RGB LED strip lights and controller - polarity issues

Does anyone have any experience with RGB LED strip/ribbon lights? I have a curious case of a lighting controller that's designed to work with these types of lights but seems to be implemented completely incorrectly.

The lights are the variety that come on a reel and are often sold in a kit with a 12v power supply and a remote-controlled RGB controller to change colors and turn the strip on and off. The controller I'm having the issue with is a separate product that provides some of the same features but can also automatically change the lighting in reaction to sound.

The light strip has 4 wires, RGB and a BLACK wire. Going by the functionality of the working controller and manual connection to a power supply, the black wire is a common ground and each of the R,G and B wires are supplied with varying voltage (+12 to 0) which determines the intensity of each light component and creates the varying colors.

The issue with the music/sound controller is that it doesn't have a common ground. It has common voltage, and each terminal for RG and B are then tied to ground, first passing through some bits on the circuit board (1 transistor each, plus some resistors, then tied to a small IC) to reduce the voltage on input from the remote. This means that the voltage is passing in reverse polarity through the LED strip, -12v instead of +12v. And of course the LED strip will not light up this way.

Which leaves me with a giant WTF!?

Reversing polarity on the power supply doesn't work to remedy the problem - no voltage can be measured on the board this way. Might this be a matter of flipping around a few components to get this to work? I only see 3 diodes on the entire board, one big through-hole and two smt.

I purchased this stuff from two sellers on Amazon, but I notice that the seller I bought the strip from also has the exact same controller on their web site. Arrgh.

Ask him for details on how it's meant to work?

Already wrote to both, but no reply. I'm expecting to get back messages from each telling me that they can't help because both sets of parts didn't come from them.

I just don't know how this controller would work for anything.

Lovely. So that controller, as-is, is useless with those LED strips.

One way to salvage the situation would be to add three more power transistors (PNP), one in series with each of the existing R-G-B trio on the controller board, except selected and wired to reverse the voltage output. Those will likely need a pair of resistors each to scale down the ouput of the existing transistors to levels that can be used to control the add-on ones.

Cheers

..assuming, of course, that the existing drivers are transistors, as opposed to SCRs or something like that.

Or.. perhaps rewire the emitter/collector paths of the existing drivers to get the correct ouput polarities. That might not be too bad, actually. Probably the best approach, depending on the exact details.

Edit: I wonder if the board layout has optional traces already to accomplish just that ?

Oh.. say, I've never seen an RGB LED strip in person, but.. aren't they the kind you just "cut to length" ?

In which case, the OTHER end of your cut strip might be opposite polarity..

Digging around some more, it appears there are also LED strips wired to match your controller, with a common (+) wire. Eg. This one from DX.

The LED strip's black wire is actually marked +12v, so it might be one of the strips you would think are wired for that controller. But it most definitely does not work if you apply +12v to that wire and then ground any of the color wires.

And the controller it comes with also definitely supplies (up to) +12 on the RGB and ground on the black.

The LEDs are all wired in parallel and connecting the wires to the other end doesn't have a different effect. It's the same from either end.

It would also not be possible to make this work only be changing around some components - the polarity at the supply would also need to flip, otherwise the black wire would still continue to receive +12 since that terminal on the outputs is connected directly to the corresponding VCC terminal on the supply side.

I'm going to take some photos of the strip and controller PCB and post them up.

In the meantime, found the same controller on Alibaba:
http://okled.en.alibaba.com/product/5212..._spotlight.html

Here are some images. Hope the traces and labels are easy enough to see.

What's the number on those transistor-like drivers? I cannot make out the part between the first "K" and the "A".

Thanks

Sounds like the factory messed up and accidentally printed up a common-cathode strip as though it was common-anode (which presumably they also make), then supplied it to your supplier as common-anode. You should take it up with your strip supplier, maybe they got a whole batch which were mislabelled...

Peter

The "missing" letter is an "I", so the part number is KIA50N06, a MOSFET transistor.

Here is a marked up image showing how to reverse the polarity to your LED strips. The black lines are where to CUT the existing traces (seven places in total), and the coloured lines show where to add new wires, with little black ticks indicating solder points.

Note that you can actually do the soldering at any existing blobs within the same strips, rather than having to scrape away a lot of green stuff to do them exactly where shown.

Cheers

Mark, it's an "I" (capital i)

Peter, I did write to the supplier, Hitlights.com (still waiting for reply), but even though the labeling may be incorrect, they are shipping them with small controllers that are wired correctly for the strips.

Since they also sell what looks like the exact same music controller, I asked about that too, so we'll see what they come back with.

Just saw the diagram Mark. I see you've made enough mods to rewire the polarity on all the terminals. I'm going to give this a shot - I think my soldering skill should be good enough with these large targets.

Any recommendations on how to best cut the traces on the board? Scrape them with a nail? Cut them with utility/x-acto knife?

Actually, the part I'm most worried about is soldering wire to points on the board where there aren't any existing pads (marked in green). Should I just scrape up a small area for the wire?

Here is a second version, same circuit as above, but showing one possible way to run the wires to existing solder blobs.

