Soft circuits experts?

Electronics
#41

I guess I need some clarification. Power is measured in watts (volt-amps). While some power is burned across resistor setting a 1mA bias current result in roughly 20x less power consumption that running the chain at 20mA with no resistor. So your battery actually runs longer. It’s actually also for safety because if one of the LED’s shorts the current draw will increase dramatically without the resistor. The resistance across a PN junction (LED) is in the mOhm to single ohm range versus that fixed resistor so for the resistor there it will be a lot gentler on the battery and likely reduce the change of operating the other LED’s beyond their operating max of 20mA.

I’d love to meet you in the makerspace and talk through these things. But in summary if we can lower the operating current per chain with a resistor it actually improves the power draw overall.

Edit:

To further clarify. Adding a resistor to the chain gives you a set resistance to baseline your current per chain off of. Using parasitic resistances is generally not ideal due to the fact semiconductor variation often has +/- 20% absolute tolerance. By adding a resistor that’s several orders of magnitude larger you eliminate this variation and thus guarantee the current per branch is roughly equal.

The other benefit is if I add a resistor and clamp current down to 1mA I can now add 19 additional chains of LED’s. Because 1*20 = original 20mA.

#42

Let’s do talk. It is a fun topic on how to maximize bling vs battery life. i’ll be around tonight wearing a maroon checked long sleeve shirt and blue jeans

#43

Agh I’ll be at run club tonight. But I’ll be over tomorrow and the weekend :slightly_smiling_face:

#44

Lol I’m just gonna sport my obnoxious but easy to spot pink shirt and try to get Level 1 in my brain

#45

Lol don’t worry. With a few basic equations we can make you dangerous. Everything I did above was algebra! :smiley:

#46

Haha arguable making me safer.

#47

@mstovenour thank you again for your patience and tutoring!

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#48

Did you (all) get it ironed out?
I’d love a follow-up with “the solution”, if you’re willing…
:pleading_face::nerd_face:

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#49

I haven’t sewn up the new circuit just yet…he was explaining the math behind the above series diagram so future circuits don’t have ridiculous numbers and burn the thread lol. I’d followed schematics but never understood where they came from

To over simplify his expertise (for brevity) and butcher it entirely in attempting to explain it…

It’s better to use series instead of parallel circuits
Work it out on a breadboard first
Keep it under 20mA

Max#ofLEDineachlineofseries: VoltageofPowerSupply >= VoltageRatingOnLED*AmountofThem
(Ex. A 9V battery can run 4 2V LEDs per trace/set. If I need 16 lights for a design I need 4 traces/sets of 4 LEDs)

Apply Ohm’s Law to remainder
(In this case 1V of unused power needs resistance. This is where you either measure the part in question or failing fancy pants DMS equipment pick a number between 1 and 10mA since its fabric but never more than 20mA)

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#50

Sounds like a very reasonable set of “rules of thumb.” (And I think you failed to butcher his teachings!
:slight_smile: ) Ohm’s Law and Kirchoff’s Law are really quite simple rules; and once you understand them, simple circuits become… well… simple!

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#51

There is a good led matrix designer here

http://led.linear1.org/led.wiz

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#52

Thanks gentlemen! @Draco really cool wizard! Will definitely add to the materials. I am glad to know where those diagrams are coming from since it will help me along the way when I add sensors and motors and whatnot! Goal is to learn enough about microcontrollers and electronics to do a couple art installations

#53

I don’t know if solution is the right word here. We did not fix the existing design. She could just add a single, properly sized, resistor to it but that design is not very efficient for something operating from a battery. The solution was that I helped her understand how to design these circuits in general. We spent most of the time talking through the design aspects of the circuit below and how to prototype a design on a breadboard so the design parameters can be measured with a meter and possibly tweaked. We talked about how to calculate voltage drop across multiple, series connected LEDs; how to use a variable power supply to measure the voltage vs. current profile for an LED; and how to select the value for a series resistor, when needed.

The design we prototyped for the 9V battery ended up using 4 LEDs and a resistor in each series circuit, similar to what Brent drew below. We chose the number of LEDs and resistor value because with the 9V battery, 5 LEDs were too dim (1.8V each) and 4 LEDs pushed the series circuit to 23mA which is above what is likely a 20mA safe operating range for the LED. On the variable power supply, we found that 1.8V was safe but dim, 1.9V was safe (6mA), and 2.0 was unsafe (above 20mA). We then calculated an ideal resistor value of 233 ohms and proceeded to try a few resistor values above and below that value observing the brightness (desired property) and current (to confirm it was < 20mA). This circuit would have operated fine without the series resistor but would have been unacceptably dim. So in this case the resistor does add value because it allowed us to safely match the power supply voltage to the LED string while also maintaining a desired brightness.

MrsMoose also learned how to measure the LED current so she can adjust the series resistor to account for the voltage drop caused by the resistance of the conductive thread once the design is sewn into the desired pattern.

All said and done, I think there will be more fun to be had with different LED colors (very different operating voltages) and different power supplies. Now MrsMoose has some idea how to design for different operating parameters.

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