Measuring 58-LED strip current

[DMCShep]

I have some strips containing 58 RGB LED's each, with wiring supplied in a common-anode fashion. They're 6-pin 5050 types, but the connector only has four pins, one for +12V and the other three for color grounds. They currently include a series resistor as they're intended to operate at exactly 12V. Providing them with a 12V power supply, I'm getting the following readings:

Green - 0.38 A
Blue - 0.35 A
Red - 0.29 A

This was using the blue Model 9205B+ multimeter you sell in your shop. Unfortunately, I can't get a more precise reading than that, but it appears it's around 380 mA tops (which seems accurate considering one LED is usually 20 mA). I'm looking to place four of these strips in a car, controlled by (but NOT powered by) a DC Boarduino. Crossing my fingers that there's no issues with it.

How do I accurately measure the milliamp requirements of the LED strip so I can match it with an appropriate LED driver IC? Or am I barking up the wrong tree entirely?

[adafruit_support_mike]

The values you got sound reasonable. What you have is an analog RGB strip.

We have a tutorial explaining how they're connected and how to work with them here:

https://learn.adafruit.com/rgb-led-strips/overview

[DMCShep]

The values you got sound reasonable. What you have is an analog RGB strip.

We have a tutorial explaining how they're connected and how to work with them here:

https://learn.adafruit.com/rgb-led-strips/overview

Which doesn't cover varying power supplies. That's the problem, I have four 12V strips that are to be used in a car. If possible, I'd like to control the current so the voltage doesn't matter, but I need an accurate mA measurement first, something my multimeter doesn't provide at that kind of level. I can only see two digits when ideally I should be seeing three. Do you have anything that will do the trick for me, or am I taking the wrong approach?

[adafruit_support_mike]

You're using the wrong approach. Just use a 12v voltage regulator for each strip.

The strip has built-in current limiting resistors on each set of LEDs. As long as you supply 12v, the current will self-adjust to the correct level.

[DMCShep]

Any suggestions on selecting a good 12V regulator? I need one that allows the voltage to be either below or above 12V (probably in the 9V to 60V range), so I can't use the 12V equivalent of the 5V or 3.3V voltage regulators you sell.

[adafruit_support_mike]

The LM7812 is a good choice for automotive applications.

An analog LED strip doesn't need a highly stable voltage, just something in the right general range.

[DMCShep]

Reading the datasheet, it lists a 14.5V minimum for it to accurately put out 4V. Considering the car I'm installing it into has voltage ranges well below that, what happens if it's supplied with a steady, say, 10 volts? (Which is not an uncommon scenario actually.) Does the regulator get damaged, or simply provide unregulated output below 12V? Also, would the 8 Darlington Transistor array you sell adequately PWM the LED strips when hooked up to an Arduino, or does that change the voltage at all?

[adafruit_support_mike]

An LM7812 fed from a 10v supply will be fine. It will just produce about 8v of poorly regulated output voltage.

The LED strip might not even be able to light the blue LEDs from a 10v supply voltage though. The nominal forward voltage for a blue LED is 3.5v, and the strip puts three LEDs in series with a current-limiting resistor. The design values will be 10.5v across the LEDs and 1.5v across the resistor, with the resistor scaled to provide about 20mA at that voltage (about 75 ohms).

Blue LEDs will still produce some amount of light down to about 3.2v, but that would reduce the circuit to 9.6v across the LEDs and 0.4v across the 75 ohm resistor, giving you about 5mA.

Start by hooking up the LED strip without any external control and see if your car can provide enough power to make the strip work at all.

[DMCShep]

Start by hooking up the LED strip without any external control and see if your car can provide enough power to make the strip work at all.

That actually gave me an idea: using a 9V 1A power supply from your shop, I was able to test the output. It's not terribly bright like it is with a 12V power supply, but it does function, even with all three colors lit up at once (and it still produces the same colors across the board, none were dimmer than the others). Since it still works as low as 9V, just not as bright, should I look into building a step-up voltage converter into my circuit to ensure it always has at least 12V? And if so, where do I look for one?

Also, I will need to conditionally turn them on or off based on four wires from the car (example: if headlights are on, turn two of the strips off), and an additional two will provide VCC and GND functions. Can something like a ULN2803 serve as a good replacement for 8 standalone transistors for powering the LED strips? Will be using a Metro Mini if it matters, and will also need a way for the Arduino to "sense" that the voltage is "High" on any of the given lines. Are transistors the best approach?

[adafruit_support_mike]

You don't want to use the ULN2803. You lose at least 1v across a Darlington when it's turned on (effective resistance is about 2 ohms @ 500mA). For this kind of application, power mosfets are a better choice. A good power mosfet's channel resistance drops below 0.05 ohms when it's fully on, and the voltage generated across the channel is small enough to ignore. The IRLB8721 would be a good choice: https://www.adafruit.com/product/355

Adding a 12v converter to your circuit would be a good idea, especially using a buck-boost converter that can accept input voltages above or below the output voltage. We don't have anything that works at 12v, but Pololu has a good selection that do: https://www.pololu.com/category/133/ste ... regulators

[DMCShep]

Forgot to mention: the Adafruit Metro Mini has a "no wait" bootloader when powered by either the VIN or 5V pins, correct? I'm hoping for an Arduino with no discernible delay between power-up and when the setup() code runs, and have my eye on the Metro Mini. Or would the bare ATMega328P chip be a better choice?

