Constant current LED dimming

[SaleB]

Hello again,

I have a new problem and need some suggestions. I have bought a few Chinese led down-lights. Any one of them is rated at 12W and constructed from series connection of 12x 1W leds. They are powered with a 310mA constant current driver delivering 27-42V DC. Seller states that the driver does not respond well to triac dimming solutions on primary side.

So, I have been thinking to use some pwm or something on the secondary side between driver and the lamp. I have found a circuit http://www.reuk.co.uk/OtherImages/ne...er-circuit.gif

This circuit may be usable, but I do not know enough to determine that. I can make a power supply for ne555 circuit.

Questions:
If I use a Fet instead of npn transistor, can i use 42v in circuit connecting leds and fet drain-source and lower voltage to power the ne555 and its circuit?
Should i separate the grounds of these to circuits?
Do I need some other protection between ne555 pin 3 and fet gate if there are different voltages in these two parts of a circuit (I have seen people using opto elements in similar situations but i do not know much about them)?

Do you have some other idea about a better way to accomplish my goal with easy obtainable components? I need to make similar circuits for eight lamps, so it should be something affordable, too.

Thankful in advance,
Sasa

[barry wilkins]

Re: Constant current LED dimming

confusus he say stick your finger in the air and try .
I dont think it would be too difficult to do ,dont forget a FET is voltage driven not current as a transistor so it might be a idea to put a transistor between the 555 output and the transistor.

Barry Wilkins

[ben7]

Re: Constant current LED dimming

Yes, just use a MOSFET and a 555.

You will need to run the 555 off a lower voltage! It's max is like 15v or thereabouts.

[999999999]

Re: Constant current LED dimming

I see a problem with this. It's not as this the input to this is a fixed voltage and current, the regulator actively increases output voltage (as much as it can given input voltage minus forward voltage drop) to maintain a 310mA output and if it can't do so, whether it runs at some unintended output level or shuts down depends on how it is designed.

What exactly do you want? Do you only want to always run the lighting at a reduced brightness, or to be able to switch between a high and low mode, or to have continuous adjustability at any later point in time?

I'm thinking the best solution is that you modify the current sense feedback loop on the LED driver board. It's probably a low ohm (< 1ohm) resistor or pair that determines the drive current so to change the current you would increase that resistance value, by substituting a higher value resistor, or series of resistors, or a switch (# of positions determines # of brightness settings) to choose between the resistance value present in the driver feedback loop.

That's a specific as I can get without a schematic or possibly very good front and back pics of the driver board.

There is another simpler option, put a basic two resistor voltage divider between the driver output and LED inputs and ground, so some of that 310mA is wasted away shunted to ground but the driver still outputs the 310mA. Another option is to just put a diffuser panel over the LED array so less light escapes.

Is the goal to make a more esthetically pleasant, lower level of light, or to use less power due to it being battery powered or another limited source, or some other reason?

[ben7]

Re: Constant current LED dimming

I see a problem with this. It's not as this the input to this is a fixed voltage and current, the regulator actively increases output voltage (as much as it can given input voltage minus forward voltage drop) to maintain a 310mA output and if it can't do so, whether it runs at some unintended output level or shuts down depends on how it is designed.

What exactly do you want? Do you only want to always run the lighting at a reduced brightness, or to be able to switch between a high and low mode, or to have continuous adjustability at any later point in time?

I'm thinking the best solution is that you modify the current sense feedback loop on the LED driver board. It's probably a low ohm (< 1ohm) resistor or pair that determines the drive current so to change the current you would increase that resistance value, by substituting a higher value resistor, or series of resistors, or a switch (# of positions determines # of brightness settings) to choose between the resistance value present in the driver feedback loop.

That's a specific as I can get without a schematic or possibly very good front and back pics of the driver board.

There is another simpler option, put a basic two resistor voltage divider between the driver output and LED inputs and ground, so some of that 310mA is wasted away shunted to ground but the driver still outputs the 310mA. Another option is to just put a diffuser panel over the LED array so less light escapes.

Is the goal to make a more esthetically pleasant, lower level of light, or to use less power due to it being battery powered or another limited source, or some other reason?

He is trying to dim the lights.

BUT, the electronic 'ballast' doesn't do well with a regular wall dimmer.

So, that won't work - then what will? This is what he is trying to find out.

Good point on the modification of the constant current ballast

[999999999]

Re: Constant current LED dimming

Heh, must get more sleep. Replace my prior mangled sentence " It's not as this the input to this is a fixed voltage and current" with "It's not as though the input to the LED arrays is a fixed voltage and current".

A dimmer prior to it would just result in it upping the duty cycle trying to keep a 310mA drive current. Same thing with a dimmer after it, unless that dimming after it diverts (wastes) some of the current to ground so the driver circuit still senses it is supplying roughly 310mA.

