Cutting Flex Strips

[tng5737]

I contacted Ray requesting that he find out just where we can cut these strips.  This is his reply:
"i checked the engineer, he told me you can cut this TM1809 digital strip each 3 5050 SMD.
but you need to add a diode which can lead to short circuit beside the resistance(6.8K),please check the PCB, and find the location of the 6.8K resistance, and then add the diode."  
I have asked for clarification sending him this photo  - the arrows point to where  I THINK he is talking about.  When I hear back I will post the response.

[urthegman]

I am NOT going to start panicking, I am NOT going to start panicking, I am NOT going to start panicikng!!! If I just want to use the first 90 nodes, Could I just leave my strips whole and NOT have to add a diode? Ugggghh

[tng5737]

As long as the strip are "whole"  they will work just fine - the problem comes when you try to cut them and run the 'cut' sections as if they were whole strings.   I think most of us assumed that we could just cut these strings anywhere the two IN/OUT pads occur.  Apparently, that is not the case!

[Corey872]

Admittedly, I don't have any strings yet, so no first hand experience, but I don't see how a diode is going to do much in a DC system beyond offering reverse polarity protection.  When you mention 'frying', it sounds like an over current situation?  Maybe the LEDs came on very bright for a second or two then died?  Or possibly the LED's were normal and the chip died?  If I read the datasheet right, the 1809 is actually a 5V chip which needs external resistance (7K) to run on 12V.

In either case, it sounds more like a current limiting resistor would be the fix?  Is it possible there are 1, 2, or more resistors along the strip to help limit / balance current and you happened to cut a section out which had no resistor of it's own?

Datasheet
You are not allowed to view links. Register or Login

[tng5737]

If youlook at ther datasheet you will see a table which indicates that to drop the voltage from 12v to 5v you need a 7K resistor.  That is the 6800 (682) resistor I believe he is talking about.  It is marked RS.  I also do not see what good a diode would do unless it is protect against accidental reversal of voltage.  Ray has replied that he will get back with me soon. 

[RJ]

I am going to look at this myself as I can not understand why you can not cut it every place marked. The Diode I would also thing would be reverse polaritity protection. I was originally told you could cut them each place.

RJ

[tng5737]

Right on Ray's site for this item (You are not allowed to view links. Register or Login)   IT even says that you can cut these every 3 LEDs.  I'd try this myself but at $53/strip - I don't want to find out the hard way that Ray posted bad info!

[Corey872]

Truth in advertising.  It sounds like they can be cut at each pad, though they still need to be connected back to the proper section of the string!

Guess I'd be curious to know: 1) does each chip appear to have it's own Rs / 682 resistor?  2) If the voltage on the + / - pads changes as you go down the string or is there 12V at each pad?  That should be pretty easy to check without damaging anything and what ever is coming out of the first section is going into the second section, so if there is a drop, then you would need to add the correct resistor or drop voltage on the 'input' of the later sections.

[chrisatpsu]

has anybody tested them yet? is each led controllable, or just a set of three operate as one. when i test the metal squares, or the plastic rectangles, they operate as sets of three.

[RJ]

Chris Modules are suppost to work all as one. They are a single pixel with multiple leds on them for brightness.

The Ribbons, ridged strip, strings all work with each led being a nodes so they work totally independant. I can control each led in the ribbons, ridged strips and strings.

RJ

[tng5737]

I heard back from Ray - he said that his engineer verified that the above photo is correct.  The two empty pads are where you are supposed to add the diode.  He did not specify WHY!!!   Personally, I believe that as RJ said, it is to proptect against polarity reversal.   In other words - the cut strings should not have burned out.

[RJ]

Maybe they are not burnt out?  Maybe they are not getting power?

Heres my thought,

  I tried to look at the flexible pcb on them and trace it but it is really hard to do.

But if you look at the wiring diagram here :

You are not allowed to view links. Register or Login

Roll down the page to find a schematic of it inside a picture of the string.

It appears you should be able to cut it anywhere marked giving you a three led segment.

I am thinking ... (this is always dangerous!) 

Maybe the power from each section does not connect to the power pads but bypasses them to the inside section of the diode pad. So you do not need a diode at each section because it would have a voltage drop and across a full string you would not want it as it would not work. You would only want one diode on the whole string to protect from hooking the string up backwards.

You can handle one voltage drop but not one for every 3 leds!

So to get power to the section with out a diode you need a diode added to pass the power. With out it you have no power applied. So a test would be to take one of the sections you think you killed and solder a jumper across the diode pads. Make sure to hook the power up the correct direction as you have no protection.

this is what the engineer means by "It might burn up with out it." He means it could be hooked up backwards on the power and be damaged with out a diode to protect it I am thinking.

 My issue is my strips need to be cut in the middle for what I am doing and this is at a spot with a diode I believe.

Let me know what you find out as I will be very interested myself.

RJ

[n1ist]

Someone should try measuring the resistance from either diode pad to +12 and to GND.  This may give a hint to whether it is a series diode (for reverse polarity protection) or a zener (for better regulation of the 5V rail).  Do the diodes have any markings on them?  I don't have any flex strips yet, so I can't do this myself.

/mike

[tng5737]

Well, I don't trust that diagram - for one thing the layout doesn't match the layout of the strings - also , you'll notice it says 24v   The 104 bypass cap isn't even shown.  It's probably just some drwaing which Ray threw up there.  
I did some probing - IT appears that the engineer is correct you can cut BUT you have to add the diode.   The +12v feeds the anode of the diode and the leds.   The diode cathode feeds the RS resistor (and 104 bypass cap) which supplies VDD to the chip.  Without the diode - no pwr for the chip.  The cathode is also connected to the cathode of every  diode pad (empty or filled) The one diode in a set of five sections feeds all the RS for each 5 section segment. (ie, between the places where the pads are soldered together.)

[RJ]

I forgot I had one extra strip from the orignal order. I cut it to try and help out with this. It did in fact not work until I jumper the diode location. 

I have confirmed that your strips are not dead you need to put either a diode or a jumper on the place marked for a diode. I have a segment setup with a jumper running on a SSC with the test firmware that cycles the colors for you and it is working fine. I will run it for a few hours and see how it does.

The power traces eun down the sides on the bottom and do not connect to the pads as I suspected. The diode is the point every one meter where the power is put into that meter of strip through a diode. Jumpering does leave you open to damaging it if you wire it backwards and a diode would drop a little voltage. I think it would be fine but you can always add a diode where you need it.


RJ