Great, mostly what I was thinking as well. I just hadn't taken the time to look for pads to use for the areas marked in green (the ground).

THANKS! It's early enough that I will probably try this today after a bite to eat.

Xacto knife: two cuts, slightly apart, angled toward each other, almost making a mini V-groove. Then just peel the copper away from between the two cuts. A gap of a millimetre (or more) is the minimum needed.



You can do it without any scraping, but for the red wires I personally would scrape away the green stuff to make room for new pads. That way I wouldn't be soldering too close to any components I might damage.

Cheers

If you do the red wires directly to the existing blobs, then use three individual wires, one for each transistor. That will be easier than trying to daisy chain a single wire like I drew above.

Cheers

When you finish rewiring, dig out an ohmmeter and measure resistance between the new RED and GREEN traces -- it should must NOT be zero!!

Cheers

Ok, just finished cutting the traces - before I saw your tip. I did something similar, but scraped the copper off between the two lines. Don't have an X-Acto handy (can't find it) so I'm using my trusty Stanley FatMax utility knife. Yeah, the same one I use for carpets, flooring, etc.

I tested for continuity before and after every cut to make sure that I was making a clean break. So far so good.

I'm not sure I can scrape the green stuff off of a flat area well enough to leave a surface suitable for soldering... I'll try.

A Dremel tool with a small grinding head might work for scraping.

When you get to soldering, USE FLUX. Even if the solder has a flux core. USE FLUX.

Cheers

Don't have any flux right now, but the flux-core solder worked well enough. Pre-tinned all the wires and didn't have any issues. I did three red wires as suggested and the point where all three attach is the ugliest part of the whole re-work

Everything seems to check out, but it's not working. The top two blue wires measure 4v and the bottom 2.5v

Can the wiring work the way it's illustrated above, with the constant voltage applied to the FET's drain instead of source? In other words, backwards from how the FET would normally work.

The current flow is from Drain to Source in the original circuit, and it remains that way in the modified circuit (unless I've goofed).

So it should work. But you did say earlier than you'd tried it with the power supply polarity reversed, and perhaps that damaged the controller?

I'll look at it again and think about it some more.

I'm unable to locate a datasheet for the transistors, so I don't know what the turn on voltage/current ratings are and so forth.

But a place to start is disconnect the LED strips, and then tell the controller to "turn everything on". Then measure the Drain Gate voltage at each transistor, the three readings should be identical.

The controller didn't get damaged - I checked voltage was working remote operated before doing the rework.

I'll check the gate voltage next. The info I browsed for FETS indicated current coming in on source, leaving on drain, controlled/modulated by gate.

It was all wired in this fashion except that instead of + it was all wired to ground instead. So the gate is wired to an IC whose PIN 1 is wired to the ground plane.

These devices can be made in either polarity. Some conduct from Drain to Source, and others go the other way round.

Given the way this one was wired originally, (+) connects to the Drain pin, and (-) goes from the Source pin. The modified circuit doesn't change that (in theory).

It should all still have exactly the same voltage drop between Source and Drain as it had before, but now there is going to be a different voltage between Gate and Drain than what was there before. I don't know if/how that changes anything.

GATE of each FET in orientation of photo (gate is the top leg of each FET) with color set to white (each FET getting constant voltage) and brightness set to maximum.

Top FET: 4.73v
Middle FET: 4.0v
Bottom FET: 2.75v

The Source leg measures exactly the same for each FET. And ON/OFF (with the remote) of the controller doesn't make a difference.

Ahh.. this might be it: on the copper side of the board, there are tiny little resistors near each MOSFET. One side of each resistor connects to the Gate, but I cannot see where the other side connects.

Have a look and figure that out for us, will ya?

If they connect to what used to be the big fat ground trace, then that's the problem. You'll need to somehow restore a true ground to that side of each of the resistors.

It's amazing how much using flux helps. I used to think I was the worst solderer ever, but then someone told me to use flux and now I'm … not the worst solderer ever.

That said, flux fumes make me ill. Either have some really good ventilation or hold your breath while it burns off.

Ahh.. I think that's it. What I thought (from the photo) were gaps, are just the black silk-screened markers for placing the resistors. So those need ground.

Which means you'll need to repair things. I recommend STRONGLY that you avoid soldering anything directly to the resistor pads -- the resistors will likely detach.. ugh.

So, three more cuts (where we should have put them in the first place), and then repairs to the original cuts on the ground:

They do connect to what used to be ground. Also connected to the same resistor on the gate end but on the flip-side of the board are those ceramic resistors which then attach to lines leading to the ICs.

I almost DID cut the board originally in the NEW locations. Dang.

Note that you can hook up one of them to try first (the top one in the photo, perhaps), and then do the others only after verification that things are working.

To repair the cuts, just scrape some copper bare on each side of the cut (Dremel tool?), and lay a small piece of bare wire across the gap as a bridge. Then solder it in place.

Cheers

Still doesn't work. Gate voltages are different now. Decreased by about .2 to .3 volts. All three still read differently.

Continuity is broken and connected where it should be as per the newest diagram.

I can still light the LEDs by shorting drain to either gate or source on any of the FETs.