[999999999]

Re: Constant current LED dimming

Previously I wrote about a two resistor voltage divider. That was incorrect, you could simply measure the voltage when the driver is powering the LEDs, then calculate the resistor value you'd need between that and ground to waste the amount of current you didn't want driving the LEDs, except that the forward voltage of the LEDs, and thus the output voltage of the driver circuit to reach 310mA, would be a little lower at the lower current the LEDs see - but again this is a wasteful way to do it.

If the driver is encapsulated to the extent that you can't access it for modification, it might be best to just buy different drivers whether they start out at the right current, or are adjustable, or can be modified.

[kikkoman]

Re: Constant current LED dimming

hey, how about a LM317T in CC mode?
Its upper limit is 40V, so you might drop in a diode in series, but all in all it should work fine with that electronic ballast.

[999999999]

Re: Constant current LED dimming

^ LM317 doesn't actually have a 40V limit, only an input output differential limit of 40V (can't drop more than 40V down to the regulated voltage level it is set to), so if this were the only issue it would work.

The problem is that the LED driver is a (crude) constant current regulating supply itself, if you try to limit the current below 310mA with a circuit after it, it will keep increasing its duty cycle as much as possible trying to compensate.

The question then becomes, what does it do when it can't compensate enough. Do the two circuits together then meet the output current goal, or does the driver IC go into a protected shutdown, or does the driver overheat, or ?? This is something that could be tested experimentally, but could take a lot of experimentation to arrive at the right current level if it works at all.

In theory all that is needed is a single resistor addition or swap to the driver circuit to set it to the target value instead of 310mA... since that's how it was set to 310mA in the first place unless it is very unusual. Practically speaking it could depend on how picky one is about arriving at an exact current value since resistors tend to come in certain incremental values, beyond which you're dealing with series or parallel resistors to arrive at a more precise value. A rheostat might've been an option but most likely the cost would be prohibitive several times over.

[ben7]

Re: Constant current LED dimming

He could always just use a simple 48volt DC PSU, and an LM317 to limit the current. There would be a max current adjustment/limit potentiometer, then there would be a 0-max (amps) control (dimming) potentiometer. With the current set down low, the voltage may exceed the max allowed voltage drop across the LM317. In that case, he could just use a comparator, and a high-power transistor, such as a TIP48. There would be a current sense resistor also.

[SaleB]

Re: Constant current LED dimming

Ok, I have read all your posts and now I understand how this constant current driver works. As 9x9 presumed, the diver is enclosed in aluminium rail and topped up with some silicone (watertight), so that solution falls out.

I want to use six of these lamps on my ceiling. I wanted to have a dimmer and switch for each circuit, so I can use different settings for various situations. The lights are very bright, for normal use maybe too bright so i wanted to use them regularly dimmed to 30-50%, and be able to give them full strength when needed.

I just got an other idea, but I still do not know how could I make it. Can I use some kind of combination matrix, that would leave me the following options: every lamp on it's own driver for max light, two lamps in parallel on one driver for 50% light, and three lamps in parallel for for 33% light on each light. Could that work?
Could I then, on previously mentioned circuit add a 5W wire would pot to give me the option for partial dimming in the range of 70% to full power? Is something like that possible?

Thanks, for all your efforts. I am following the thread carefully
SaleB

[999999999]

Re: Constant current LED dimming

We'd need to know (a link?) which driver, the area it is lighting, etc... far more details about the project.

The short version is that it seems like the equipment you have is suited to only one purpose and trying to alter it, may not be a time or cost effective alternative (based on more details about the project). The other option to decrease light is to just jumper (or use a switch to) remove some LEDs from the circuit so when there is 310mA, fewer LEDs are being driven.

[SaleB]

Re: Constant current LED dimming

Ok, the lamp and the driver look similarly to this: http://www.dhgate.com/white-super-br...2b42d763e.html

I have bought a few of them, and every one was little different (color of the driver, text in front). On few of them there is an information about the current 300mA +/-10%, on some there is the info 310mA +/-5%, on some only says constant current driver, output voltage 27-42VDC. They all are similar in dimension and identical in function. All the lamps are identical, 12x 1W serial connection.

I have a concrete ceiling, so i cannot insert the lamps like in it like it is intended. So, I have made an wooden rectangle in the middle of the room intending to add an MDF board on the underside, and to add lamps in MDF. On the ceiling i have one switched AC lead form the switch, so every combination I make about selecting individual light and dimming them, I must do it on the ceiling.

I would like to have full control of intensity of light(s) and if some of them is switched of or on, but I'll settle for partial control.

[SaleB]

Re: Constant current LED dimming

There will be six individual lamps up there each of them with their own driver.

[999999999]

Re: Constant current LED dimming

What you are asking is a complete project more involved than the product itself. If you can get the driver board out of the enclosure and post good top down and bottom up pictures of the PCB, then it may be possible to modify it if you have the ability to determine component values on it.

