Using a sliding potentiometer (dimmer) on LEDs??

[chuckdallas]

My wife is an artist who reverse paints on glass to make artistic individual chandeliers. If you would like to see her website, please PM me. I bought one from her with an aquatic scene at a Home Show before we married.

I need a small width, long strip of LEDs in my canopy to supplement my T5s. I'm looking at the Reef Brite line. They have a nice straight 48 inch in-line set of white, blue and 50/50. I've been told the white set isn't dimmable.

My wife has a ton of lighting and electrical cord pieces from the lighting fixtures she sells and asked why we couldn't use one of her sliding potentiometers on the LEDs. We know that fluorescent bulbs can't be dimmed, but why couldn't we connect the sliding dimmers between the end of the LED cord and the wall outlet to dim the white LEDs?

Has anyone tried this or some other method to make a fixture dimmable?

[chuckdallas]

Anyone try this before?

[Shard]

It isn't exactly as simple as what you are thinking of doing. I would have to look at the fixture to give you an idea. In short, you really need to have dimming LED drivers, and use either a 5k ohm, or 10k ohm potentiometer, depending on the driver.

Its not really the type of potentiometer that matters, it is the ohm rating.

I am having a little bit of difficulty visualizing what you are talking about. Are you talking about putting a potentiometer on the AC line that goes into the driver? Bad idea I think.

Hope this helps,
Landon

[Takitaj]

+1 You can't dim a non-dimable LED driver with a lamp dimmer on the A/C side and I would HIGHLY recomend NOT putting just a potentiometer on the A/C side unless you have a fire extinguisher standing by ;) I'm not exactly sure of what would happen if you tried to use it on the LED output side although I would'nt recomend that either. By changing the resistance of the load enough you could fry the driver. You really need to have a dimmable driver with the potentiometer and 9-10v dc supply source connected to the dimmer port on the the driver.

[Shard]

You really need to have a dimmable driver with the potentiometer and 9-10v dc supply source connected to the dimmer port on the the driver.

Depends on the driver.... your are referring to meanwells and the like. The buckpuck drivers have 0-5v dimming controls, with one of the two leads being a 5v reference, so no external power source needed, just a potentiometer (5k).

[Takitaj]

Yes, thanks for catching that! I use the meanwells, forgot about all the others :crazy1:

[chuckdallas]

I was talking about a ReefBrite LED strip. What do I need to do to check and see if this will work??

[chuckdallas]

Has anyone used a sliding dimmer switch for a lamp on an LED fixture?

I want to connect an extension cord we have that has a sliding dimmer switch between the wall and the cord to a Reef Brite LED strip.

Has this been tried?

[Shard]

No. Don't do it. You are misunderstanding the fundamental way in which LED drivers work. You cannot use a sliding dimmer on LEDs, just as you can't use them on CF bulbs.

LED drivers take an input and convert it to a specific output. The ones with dimming functions have a reference circuit where the current is changed to change the output. This reference circuit is NOT the AC in. You do not change the output on a LED driver by changing the main input. That is what a wall mount slide dimmer does, it changes the main input.

So, just as Takitaj and I have previously said. Don't do it, unless you want to waste your money and burn up a LED fixture.

Hope this helps,
Landon

[hardin4019]

Hi guys. My personal suggestion would be go with an LED strip and Driver meant to dim just to be on the safe side of burning out pricey electronics.

On the other hand, I have a degree in electronics and feel it necessary to point out that this may be entirely possible. When you put a dimmer in, you don't change the voltage of the entire system, however you do change the % of the voltage that is applied to the LED strip as the rest is then applied to the dimmer. This also changes the amount of current going through the LEDs. Changing the position of the dimmer then raises/lowers the current of the entire system making LEDs brighter/dimmer as you change it. LEDs also need a particular amount of voltage to work in the first place, and putting a dimmer not designed for DC electronics, or low voltages may completely drive the voltage so low the LEDs then do not lite... Any way, as I mentioned previously, I suggest sticking with an LED set you buy with a driver that is meant to be dimmed.