You don't want to use the ULN2803. You lose at least 1v across a Darlington when it's turned on (effective resistance is about 2 ohms @ 500mA). For this kind of application, power mosfets are a better choice. A good power mosfet's channel resistance drops below 0.05 ohms when it's fully on, and the voltage generated across the channel is small enough to ignore. The IRLB8721 would be a good choice: https://www.adafruit.com/product/355

Definitely meets my power requirements! Will be going that route, thanks!

Adding a 12v converter to your circuit would be a good idea, especially using a buck-boost converter that can accept input voltages above or below the output voltage. We don't have anything that works at 12v, but Pololu has a good selection that do: https://www.pololu.com/category/133/ste ... regulators

I only saw one on their site that looked like it would work. I'd like to integrate the final result into my own circuit board, instead of relying on external circuits that would need to plug into mine, so I started looking at DigiKey's PMIC - Voltage Regulators - DC/DC Switching Regulators page, with the following filters:
1) Through Hole**
2) Current-Output of 4A or more
3) Function of anything listing "step-up"
4) Voltage Input (max) of 40V or more

[** Only because I still haven't learned how to solder surface-mount devices yet. Might not be a bad time to learn though, especially if a better alternative exists in surface-mount form...]

And I'm left with about a page and a half of results to select from (before it becomes a minimum order quantity of 90, that is). Randomly sampling some of their datasheets, it appears most (if not all) of them require external components, which is fine.

How do I narrow down the results from here? It looks like some of these guys can give a constant 12V, which would be ideal, but I might be able to get away with a 12V minimum if I do it right.

[adafruit_support_mike]

Always remember to click the "In Stock" button.. that's an easy way to eliminate a good chunk of the listings. Then sort by price or part number to group all the variants of a particular chip.

Once you've done that, you're down to a handful of actual chips:
http://www.digikey.com/product-search/e ... ageSize=50

The TI LM2587 and LM2588 are basically the same, but the LM2587 has a fixed-frequency internal oscillator and the LM2588 has an adjustible oscillator. For your application, no big deal. The LM2587 is probably easier to use.

The Linear LT1070 and LT1170 are also about the same. The LT1070 looks like an older version and uses a slower clock than the LT1170.. 30kHz versus 100kHz. Again, no big, but higher clock speeds mean you can use smaller external components, so the LT1170 edges ahead slightly.

The LT1074 is a false positive.. it's a step-down regulator that can be used as an inverting boost converter (9v to -18v or something). It isn't relevant to what you want to do, so you can ignore that one.

The On Semi MC33167 and MC34167 have the same datasheet, and are variants with different ranges of operating temperatures. The MC33167 has a wider range, so if the prices aren't too far apart you might as well use that one.

So despite the 37 entries in the list, you're really looking at three basic chips. Read the datasheets to see which one makes the most sense to you, price the external components to see how much those will cost, and if you still haven't collected enough information to make you prefer one over the others, get the one that's cheapest.

[DMCShep]

Okay, so I did quite a bit of research into this, and realized it was simply far too complicated to actually do on my own. I've decided on the Pololu 12V guys you linked me to (thank you for that by the way!), using one for each pair of strips to distribute the load between them. I've got almost everything I need, except for one final thing: I need to be able to read voltage inputs from the car, and use them as inputs on the Metro Mini, in addition to powering it off of some of them. I'm currently using UC3116N Diode Arrays to separate the two functions and the inputs from themselves like so:

UC3611N_Hookup.png (6.73 KiB) Viewed 451 times

Two (hopefully final) questions:

1) Is it safe to connect the Metro Mini's VIN directly to a car's VCC like that, or would I need some kind of buffer first (maybe the Pololu units I got?)

2) How would I perform a digitalRead() on a voltage above 12V that would, if directly connected, otherwise fry the chip? I read a simple-ish approach here that mentioned opto-isolators, but I'm sharing grounds, so I'm not sure if it'll work. And if opto-isolators would work, how do I select one?

[adafruit_support_mike]

1) If you're absolutely sure the voltage from the car will stay below 16v, you can connect it directly to the Metro Mini. Personally, I'd use a step-down converter to avoid having to worry about it.

2) Optocouplers are a good choice, and work whether you have a common ground on both sides or not. You could also use a transistor clamp like this (which I just drew for another thread):

clamp.jpg (16.31 KiB) Viewed 442 times

[DMCShep]

1) If you're absolutely sure the voltage from the car will stay below 16v, you can connect it directly to the Metro Mini. Personally, I'd use a step-down converter to avoid having to worry about it.

That's what I was thinking too. Particularly with transient voltage spikes being common in automotive use. Would your 5V 1A TSR12450 work, connecting 5V out from the TSR12450 to the 5V pin on the Metro Mini? I know quite a few Arduinos can work by hooking up a 5V regulator to the 5V pin, thereby bypassing its built-in voltage regulator in lieu of an external one, but I'm not sure if the Metro Mini works the same way or not.

Also, how do I protect against spikes above the 32 range? Stable range is somewhere around 9V to 15V, but there are spikes every now and then as much as 60V (or so I've read, not sure how accurate that is). Or should I not worry about that?

2) Optocouplers are a good choice, and work whether you have a common ground on both sides or not. You could also use a transistor clamp like this (which I just drew for another thread):

clamp.jpg

I'm digging this approach far better actually. Very simple to integrate into my current circuit. Thank you!