(MicroLumina) current limiters?

[Bernd]

Now I'm back to being confused.  I thought each LED eats 3v, so 4 of them would totally consume 12v, so no resistor needed?  But how do I determine or measure the voltage drop for a single LED?

dave

If they are wired in "series" they consume the 12v. Wired in "parallel" each will only take 3V.

Bernd

[Bernd]

This lovely creature will explain how it's done.

Bernd

[deemery]

This lovely creature will explain how it's done.

Bernd

Thanks, that's a great video for dummies.  The one remaining question I have is how to determine the voltage for the various LEDs I have.  For some, I can find the spec sheet.  After searching, I found the "forward voltage" range for the 0805 LEDs, it provides a range:  " Forward Voltage VF 2.7 3.3 3.7 "  (min, typical, max), so I presume I should use 'typical'?    Is there a way to determine the voltage for an unknown LED (using a voltmeter and power supply where I can set both voltage and max current?) 

But if I'm right and 4 LEDs each consume 3v, I don't need any resistor?

dave

[deemery]

Here's a page that talks about how to figure out voltage for an unknown LED:  https://electronics.stackexchange.com/questions/173468/easy-way-to-figure-out-a-leds-vf-in-order-to-pick-an-appropriate-resistor   The simple answer is for me to get a cheapo voltmeter that includes an LED measure    

dave

[jbvb]

My lab power supply lets me set voltage and current independently. The gauges claim a precision of 0.001 but there's no expensive sticker on it confirming that precision. Most frequently, I set the "typical" voltage and slowly turn up the current, stopping when I get to the brightness I want. Then I choose resistors based on the sweet spot voltage & current.

You could make up your 4 LED series string temporarily with clip leads and see what voltage it actually drops, and what current gives you the brightness you want.

[deemery]

From the Department of Things I Don't Understand:  Turns out my old voltmeter has an LED setting.  I tried the new and old voltmeters on a set of 5mm LEDs (nice and big, easy to apply the leads.)  I tested the LEDs, they light up with the MicroLumina LED tester.  But when I apply the meter leads, no matter which way I apply them, I get "0L".   What I expected to happen was the LED would glow a bit, and I'd see something around 3 volts.  

dave

[Bernd]

From the Department of Things I Don't Understand:  Turns out my old voltmeter has an LED setting.  I tried the new and old voltmeters on a set of 5mm LEDs (nice and big, easy to apply the leads.)  I tested the LEDs, they light up with the MicroLumina LED tester.  But when I apply the meter leads, no matter which way I apply them, I get "0L".  What I expected to happen was the LED would glow a bit, and I'd see something around 3 volts. 

dave

From the Department of Electrical Devices Research play with this calculator.

https://ledcalculator.net/

Bernd

[deemery]

From the Department of Things I Don't Understand:  Turns out my old voltmeter has an LED setting.  I tried the new and old voltmeters on a set of 5mm LEDs (nice and big, easy to apply the leads.)  I tested the LEDs, they light up with the MicroLumina LED tester.  But when I apply the meter leads, no matter which way I apply them, I get "0L".  What I expected to happen was the LED would glow a bit, and I'd see something around 3 volts. 

dave

From the Department of Electrical Devices Research play with this calculator.

https://ledcalculator.net/

Bernd

The second thing it asks for is "LED Voltage Drop".  That's what I was expecting to get from the multimeter.   And this presumes a string of identical LEDs, but I'm hoping to do 3 of one type and 1 of a smaller type.   Tomorrow, I'll solder together the string of (4) LEDs and try James' "successive approximation" approach.  Today I practiced soldering very thin wires, for me soldering is a perishable skill!

dave

[jbvb]

Several VOMs (Voltmeter/Ohmmeter/Milliampmeter) I've owned have had a little diode picture at one dial position.  What that does on mine is tell me if the diode was blown (didn't conduct either way) or functional. The test current is enough to make all the LEDs I've tried glow, but I'd have to read the instructions to say what the display shows while doing this.   Do you still have the instructions for your VOM?

Clip leads are inexpensive and more convenient than soldering while you're still testing things.

[deemery]

Several VOMs (Voltmeter/Ohmmeter/Milliampmeter) I've owned have had a little diode picture at one dial position.  What that does on mine is tell me if the diode was blown (didn't conduct either way) or functional. The test current is enough to make all the LEDs I've tried glow, but I'd have to read the instructions to say what the display shows while doing this.  Do you still have the instructions for your VOM?

Clip leads are inexpensive and more convenient than soldering while you're still testing things.

I have the instructions for the new VOM, and followed them.  An on-line discussion said 'connect the black lead first', but the order in which I applied the probes didn't make any difference with either meter.

dave

[Bernd]

I have the instructions for the new VOM, and followed them.  An on-line discussion said 'connect the black lead first', but the order in which I applied the probes didn't make any difference with either meter.

dave

A simple question to Google asking for forward voltages of LED's gave a cheaper answer than buying a VOM. Probably asking ChatGPT would have helped too.

https://www.circuitbread.com/ee-faq/the-forward-voltages-of-different-leds

Bernd

[jbvb]

Connecting black before red is correct with the grounded side of a live circuit, but not applicable to this case.

I tried the diode test function of my fancy VOM with a purple LED.  It didn't report continuity with either polarity, but it did light the LED with the correct polarity,.

Bernd, I'd trust the manufacturer's spec unless something blew up when I followed it. But there are a lot of oddball parts out there; I'd always verify any general result from web search. And AI can be completely sure about a totally bogus result, so I treat it like that type of person.

[deemery]

Since I can't seem to use my meters to measure the forward voltage through LEDs, I think what I can do is use the information I do have (including Bernd's link), and assume (a) the LEDs will all take 3v (b) if the smaller LED takes less, the larger LEDs will be able to 'eat' the extra voltage.  So then, it's just a case of determining the resistor for 4xLED in series.



Now if I assume the big LEDs are 3v and the little one is 2v, then the average is 2.75, and here's the result:


But once I get things assembled, I'll first test with the lab power supply, setting it to 12v and then slowly increasing current until I'm happy with the result.  Assuming the magic smoke stays in place, then I'll figure out the actual resistor value from the current on the power supply.

dave

[jbvb]

From my Velleman E12 Series resistor assortment (610 pieces), your choices would be 47, 56 or 68 Ohms. I'd chose higher over lower.  68 will be easier to find than 56 because 68 is in the E6 series.  I plan to go to Seashore Thursday, could drop one off on my way home.

[deemery]

From my Velleman E12 Series resistor assortment (610 pieces), your choices would be 47, 56 or 68 Ohms. I'd chose higher over lower.  68 will be easier to find than 56 because 68 is in the E6 series.  I plan to go to Seashore Thursday, could drop one off on my way home.

I have some resistor sets, no need to stop by.  But at some point, we'll have to do a huddle using your meter and my meter on LEDs to see if we can figure out why I wasn't getting my expected results.

dave