Dimming an LED module intended for 12V nominal 1

[dubc4]

First let me preface this by stating that I am a mechanical engineer not an electrical engineer. I have some basic knowledge of electronics so hopefully we can communicate in the same language to resolve this

I have a 12V LED module (only has two leads for power and gnd) and when I took apart the module it turns out that it has its own programmable controller inside (running at 5V) that uses mosfets inside to flash the LEDs in programmed patterns, but it has no method of dimming the LEDs (it has a bunch of other components for vreg for the controller, current limiting resistors for the LEDs etc). Before taking apart the module I thought perhaps I could PWM the 12V power and dim the LEDs but I no longer see this working as I will likely just be rapidly turning the programmable controller on and off very quickly and messing with the flashing patterns.

So that brings me to this post.

Although the module nominal input is 12V, when I drop the input voltage using my power supply, there is no appreciable difference in perceived brightness down to 9V (current also drops from ~190mA to ~100mA). Below 9V I would estimate a 50% brightness drop when lowering input voltage to ~8.2V (current drops to ~25mA)

So I have some 1n4002 diodes here and I believe on the datasheet it shows a forward voltage drop of 0.6-0.8V with current between 0.03A to 1A. If it put one in series with the LED module and run it at 9V I achieve the ~50% dimming I am looking for (voltage across the LED module should be in the ballpark of the 8.2V I'm assuming). If I used a ~30ohm resister in series instead of the diode I get roughly the same result as I am limiting the current.

Here in lies my first question, by using a diode in series to drop the voltage, or using a resistor to further limit the current (since it already has resistors built into the module) what is the advantage or disadvantage to either solution?

Second question, someone suggested to use a constant current LED driver to dim the LEDs but this was before determining all of the other components inside the module. I'm not sure what effect it would have trying to "push" a constant amount of current through the module with everything else going on in there (programmable controller etc.)

Third question, I actually have three of these LED modules in red, amber, white... which I would like to control individually (on/off). The brightness of all LED modules can all be tied together as I have no reason to run them at a different brightness simultaneously. I came up with the following circuit (see attached) using MOSFETS for the ON/OFF and controlling the dimming with the diode as mentioned in my first question above to achieve two discrete brightness levels. The red/yellow/white inputs from my controller would be normally low to turn off the lights, and then driven high to turn each of the colors on. A common dimming signal would be driven high so that path to GND avoids the diodes (or resistors, see question 1). Low signal on this line would put the diode/resistor in series to dim the lights. Does this look like it should work, or is there a better way to do it with less components?

Thank you all for your patience with all of my questions... there are definitely some gaps to fill in my fundamental understanding of electronics.

 

[itsmoked]

Trying to run the module with less voltage is fraught with issues. You are essentially removing the controller by doing that because it's getting less than its required voltage to 'think correctly'. If it's a switcher you may soon damage it.

There are countless modules out there to do anything you want with any voltage, colors, and dimming up the wazoo. Figure out exactly what you want to do and find it.

Start at DealExtreme

Search for LED lighting or LED novelties or LED drivers.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[dubc4]

Hi Keith, thank you for your reply.

The controller runs at 5V. I removed the epoxy covering the chip and identified its regulator with the part number. Running the module above 8V should give the regulator more than enough over head to power the controller. I have the part numbers at work, I will link to them here tomorrow.

I understand there are a huge variety of modules out there... but sometimes you have to work with what you have. We (company) have contracts with certain suppliers and have to work within their catalog. If you're wondering how a mechanical engineer got stuck doing electrical work, I'm wondering the same thing

 

[itsmoked]

Not clear I guess on the situation enough to help much. If you're interested in chip-level solution there are a bazillion of those too. Go with the same brand but pick one with a dimming input. Keep in mind the amount of dimming available and whether or not it's enough to provide what you need. Some have really slow dimming frequencies that will make everything look like it's blinking if it moves or your eye sweeps past it. (Super freaking annoying)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dubc4]

Thanks again Keith...

So to bring more clarity to this topic, I took a picture of the inside of the module (back of the board is unpopulated).

Main components I could identify are as follows:

Controller:
PIC12F683
[URL unfurl="true"]http://ww1.microchip.com/downloads/en/DeviceDoc/41211D_.pdf[/url]

Likely powered by 5V, 30mA regulator:
ZXTR2005Z
[URL unfurl="true"]https://www.diodes.com/assets/Datasheets/ZXTR2005Z.pdf[/url]

There are two N channel MOSFETS:
DMT6008LFG
[URL unfurl="true"]https://www.components-mart.com/datasheets/82/DMT6008LFG-7.pdf[/url]

There are 6 LEDs that I cannot identify... and what would appear to be 6 very small transistors (my guess as they have a 'Q' designation).

