# DIY low power light from LED using 120VAC



## primuspaul (Jan 25, 2018)

I have a low power "floodlight" type light I use to provide some ambient light to a room. It's a 10W LED, but unfortunately is too powerful for what I want. It also heats up quite a bit, which I can tell from the heat sink/enclosure. I have some spare 12V LED chips and was wondering if I could pump less voltage into them, like 10-11V or even less, to get them to produce a less intense light.

What kind of power supply would I need for this? Would I still require a heat sink?


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## primuspaul (Jan 26, 2018)

I was thinking about a 500ma 120 to 12v converter and 2 12v chips in series.


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## FRITZHID (Jan 26, 2018)

LEDs need controlled current, not controlled voltage. you can find a 10w 120v ac dimmable LED driver and set it to what ever brightness you'd prefer but you still need a heatsink for the LED to prevent it from thermally degrading.


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## primuspaul (Jan 26, 2018)

What is the disadvantage of using something like this with 2 x 12V led chips in series?


https://www.ebay.com/itm/AC-US-Plug...ghter-Converter-Adapter-90V-240V/253284028430


It's 12V0.5A, which translates to 6W. The 12V chips will split the voltage, getting 6V each. At half voltage, it will use 1/4 of the 20W, bringing the power requirement down to 5W (which is less than the 6W the converter can provide).


There should be no risk of burnout or overheat due to excess voltage since the 6V provided will be far below the specs of the LED chips.

The only problem I can see then is too little light (though I tested a single chip with a 6V battery and it was sufficient), am I correct?


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## FRITZHID (Jan 26, 2018)

LEDs have a Forward Voltage at which point current will pass and light produced. They don't function like a filament bulb where even a 12v bulb will glow a little at 2v. LEDs do not work this way. below the Vf, no light will be produced. 
what you NEED to do to accomplish what you desire is to simply get a 120v LED Driver that has a Current Adjustment (dimmer) in order to set your LED output at the level you want without harming the LEDs or the controller, OR YOURSELF.


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## primuspaul (Jan 26, 2018)

FRITZHID said:


> LEDs have a Forward Voltage at which point current will pass and light produced. They don't function like a filament bulb where even a 12v bulb will glow a little at 2v. LEDs do not work this way. below the Vf, no light will be produced.
> what you NEED to do to accomplish what you desire is to simply get a 120v LED Driver that has a Current Adjustment (dimmer) in order to set your LED output at the level you want without harming the LEDs or the controller, OR YOURSELF.


I understand what you're saying and completely agree that that is the right way to do it. I was just wondering what would happen if I used the constant voltage PSU instead to send 12V through 2x12V chips in series (6V a piece). I already tested it and know that there is sufficient light produced for what I need.

What I want to know is whether running an LED underpowered in this manner will damage it or shorten its life. I would think that it would prolong it, if anything. I suppose my question is more one of DIY curiosity than of standard practice (which you described quite well).


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## Lynx_Arc (Jan 27, 2018)

Best thing to do is use a 12v supply and a proper resistor to reduce current or a linear regulator. Wire the LEDs in parallel as Fritz said, LEDs don't operate over a wide voltage range constantly dimming to 0v rather then operate about 1/3 to 1/4 of the range like a 3v LED will operate down to 2v or so but at 1.5v won't put out much at all a firefly wouldn't be attracted to it.


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## Ken_McE (Jan 27, 2018)

primuspaul said:


> What I want to know is whether running an LED underpowered in this manner will damage it or shorten its life.



Under driving an LED will do it no harm.


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## ssanasisredna (Jan 27, 2018)

I guess using a $2.00 - $3.00 LED lightbulb and an AC dimmer would be too obvious a solution? Anything you try to put together yourself is likely to just end up costing more.


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## Lynx_Arc (Jan 28, 2018)

Ken_McE said:


> Under driving an LED will do it no harm.


I agree with this... in fact more often than not underdriving an LED lengthens its life.


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## primuspaul (Jan 28, 2018)

Lynx_Arc said:


> I agree with this... in fact more often than not underdriving an LED lengthens its life.



Well, I tried it with a 12V wall wart and it produces plenty of light for what I want (a night light). I even added a plastic cover to make it a bit less intense. Before attaching it, I ran it with no heat sink at all for a few minutes and it was barely warmer than room temperature when I felt it with my hand. I imagine you can run this thing indefinitely with no heat sink at all.

Power-wise, it's getting 6V to each cell since they're 12V cells and I wired them in series and powered them with the average 12V wall wart. I tried testing the amps before mounting it, but for some reason my clamp meter put out 0 amps (and yes, I tested in DC mode at the place were the + and - wires were separate).


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## Lynx_Arc (Jan 28, 2018)

primuspaul said:


> Well, I tried it with a 12V wall wart and it produces plenty of light for what I want (a night light). I even added a plastic cover to make it a bit less intense. Before attaching it, I ran it with no heat sink at all for a few minutes and it was barely warmer than room temperature when I felt it with my hand. I imagine you can run this thing indefinitely with no heat sink at all.
> 
> Power-wise, it's getting 6V to each cell since they're 12V cells and I wired them in series and powered them with the average 12V wall wart. I tried testing the amps before mounting it, but for some reason my clamp meter put out 0 amps (and yes, I tested in DC mode at the place were the + and - wires were separate).