[Shard]

On the other hand, I have a degree in electronics and feel it necessary to point out that this may be entirely possible. When you put a dimmer in, you don't change the voltage of the entire system, however you do change the % of the voltage that is applied to the LED strip as the rest is then applied to the dimmer. This also changes the amount of current going through the LEDs. Changing the position of the dimmer then raises/lowers the current of the entire system making LEDs brighter/dimmer as you change it. LEDs also need a particular amount of voltage to work in the first place, and putting a dimmer not designed for DC electronics, or low voltages may completely drive the voltage so low the LEDs then do not lite... Any way, as I mentioned previously, I suggest sticking with an LED set you buy with a driver that is meant to be dimmed.

If you read his post, he is talking about putting a dimmer on the AC line that goes into the driver, not the DC line coming out that drives the LEDs. So, he is talking about trying to use a dimmer which changes the voltage going into the overall system, which is flawed, because although 110V is coming in, much less is going out to the LEDs. Changing the voltage coming in will not have the effect he desires. Most drivers can take fluctuations in the line in, and still supply a steady DC voltage out. Playing with the AC line in is really just going to stress the driver, as it is specifically built to try and provide a level flow out the other side.

You are right that the current is changed to affect the intensity of the LEDs on fixtures with dimming, however you are wrong about it changing the voltage.

First off, if you try to put a dimmer in the DC line that holds the LEDs on the driver, you may change the impedance, therefore putting undue stress on the driver, which is not made to handle dimming.

Drivers that are equipped with the dimming feature, dim by using a reference circuit with a pot or other device to change the voltage on the reference circuit itself. They do not use dimmers in the DC circuit that runs the LEDs, or the AC line coming in which powers the overall system. This is the case with both Meanwell and Buckpucks, two of the most commonly used drivers for DIY in this hobby.

Also you talk about the voltage driving the LEDs changing with dimming. As I understand it, you are wrong on this. This does not occur, at least not significantly. You must remember I=V/R? Ohms law is that the current (I) is equal to the (V)oltage divided by the (R)esistance.

So when the current is changed, the resistance is changed to compensate, and the voltage stays the same, or very close to it, so you can run the LEDs at 350ma to 1000ma on a driver without changing the voltage.

I would invite one of the ones with more electrical engineering experience like der_wille_zur_macht to come review my post and correct accordingly, because misunderstandings about electronic circuits can have a bad effect. For both your reef, and your wallet.

Hope this helps,
Landon

[der_wille_zur_macht]

I wouldn't claim to have a ton of EE experience ;) but Shard's post is more or less correct. You CAN NOT dim or control a typical LED rig by modifying the 120v AC power delivered to it. In other words, you CAN NOT use a typical "lamp dimmer" or rheostat on the 120v AC power running to the driver in order to dim the LEDs. LED fixtures (typically) have active electronics inside them that are made to function given a certain range of AC voltages. If you're within that range, it'll work as designed. If you're outside of that range, it simply won't work, or at best it will behave unpredictably. There is no direct linear relationship between input voltage and light produced as there is with a simple resistive light source (i.e. an incandescent lamp).

Further, you (typically) CAN NOT dim or control a typical LED rig by adding a resistor or potentiometer (sliding or otherwise) to the DC output, i.e. in series with the LED string. 99% of commercially produced HP LED rigs use an active circuit to drive the LEDs, which means it self-adjusts to maintain a preset current level. If you tried to "reduce" the power in the LED string by burning some of it off in a potentiometer, the driver would simply adjust the voltage level in the string upwards until the current in the LEDs returned to it's target value - it would completely thwart your attempt to dim the LEDs. This may be confusing for some people but it's important to remember that LED drivers are inherently constant-current devices, as opposed to the vast majority of other drivers/power supplies typically toyed with in hobby electronics which are inherently voltage-regulated, not current-regulated.

What it boils down to is simply this: The only reliable and surefire way to dim a string of LEDs driven by an active constant current driver is to use the driver's built-in dimming functionality. If the driver has no such functionality, you're SOL and the only realistic way to proceed is to replace the driver with one that is controllable or simply replace the entire rig with one that is meant to be controllable.

[chuckdallas]

Thanks everyone. I appreciate your help. I understand better now, thanks to the electronics and electrical engineering gurus.