I don't see any evidence of any other components like inductors that I have seen in switching regulators. Only small caps, resistors and a few diodes present.

My initial guess was that the PIC controller is switching the larger MOSFETS to turn the LEDs on and off, but the coincidence of 6 LEDs and 6 smaller transistors is throwing me off. These light modules are intended for automotive use (emergency vehicle light bars etc)... so perhaps there is something going on to protect the LEDs from voltage spikes produced by the vehicle's electrical system.

Let me know what you think. Really appreciate the responses so far

 

[VEBill]

General approach might be to separate the power supply to the LEDs from the rest of the circuit; this is done by tracing the PCB, cutting traces and soldering jumper wires. Then you can adjust the LED voltage to change the brightness without impacting anything else.

Linear adjustment may lead to thermal considerations. So you might instead add a PWM dimmer circuit onto the LED power rail.

 

[dubc4]

Thanks VE1BLL,

I would like to avoid modifying the board if possible. If I have to put together 100 of these things I'd rather not have to get into that sort of work.

At the end of the day, it may just be easier to design my own board and have them made. I've done a few simple PCBs in the past, I could do it again. I was trying to avoid it because then I would have to source out some kind of optics/lens for the board etc.

 

[JG2828]

Could you just mechanically dim them by inserting something semi-opaque between the LEDs and the cover?

 

[dubc4]

I suppose I could try and put some kind of "flip screen" to cover the LED module when I want to dim it, but the way the guys treat the equipment.. it would probably get ripped off or lost rather quickly.

A little more info... so the brand of lights is Whelen if you guys didn't see the little label in the picture I posted. They have a programmable flashing module that apparently we just so happen to have. So I was playing around with it and it can dim the lights with only the power and gnd connected... I started prodding the connections with a Fluke 177 true RMS meter.

My power supply is putting out 12VDC ... The readings I got at the output for the LED module for each brightness level are as follows:

100% brightness --> 12VDC --> N/A VAC --> 200Hz AC --> 0Hz DC
70% brightness --> 9.6VDC --> 3.8VAC --> 200Hz AC --> 0HZ DC
50% brightness --> 7.5VDC --> 4.75VAC --> 200Hz AC --> 0HZ DC
20% brightness --> 4.12VDC --> 4.33VAC --> 200Hz AC --> 0HZ DC
0% brightness --> 2.7VDC --> 3.45VAC --> 182Hz AC --> 0HZ DC

I set the meter to both AC and DC to see how the readings would differ. I'm not sure if the AC setting would respond different to a PWM signal or not. I'm not experienced in taking these sorts of measurements but I did read that using the AC setting to check DC voltage would not damage the meter.

Now where I am confused is that if I remove the module from the programmer and just wire it directly to my power supply and adjust the DC voltage to lets say 4.12VDC as was read for the 20% brightness... the LEDs do not come on. They turn off around 7VDC direct from the power supply... So when I took the above measurements, am I reading some kind of average 'ON' time from a PWM signal perhaps?

And since I have the programmable module, before anyone suggests just using that... its quite large. Too large to fit into a portable device. I'll see if someone in my building has a scope to see if it is a PWM signal then?

 

[dubc4]

Got it on the scope ... see attached. So yes, with 12V supply and the programmer set to 40% brightness, there is a 200Hz frequency, 5ms period... 40% on time

So now I have to revisit the 555 timer circuit to do this

My last little bit of curiosity is this... when I first power on the LED module (no PWM), my power supply reads a higher current draw and then after about 5 seconds of being on, the LED modules settle at about 1/2 the peak current of when it was first turned on... Now when I PWM them at a lower brightness (ex: 40%), the current draw is still slightly higher than the "settled" current when I just directly connect the 12V supply to the module. I thought PWM should reduce current consumption since there is a considerable "off time?"

 

[itsmoked]

Need a clearer close-up picture. NOT taken against a WHITE background so the camera doesn't stop way down making the board extra dark. Maybe just attach it to the post so it can be more than a Mbit.

Got a part number of this thing?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dubc4]

See attached.

I ended up with an acceptable dimmed state by using this calculator/circuit:

https://houseofjeff.com/555-timer-oscillator-frequency-calculator/

Put in my required 200Hz frequency... ended up with the following values because I had them on hand to breadboard

C1:0.01 µF R1:56 KΩ R2:330 KΩ Period:0.005 sec. f:201.5363 Hz. Duty Cycle: 53.9 %

Was still kind of bright... found this info here:

https://electronicsclub.info/555astable.htm

Put a diode in parallel with R2 and the dim state looks great. Looks like the frequency is somewhere around 300Hz + (have to scope it again) but it works...