LED Dimming

Do you have a schematic picture of the current driver you can post here?

I don't have a schematic, I wish I did... Here are a couple pictures of the actual driver if it helps...

Is there anything written on the boards (part number or something; anything)?

Do you know who made the unit (or a model number).

What colour are the LEDs?

It looks as though the unit is split into three sections: an AC-DC power supply and two current drivers.

I think I'd be tempted to forget about the current drivers: just tap off the DC voltage, and, stick a resistor in series with each string of LEDs; adjust the resistor value until you get about 15 mA per string.

If the LEDs look like bog standard ones then 40 mA is way over the top.

Sounds like the LEDs are in series, given the 40mA current observed.

Voltage control is very poor for LED intensity, but there are many frequency modulation methods that work fine.

A simple oscillator circuit (555 or modern equivalent) and MOSFET with a variable resistor to create a chopper circuit should be OK. (This can create electronic noise that might affect other nearby devices.)

There are other circuits that use threshold detection in AC signal to change power duty cycle, but reverse voltage can be an issue at the levels that you are talking and cause complete failure.

Thanks, do you have any example circuits of this? I don't think the noise will be an issue as these are about 10' away from everything else. I would take any help because I really don't want to burn these things up.... The input is standard US 110 volts 60 Hz.

Thanks again!

Sorry, no circuits available. Left that industry 4 years ago, but we were manufacturing around 10,000 lamps per day with typically 4 LEDs per lamp. (Automotive)

Oh the good old days.

I know they are out there but don't have any on me using the latest techniques and components.

Try an internet search, you may be able to find an existing product you could use.

? FM 450V

Try using a variable transformer to reduce the primary input voltage. Depending on how the power supply is designed it might work.

If you were to connect both strings in series you would halve the voltage across each LED which will reduce their light output considerably, but this is a fixed reduction, and dependent on the characteristics of the LEDs, may be less than half brightness, a suitable series resistor would do similar but would need to be at least 50 watt rated.

You don't mention what the input supply is, but assuming that it is AC, and depending on the input current demands, then a simple Triac wave chopper dimmer or a Variac on the input may work for you. Be aware that colour rendition will be altered with the above forms of dimming

The wave chopper could produce a bit of interference whereas the Variac would not.

Both will reduce the input voltage which may be a problem if the driver has some sort of voltage regulation that relies on the input being at a certain value.

A pulse width modulated controller on the output would produce a better result as it won't greatly affect the colour of the LEDs due to the fact that they will still draw full current when the pulse is on and no current when it is off, but due to the voltages involved, an off the shelf product will likely not be available.

The switching rate of the controller would need to be kept above about 0.25kHz to eliminate visible flicker.

do you have the LED/ or LED data sheet?

How many of each one , or all are the same or equal.?

For power supply

maybe you can ask

http://www.oksolar.com/led/led_color_chart.htm

LEDs are just what the name says: light emitting diodes. Like all diodes, they have an exponential I-V curve around their turn-on threshold. As others have noted, that makes them difficult to control through a programmed voltage.

So no problem, just use a programmed current source instead. You need a circuit to sense the current, and ramp the voltage supply up or down to maintain the desired current level. A search on programmed current sources will turn up any number of ways to do that. I'd be tempted to use an Arduino SBC. That's gross overkill, unless you want to be able to control the lights remotely, but it's cheap.

If you don't want to bother with all that, you could also just put a variable resister in series with the string, and turn the pot to adjust the light level. You'll lose some efficiency that way, but you can't beat it for simplicity.

Could you not break the 144 diodes down into banks of 12, 24 or 48 or any other multiple of 144 so that each bank can be switched on independently. The intensity would then be determined by how many of the leds were on.

Try this site: www.dealnay.com

I tried to insert the image but was unable to do so.

http://www.ams.com/eng/led-driver/AS1119 is likely not what you need -- un-/re-soldering the leds may damage them especially if the pins are too short ... however you may "manufacture" a "cascading PCB" , cut/grind LED-mounts to separate, solder the "cascade" - or design your own voltage regulator knowing the I to Intensity curve of your LED-s -- most regulated supply designs are not efficient over the voluntary output -- you need to pay attention on the circuit from ~240V to V.LED(s)

how about a polarized filter you can adjust manually or with the stepper and controller