Well, it is possible that Gate-to-Source voltage "matters", then. I don't know enough about that stuff.

Probably best to set it aside for a day and read/think about it some more.

It does matter. And it's the only possibility left anyway, so..

One could conceivably correct for that with new resistor values, but I think your best bet now might be to just restore the original circuit, and get LED strips with the correct polarity.

Or get a controller that works with your existing LED strips, if they exist (I didn't find any).

Oh well. We tried.

Actually, if you want to pursue this further, we should be able to calculate the correct resistance values, and it might work out that only the larger three (component side of board) resistors need changing.

To do this, we need to know what resistance values the three tiny ones (copper side of board) have, and similarly for the three larger (component side) resistors. And measure the voltage input into the larger resistors from the chip that controls them.

It might work out nicely, or not.

Edit: Background: each pair of resistors for each transistor, form a simple voltage-divider circuit, scaling the output from the control chip into a voltage appropriate for controlling the transistor. The voltage we need is whatever is appropriate to achieve the correct Gate-Source differential for the transistor. We don't know that value (no datasheet), but we can figure it out from the measurements above and those already posted earlier.

I don't have any spare resistors around here, so unless I can extract some suitable ones from some other piece of junk kit I might have lying around, it means I have to go out and get them.

But, for shits and giggles, let's see if this makes any sense....

The SMT resistors say "103" on them. The ceramic ones are 390 ohms according to an online calculator. 4 bands, orange, white, brown and gold.

Okay, so this multiplies the control signal by:

10000 / (10000 + 390) = 0.96

Mmm.. that hardly seems worthwhile, but, okay
(the resistors also provide some protection to the controller).

And what is the (full brightness) control-voltage you see, on the input to the larger resistor from the control circuitry?

And what is the voltage (with respect to ground) that you see on the rewired Source when the LEDs are switched completely "off" by the controller?

The idea is that we then modify the first resistor (if possible) to achieve a difference of (0.96 x control-voltage) between Gate and Source.

Cheers

I don't think you will be able to get it to work without some more modifications.

The 10k resistor is a pull-down resistor and the 390ohm resistor is just protection for the uC.

MOSFETs are voltage-controlled, while regular bi-junction transistors are current-controlled.

The MOSFET is connected between the cathode-side of the diode and ground, so you will have a voltage 0-5v between source and gate depending on intensisty of the LED. When it's connected to the anode-side of the LED, as the modification is doing, you would still need 0-5V difference between source and gate, but now you have the LEDs raising the reference for the source and you will never get more than 5V out from the uC, making it not work.

Unless I'm mistaken, what you need to do to get it to work, is to replace the MOSFETs with their opposite equivalent connected between anode and Vcc, and a transistor/MOSFET between uC and gate (with neccessary resistors).

We come across this every so often, we supply equipment that drives the LED signs you see by the side of the road.

From what we see customers doing/using, the most "common" way that these are constructed is with a common voltage and you switch ground in and out via a PWM, occasionally we've come across signs that are configured the other way, where the grounds are are common.

To be able to drive the common ground configuration, we have to supply an adaptor board which like StigOE points out, has the "opposite" FET configuration. (i.e P & N channel FETs swapped)

The alternative method, is to swap the LED's so that the A & K are the other way round, but that's not possible with SMD RGB LEDS because there's no rotational symmetry.

The guy I bought the controller from says all his LEDs are common voltage - haven't heard back from the other person yet.

I think the easiest thing to do at this point is to rework the board back to original function before it gets too complicated to do so, and pick up second set of LEDs. I can hopefully pass the current set along to a friend who doesn't need to use the alternate controller.

Everyone, especially Mark, thank you so much for the assistance. Even though the rework didn't work, it was fun to do and good practice. Even though I really need a new soldering iron, or at least a new tip on this crappy one, I think I did a half-decent job. The anticipation of success by performing the mods was also a treat on what would have otherwise been a relatively dull day - weather was crap which pre-empted a few projects.

The easiest way to change the function would be to use 3 external PFETs; currently it's a low-side switch, with the gate voltage going above ground to turn the FET on and let current flow from D to S.

To change this to a high-side driver (enabling the current flow at the supply voltage side), you'd take a PFET and wire it like this:

- Source of PFET to LED power supply (body diode cathode points to power supply)
- Gate of PFET connected to PFET Source with a resistor - say 10k or 100k
- Drain of PFET connected to R/G/B LED string power line

...then, connect the gate of the PFET to the appropriate R/G/B of your power control board.

The PFET will turn on when the gate drops below the source by >Vgs. When the lighting controller NFET is off, the PFET gate is pulled up to the source by your resistor - ie, no current will flow.

When the NFET in the LED controller turns on, the gate of the PFET is pulled low, meaning Vgs goes negative (Vgate < Vsource, as Vgate will be near ground and Vsource will be your supply voltage). Current will then flow and the LED will light.

Ok, so you'll need to find yourself 3 PFETs (at least 20V rated; getting 60V ones would be best for robustness), but you should be able to do this all "in the wiring" as it's just a single FET and a single resistor per channel. You can also try just one channel to see how it works.

Ok, so call me cynical but this has all the hallmarks of "pre-spam".