacitor to eliminate any possible flicker if this is used around moving machinery. I don't know how many LEDs or what type you're using, so you need to figure out how to wire them up (series/parallel), then calculate the total voltage and current for the bunch of them.

That circuit won't work. You need a reverse path for the current in the current limiting capacitor. That is why it is best to use a bridge rectifier. Alternatively a reverse diode that bypasses the LEDs, but that means you need more current as half of it is lost, and a bigger filter capacitor as the time between forward current is much larger.

Also the LEDs should be in series. If they are in parallel you will need more current and a larger current limiting capacitor, and the LED brightness will be uneven.

A 1 MΩ resistor in parallel with the current limiting capacitor is a good idea to discharge it when the circuit is disconnected, although any load on the same wires will also do that.

Edit. Colin's circuit is very good. A small resistor in series with the 0.68µF capacitor can help to limit the inrush current when the circuit is turned on.

 

[jgeesen]

I am at nearly a complete loss. I assume no one likes the using the 12v LED drivers to drive the 12v strip lights.
I was just hoping to get the 277v down a little bit before the LED drivers.
**broken link removed**
And I got one of these
**broken link removed**

 

[Diver300]

Those would probably be fine on their own.

I know 277 V is pushing it a bit, but not much.

To cut the voltage down with a resistor is possible, but it is difficult to know what resistor to use, as the power supply is likely to have very non-linear characteristics.

I've just thought of a reasonable way of getting a voltage in the right range by connecting a small transformer as an auto transformer.

Get one of these:- MYRRA|44060|TRANSFORMER, 1VA 230V 2X 24V | Farnell United Kingdom

The data sheet is https://www.farnell.com/datasheets/484366.pdf

Connect pin 1 to pin 6
Connect pin 7 to pin 9

If you put the 277 V across pins 5 and 10, you will get about 230 V from pins 1 and 5

It will not be isolated.

It does not matter that the transformer is only rated at 1 VA. The output windings, which are rated at 2x24V are therefore rated at about 20 mA. Your 3 W load is taking about 13 mA at 230 V, so the transformer should be fine. You can use a larger transformer if you want, but you need to get the connections correct, and the one that I have suggested has the phases of all the windings shown on the data sheet. Those are the black dots on the winding ends.

 

[carbonzit]

What is discharging the capacitor????

Aha; OK, fixed that:

**broken link removed**

As you get more LEDs in parallel, the capacitor has to be increased in size. 12v strip lights have only 3 or 4 LEDs in series.

So make the capacitor bigger. No problemo.

 

[Mr RB]

That's a good fix Carbonzit, and that's a good working circuit for times when you need the LEDs to be low potential (near neutral potential).

But the bridge rectifier circuit gives double the LED current for the same capacitor, and it's the high-voltage non-polarised cap that is the largest and most expensive component. And then the LEDs will be "fully" live, although for both of these circuits it would be very wise to assume all parts are at lethal potential.

Speaking of that, on the other forum (allaboutcircuits) they have banned all talk of offline mains powered devices and this thread would have been shut down by now by the moderators as being "unsafe".

 

[jgeesen]

I intend on only having 12vdc leaving the high voltage box.

 

[carbonzit]

That's a good fix Carbonzit, and that's a good working circuit for times when you need the LEDs to be low potential (near neutral potential).

But the bridge rectifier circuit gives double the LED current for the same capacitor, and it's the high-voltage non-polarised cap that is the largest and most expensive component.

Yes, but even so, the capacitor is going to be pretty cheap. Like this one (0.68 µF 630V metal poly from DigiKey): only $1.05, so I don't see what the big deal is.

And I wouldn't sweat the potential-lethal-voltage aspect of such a unit, IF and only if it were very well constructed and totally enclosed in a metal box with the proper connectors used. Otherwise, not so much ...