# 20W COB Spotlights - Dimmable?



## machineage (Jul 14, 2021)

Hi all!

OK - I have four spotlights which use 20 watt COB LEDs, powered by a driver which accepts 10 - 30v input.

I'm wondering if there is a way to make these dimmable, would a PWM dimmer work for example?

I would attach a photo of the COB & driver board, but it seems I'm unable to do so.

EDIT: I think I managed to include the photos!

Thanks!


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## LEDphile (Jul 15, 2021)

Unless they are marketed as dimmable by the manufacturer, they are unlikely to be dimmable. Given that input voltage range, that driver looks to be a boost circuit, and will treat a dimmed input as either a lower voltage input (and function properly without dimming), or flicker/strobe when attempting to dim. If the fixtures are not dimmable, you also run the risk of damaging the driver and/or the dimmer you use if you try to dim the fixtures.


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## Dave_H (Jul 15, 2021)

Such lights are not intended to be dimmed but I have successfully done so with a couple of 12v spotlights. It all depends on the driver. Buck drivers like PT4115 and others like it have an input which can disable the light, or used for analog or PWM dimming with 0-5v signal.

I initially hooked one up to a dollar-store LED 3xAAA light which has high/low modes, confirmed to be PWM (using a scope). It required a small interface circuit, transistor plus two resistors. Worked nicely, and I also got "flash" mode out of it.

My biggest issue was getting these sealed lights open to expose the components, without undue damage. I end up prying off the front lens, causing some minor damage. As these are low-cost and for experimental use, no concern with resealing them.

Some had PCB with components potted with clear coating which is next to impossible to work with. Your light is obviously free of this, open and exposed.

I see two inductors which may indicate buck-boost configuration. If you COB is 12v, with input range 10-30v it would need buck-boost (you could measure LED voltage).

Can you read markings on the small 8-pin (SOIC) IC "U1"? If it's identifiable and can find a datasheet, you can determine if there's a PWM input, which pin, and its voltage levels.

Dave


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## machineage (Jul 15, 2021)

Thank you so much for commenting!

@Dave_H - the 8-pin chip is marked '5305 1803' - see attached image, which does appear to be a LED boost driver.

I thought too they would be buck-boost, given the wide input voltage. One of the things the images don't show, is the boards are held down with clear but solid epoxy type glue. The power cable is also sealed into place with this glue, making it nigh on impossible to get extra cables into the housing via that route, leaving the option of drilling a new hole for any control input.

A thought I had, was to eliminate the internal driver board completely, by severing the tracks from the power input cables, possibly just DC+, then unsolder the COB wires from the PCB and solder them direct to the power inputs. That way I could use an external driver board?

Thanks again!


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## Lynx_Arc (Jul 16, 2021)

You would need to know the operating voltage of the COB LED before you replace the board. One option would be to possibly cut the leads to the LED and put some sort of linear regulator inline to reduce the voltage to the LED from the circuit board that you can adjust with a control. Using a variable resistor would work but the problem is LEDs don't translate voltage drop linearly to 0v they quit putting out much light at all below about 2v per LED and COB LEDs sometimes are series along with parallel you may have a 12vdc COB or just a 3-4vdc COB they make them in all sorts of voltages.


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## Dave_H (Jul 16, 2021)

Measure the LED voltage with lamp operating...this will help answer some of the questions especially if you decide to use a different driver; but may not need to.

I found QX5305 LED driver datasheet. You are correct, it is boost driver. There is no obvious method to PWM/analog dim the output directly. Enable (EN) pin appears to shut down the device but no indication of ability to accept PWM. 

Inserting any circuit in series with LEDs will interfere with the driver's current regulation and it will try to compensate, up to some limit.

However, you can lower LED current effectively by changing the current-setting resistor (Rfb in datasheet). I suggest one or two lower settings selected by switch, not a continuously-variable pot in circuit. Just switch between resistors. Sometimes this is made of series/parallel combinations of low-value resistors so may be a bit tricky.

I also recommend not increasing LED current.

Circuit has two identical inductors (33uH) apparently in series, as datasheet shows 68uH. 

On caution: hopefully this is not used for on-road vehicle use.


Dave


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## Dave_H (Jul 18, 2021)

A lot of 12/24v LED lamp drivers with 10v-30v input are buck, driving LED strings up to maximum three in series. A defective 9-LED light I repaired had three such strings in parallel off the same driver. One LED failed; finding a replacement very difficult as it would need to be matched closely with the others. So I decided to run only two strings; but to not overdrive these, current had to be reduced to 2/3. By changing feedback resistance, this was done. It puts out less light, but lower current, still somewhat useful.

Dave


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## Dave_H (Jul 18, 2021)

machineage said:


> Hi all!
> 
> OK - I have four spotlights which use 20 watt COB LEDs, powered by a driver which accepts 10 - 30v input.
> 
> ...



Also if you decide to do any mods on these lamps, I recommend verifying both COB voltage and current beforehand. In my experience with some of these products the LED power rating is overstated; I actually view this as misleading/deceptive. One case a 9-LED lamp claimed "27W", I measured closer to 18-20W, which includes some losses. Appears it is 9x3W LEDs run around 2W each, which is actually a good thing. There is margin in the design, and if it delivers the stated brightness at lower power, so much the better.

So just curious about this 20W COB, is it:

- 20W COB run at 20W
- 20W COB run at less than 20W
- higher than 20W COB run at 20W

Is a part number visible on the COB? (you'd need to disassemble a bit further)


Dave


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## Dave_H (Jul 27, 2021)

So, did anything happen on this...?

Driver is not inherently dimmable, best I could recommend if high/low is sufficient control, is a switch to select feedback resistor. You would need to keep wiring short. Resistors are likely a series and/or parallel combination of low values (less than 1 ohm), can't make out from the images.

Dave


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