Hi Mike (and all you other helpful folks),
OK! I did a lot of work on this last night and have everything typed up to report.
I got this message about a schematic now that I'm back at work. I don't have one, but I've notated this image of the breadboard I'm using to describe every connection on it:
I did all the tests we all talked about last night. To me, the results were inconclusive. I'll share them here to see if they make anything stand out to anyone.
In all tests below, when I say I'm measuring voltage before the regulator, I'm putting the positive and negative probes of my multimeter on the breadboard rows connected to the biggest cap on the left. For "after the regulator" it's the one on the top right. At Vin means I put the ground of the meter on the ground rail at the top of the board, and the positive on the row with those blue and green wires at the top left which are connected to Vin of the Mega.
Before I added the caps Grumpy_Mike suggested:
Voltage coming into my DC/DC regulator (from wall wart) with circuit completely off: +12.44V DC
Turn on all 9 lights at full brightness / full current: 12.07 V
Once I disconnect the wall wart, the voltage before the regulator takes a very long time (30 seconds a volt) to dissipate. I'm assuming this is because of the caps in the regulator.
If I measure directly after the regulator with lights off, I get the expected voltage, 7.5V
Lights all on full I get 7.05V
@Vin to Arduino Mega: 7.5 off, 6.74V lights on
With just the 2200uF electrolytic cap across the input side of my regulator:
Input to regulator: 12.45V off, 12.02 lights on
Output from regulator: 7.53V off, 7.1V lights on
@Vin, 7.5 off, 6.74 on
With 2200 uF on output side as well:
Input to regulator: 12.44 off, 12.09 on
Output: 7.5V with nothing on, 6.43 lights on
@Vin, 7.5off, 6.08 on
Adding the .1uF ceramic caps on input and output to regulator:
Input to regulator: 12.44 off, 12.1 on
Output: 7.53 off, 7.26 on
Measurents of current:
I put my meter in line with the power going off to the LEDS.
In the “lights off” state, it pulls .028 amps.
1.94 with lights on.
I then set the "dot correction" registers on the LEDs to a value of 10. (Max is 127)
The lights then pulled 1.19 amps.
At "0" dot correction, they pulled 1.058
If I set the dot correction to 1, and measure current at the input to the regulator, I get: .085A off, .784A on
At the output of the regulator, I get .110A off with Arduino on (.026 if I unplug Vin from Arduino) and 1.131A with lights on
At dot correction 127, I get 1.89A at the output of the regulator
(The data sheet for the Alegro PWM drive on the lights made me think settign dot correction to 0 would result in 36.5% of my current at dot correct 127. So I was suprised I was seeing 1.131 at 0 and 1.89 at 127, since that is only 60% of max current. Makes me think something is fishy or I'm measuring current at the wrong spot or somehow incorrectly, or reading the data sheet wrong)
One more voltage test, with all the caps and the dot corretion set to lower current (value of 1)
Input to regular: 12.45V off, 12.28 lights on
Output to regulator: 7.53V off, 7.4V on
(***I'm probing at the two breadboard rows that big cap in the top right is plugged into. I don't understand why if I leave the positive probe in that location and move the ground probe to the groudn on the breadboard rail, I see a voltage of 7.21V with the lights on [rather than the 7.4V I see with the probe right at the cap. That confuses me a lot.)
@Vin: 7.47V off, 6.99 lights on
I'm looking to hear if anything stands out as unusual, and to come up with some next steps to try.
The only things left I can think to try are:
-- Using the other DC/DC regulator I have sitting around
-- Replacing all the wires that are carrying the high current 7.5. They are currently a combination of the breadboard, some 26AWG male/male prototyping wires, and some 24AWG stranded hook up wire that is technically probably all too little for this current. [Though I don't know how to easily avoid using the breadboard to carry this current)
One thing I have noticed is that the first light in the chain blinks a fair amount unless I connect the 7.5V volts to Vin on the Mega. That's why I started going down this route of trying to power the Mega off the same voltage as the lights in the first place, and why I've been caring about making sure the Vin voltage stays over 6.6 on Vin so the Mega doesn't switch to USB power. I'm not exactly sure why that light flickers when the Arduino isn't being powered by the same 7.5V as the lights, I had initially planned to do something like macegr's plan to power the Arduino on USB and the lights on the 7.5VDC. But, maybe all these problems are related. One way or another, I just don't understand why I'm seeing the voltage drops I'm seeing where I'm seeing them.
Grumpy_Mike: Just a note to you - you mentioned earlier about me wanting star wiring. Usually, I have direct runs of power / ground pairs every three lights in the chain coming back to the power rail on the breadboard. To simplify this test, I removed all those and just have the first light hooked to the power (where indicated in the image above on the left). That power is then chained through the leds. I still feel like my Vin and other stuff is properly hooked up in parallel...