Depending on the wall wart some older ones actually have higher voltage output with less than nominal loads they have to have a rated load on them in order to see around the voltage listed on them so if you are running at low current the voltage could possibly be more like 15v or so. I've not used a clamp meter it is possible that they don't work well with low voltage DC.


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## CuriousOne (Jan 29, 2018)

If your existing led fixture uses driver, you can find current limiting resistor, increase it's value, and reduce light output, without need to modify anything more.


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## primuspaul (Jan 30, 2018)

CuriousOne said:


> If your existing led fixture uses driver, you can find current limiting resistor, increase it's value, and reduce light output, without need to modify anything more.


Could you expand on this? What sort of resistor am I looking for? It's a fixture designed for a 12V LED, but with a driver that provides 24-36V.

I only have [FONT=&quot]1.3D-13 NTC Power Thermistor Inrush Current Limiters.[/FONT]


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## DIWdiver (Jan 30, 2018)

If you want good control of them, LEDs are best driven with a particular current, not a particular voltage. This is because a very small change in voltage can produce a very large change in current, and thus in power and light output. Couple this with the fact that the voltage characteristics of LEDs are pretty loosely characterized, and vary from one to the next, and you can see why we usually talk about current in LEDs instead of voltage.

You can luck out and put 6-8V across a "12V" led and it might work well. The next one might not. This can depend on the design and/or the individual unit you get.

Many LED drivers, especially ones used for single-die LEDs, use a low value resistor to 'sense' the current in the LED. If the driver senses the current is a bit low, it increases the output, and vice-versa. If you were to cut this resistance in half, the driver would sense only half the current, and thus would double the output to compensate. Conversely, if you were to double the resistance, the current would be half to compensate.

If you have a driver that is built this way, changing the current sense resistor is a good way to change the output. Keep in mind that increasing the output current tends to increase stresses on many of the components, and can result in reduced life, even to a few seconds or less. On the other hand, reducing the output tends to reduce stresses on the components, and may increase driver lifetime. This is not an absolute rule; in certain cases the exact opposite may occur. In general, an "expert" familiar with the particular design can be pretty good at predicting which is which.


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## KENN MOSSMAN (Sep 12, 2018)

primuspaul said:


> I have a low power "floodlight" type light I use to provide some ambient light to a room. It's a 10W LED, but unfortunately is too powerful for what I want. It also heats up quite a bit, which I can tell from the heat sink/enclosure. I have some spare 12V LED chips and was wondering if I could pump less voltage into them, like 10-11V or even less, to get them to produce a less intense light.
> 
> What kind of power supply would I need for this? Would I still require a heat sink?



I am too lazy to read all the replies..... but are you really using 120VAC directly???? It seems not.

You can use PWM, CV, or CC to control the LEDS. CC is often best and can be done easily (DIY) but does require some expertise to done successfully.


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## KENN MOSSMAN (Sep 12, 2018)

primuspaul said:


> Well, I tried it with a 12V wall wart and it produces plenty of light for what I want (a night light). I even added a plastic cover to make it a bit less intense. Before attaching it, I ran it with no heat sink at all for a few minutes and it was barely warmer than room temperature when I felt it with my hand. I imagine you can run this thing indefinitely with no heat sink at all.



There seems to be a phobia about heat. Yes heat is bad for LEDS but the derating curves start at 50 degrees C.


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## KENN MOSSMAN (Sep 12, 2018)

DIWdiver said:


> If you want good control of them, LEDs are best driven with a particular current, not a particular voltage.



NOT always true.
. If you need to drive different LEDs at different currents then CV can be more practical


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## MeMeMe (Sep 13, 2018)

KENN MOSSMAN said:


> NOT always true.
> . If you need to drive different LEDs at different currents then CV can be more practical



CV will never give you control of the current in an LED. It may allow the LED not to fail, i.e. if the voltage is well below the voltage of the rated current. If you add a resistor in series, it's not CV, it is really CC. The driver may be CV, but the circuit is CC.


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## DIWdiver (Sep 13, 2018)

KENN MOSSMAN said:


> NOT always true.
> . If you need to drive different LEDs at different currents then CV can be more practical



YES always true, but we may be arguing semantics.

To get GOOD control of the led using a CV supply you need a resistor in series, which means the led is NOT being driven CV. In that case it's not really CC either, but to get GOOD control it should be closer to CC than to CV.

It is true that to drive different leds at different currents it is often practical to start with a CV supply and use different resistors for each led. I often do this when using different color leds in a project. A 5V supply may be used to drive a 3.3V white led, a 1.7V red led, and a 1.1V infrared led (typical of optical isolators). This is NOT the same as driving the leds CV. It is also only practical in situations where the overall efficiency of the device is not a major concern.


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