If you can do that and the driver is designed as I suspect, a multi-throw rotary switch could select between different brightness levels by changing the resistance value in the driver feedback loop. For practical purposes this would (probably, unless there are leaded resistors in the values needed) need a copper clad circuit board, or some creative ways of mounting surface mount resistors without any pads to solder them to... which isn't impossible, just unusual.

There's also the issue of where to acquire these parts in Serbia as I have no idea what suppliers are available there and cannot advise on this.

[999999999]

Re: Constant current LED dimming

He could always just use a simple 48volt DC PSU, and an LM317 to limit the current. There would be a max current adjustment/limit potentiometer, then there would be a 0-max (amps) control (dimming) potentiometer. With the current set down low, the voltage may exceed the max allowed voltage drop across the LM317. In that case, he could just use a comparator, and a high-power transistor, such as a TIP48. There would be a current sense resistor also.

If no other option is found then this may be the right solution. I just hate the idea of using LEDs but powering them with something as inefficient as a linear regulation circuit from a transformer, or the resistor to ground idea I mentioned that also wastes a lot of power.

On the other hand these are both clear paths to accomplish the goal.

[SaleB]

Re: Constant current LED dimming

I have other concerns about the driver. Driver is installed inside the aluminium rail. On the back side it is sealed with some silicone based compound. When I tried to see the insides of an HV transformer of some automotive hid light, there inside was the whole board in that compound and I could not get to the board.
I am not avoiding the process of disassembling of the driver, but I fear that the situation will be the same one. Is there any other way?

I have been thinking, about some other solution. I have found locally an voltage supply 220V to 48V, 300VA. Would it be possible to add an voltage regulator with some kind of current control at it's output to get the needed approx 40V with regulation of current for six different 300mA circuits, so I can add a little control panel to enable/disable any light and to control individual current levels for every part of the circuit. What do you think about that solution?

I presume that lm317 could be used for voltage regulation 48 to 40v. Max needed power is 6x 300mA, so 1.8 amps should not be a problem for a to220 regualtor. But in that case I would need your help for creating an adjustable current controls for those six output circuits. That would also be a more elegant solution because all the electronics would be in one box and not six small boxes. But my question is would it work?

[ben7]

Re: Constant current LED dimming

I have other concerns about the driver. Driver is installed inside the aluminium rail. On the back side it is sealed with some silicone based compound. When I tried to see the insides of an HV transformer of some automotive hid light, there inside was the whole board in that compound and I could not get to the board.
I am not avoiding the process of disassembling of the driver, but I fear that the situation will be the same one. Is there any other way?

I have been thinking, about some other solution. I have found locally an voltage supply 220V to 48V, 300VA. Would it be possible to add an voltage regulator with some kind of current control at it's output to get the needed approx 40V with regulation of current for six different 300mA circuits, so I can add a little control panel to enable/disable any light and to control individual current levels for every part of the circuit. What do you think about that solution?

I presume that lm317 could be used for voltage regulation 48 to 40v. Max needed power is 6x 300mA, so 1.8 amps should not be a problem for a to220 regualtor. But in that case I would need your help for creating an adjustable current controls for those six output circuits. That would also be a more elegant solution because all the electronics would be in one box and not six small boxes. But my question is would it work?

As 999999999 said, it would be inefficient to use a liner regulator though. Perhaps you could use a switchmode (buck) regulator set up in constant current mode.

[SaleB]

Re: Constant current LED dimming

In the mean while I have found a factory unit that will help me with all my needs, 42V power supply, 2.2 Amps, dimmable with pwm, resistor or 1-10V signal and it's not too expensive. So, I will be suspending the project. I would maybe want to make it, but induction coils are big problem in this country so it would be a problem to assemble the parts, because every circuit that I have found on the web using switching reg also uses at least one coil.

Thank you for all your time and help.

I have just one follow up question about switch mode regulators. I have been reading about them in recent days and one parameter always pops up, but I cannot understand why is it so important. It's switching frequency. Why is switching frequency so important in switch mode regs and what is lowest acceptable limit for it?

Thanks again,
Sasa

[zaero]

Re: Constant current LED dimming

...induction coils are big problem in this country...

A great source for inductors (low value though) is old computer power supplies. They are not great, but for some applications they are ok. If you just need low current (150-200mA) you can use the ballast from the CFL lights (2-4mH).

...Why is switching frequency so important in switch mode regs and what is lowest acceptable limit for it?
Sasa

Switching frequency is related with the required inductance. Minimum is usually 20kHz, to avoid audible noise in inductors. The high value is usually limited by switching losses (whenever transistors switch). It really depends on the application.

Some MR16 drivers available on ebay use intended chips that you can use up to 28V/1A, and they allow dimming, even if it is not implemented in the board. Such boards are found them for less than $3. Some others use the MC34063, just a switching regulator, which doesn't allow dimming, but up to 35V/1A

The truth is that thousand of things can be done, depending on your exact requirements.