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Leds With Resistors (parallel) Reducing Source Voltage

#1 User is offline   LiveBrianD 

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Posted 21 November 2012 - 04:39 PM

I cannot find anything on this ANYWHERE on the internet, so I'm posting it here. I bought a bunch of 20cd 3.2-3.8V 30mA white LEDs, and a ton of 56ohm resistors. I connected each LED with a resistor, repeat 12 times, all in parallel. The source is 5V. (USB) Each LED gets 3.3V, good enough, but the source is now down to 4V. (a USB phone charger I had lying around - the more I add to it, the lower the voltage gets) The odd thing is that my multimeter tells me the LEDs are pulling around 108mA (yes, for 12 of them - the current seems to be proportionality less as I add more LEDs, as it was 24mA with one, 40mA with two, 55mA with three, etc), and the adapter is rated for 700mA, so this should not be an issue. My watt meter (120V) isn't even detecting this, so I don't think I'm shorting it (I know it'll detect 3W or so of draw, for instance). Any thoughts?

Note: I'm doing this on a breadboard at the moment, but will eventually be soldering it together. I already drilled some holes in one of those plastic CD protectors that come in a pack of CD-Rs or DVD-Rs for the LEDs.

This post has been edited by LiveBrianD: 21 November 2012 - 04:50 PM

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#2 User is offline   waldojim 

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Posted 21 November 2012 - 05:09 PM

No offense, this is where a basic electronics course comes into play. You are missing quite a few of the basics here, which is why you aren't understanding the voltage drops or the current drops.

Start here, this will help you understand what is going on.
http://www.clear.ric...basic_elec.html

Also note, that most of those 5 volt phone chargers are NOT designed correctly. Under different load types, they don't necessarily deliver 5V. This has become a serious concern with the Raspberry Pi. As such, use a dedicated high capacity, high quality USB power adapter.
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#3 User is offline   LiveBrianD 

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Posted 21 November 2012 - 06:23 PM

I'm getting a voltage drop to around 2-3V when using that. Interestingly, I got the same issue when doing this off my desktop. I've got 24A on the 5V rail, so this shouldn't be a problem, but I'm getting the same drop, measured from that USB port - perhaps this is due to the USB current limit? (however, according to software anyway, the PSU didn't blink an eye - and if the 5V rail had dropped like that, the PC would've crashed)

When I ran this off a heavier duty PSU (2A, variable voltage), at 4.5V, it was pulling around 180mA, and was brighter. (and the voltage was actually 4.5V) Sure, maybe cheap chargers can't deliver the full rating, but I shouldn't have been even close to the limits. I used a big resistor to put load on the USB charger (v=ir, r = ~7ohms), and again, noticed a huge voltage drop.

Anyway, I'll read more about this.
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#4 User is offline   mjd420nova 

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Posted 21 November 2012 - 07:49 PM

I cringed in my seat when you mentioned USB 5V as your source. Now I understand. If you have a constant voltage/constant current source, set it to 5 volts and check the current. Those LED also create loads of heat when at the full current draw at maximum intensity. A reduction of 1.5 volts doesn't show much intensity reduction but the current does reduce proportionaly. Also a lower current will lengthen the turn on time. Sometimes a smaller supply can be bolstered with a couple large capacitors. You could also make some variations in the balast resistors but a good current source with limited drop on load would go along way.
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#5 User is offline   LiveBrianD 

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Posted 21 November 2012 - 08:00 PM

The only 5V sources I have are my PC, and some USB chargers. The other power source I have is up to 2A, but has settings of 3/4.5/6/7.5/9/12V. (no 5V) I'm not running the LEDs at the max - they're rated for 3.2-3.8V forward voltage, up to 30mA, and I found I'm realistically giving them about 3.3V.

The thing is, I figured that, worst case, 30mA*12=360mA, still well below the limits of the 700mA adapter I was using. I know that can charge a phone without a problem.
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#6 User is offline   waldojim 

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Posted 21 November 2012 - 08:33 PM

View PostLiveBrianD, on 21 November 2012 - 06:23 PM, said:

I'm getting a voltage drop to around 2-3V when using that. Interestingly, I got the same issue when doing this off my desktop. I've got 24A on the 5V rail, so this shouldn't be a problem, but I'm getting the same drop, measured from that USB port - perhaps this is due to the USB current limit? (however, according to software anyway, the PSU didn't blink an eye - and if the 5V rail had dropped like that, the PC would've crashed)

When I ran this off a heavier duty PSU (2A, variable voltage), at 4.5V, it was pulling around 180mA, and was brighter. (and the voltage was actually 4.5V) Sure, maybe cheap chargers can't deliver the full rating, but I shouldn't have been even close to the limits. I used a big resistor to put load on the USB charger (v=ir, r = ~7ohms), and again, noticed a huge voltage drop.

Anyway, I'll read more about this.


For the record, most of those chargers barely deliver half of what they are rated for. I found this out with my Pi. 1A chargers couldn't keep it from crashing (it only needs .7A). Then I moved to a 2A quality unit, and never had trouble since.

Also, your PC may have 24A available at the 5V rail, but your USB controller will limit the current to the ports. Usually, to 500mA.
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#7 User is offline   LiveBrianD 

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Posted 21 November 2012 - 08:46 PM

I figured the current to the port was limited, but why is this being an issue when, worst case, I shouldn't be anywhere near the limit? I'm pretty sure those should be able to provide the full current, since the PSU has plenty.

Now, don't most phones actually draw the full 500mA (or 1A) the charger claims to provide? Or are they getting it, but with a really low voltage?
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#8 User is offline   mjd420nova 

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Posted 22 November 2012 - 08:51 AM

That's why I held my breath when the USB came up. Drawing anything over 100 MA makes me realy nervous. You could select a higher voltage and see what that drops to when loaded. The universal adapters are not the best source but for LEDs it'll be okay. I won't buy a USB device that draws power from the port. I've done enough repairs on my own boards and quite a few clients. I've tried fuses to protect the ports which works okay but lifted traces and smoked regulators are not uncommon. I've played around with LED arrays the alpha-numeric displays while teaching basic electronics and use the seven segment display elements to teach hexadecimal counting. This is the foundation for logic gates.I use a lot of LEDs tied to data lines, control lines and such so the actions of a training processor board (only 4 bit) can be seen and I've even modified the clock so the action can be slowed down and the actions analyzed.

This post has been edited by mjd420nova: 22 November 2012 - 08:55 AM

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#9 User is offline   LiveBrianD 

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Posted 22 November 2012 - 08:58 AM

If the port should be able to do 500mA, why would 180mA or so be an issue? And I don't need the cleanest power here, since this isn't as sensitive as something like waldojim's pi.

My intention was to eventually make this into a little desk lamp, and I'd rather use the adjustable power supply for other things. I have a spare Lacie power brick, but it makes high pitched noises when on - those are known for bad caps. (I disassembled it, and the caps actually do look like they aren't entirely flat.) I certainly don't know what world the 12V line lives in. (The 5V looks better.)

This post has been edited by LiveBrianD: 22 November 2012 - 09:03 AM

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#10 User is offline   LiveBrianD 

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Posted 22 November 2012 - 11:14 AM

Damn it... I see - a device needs to negotiate with the computer to get more than 100mA. The problem is, according to wikipedia, a charging port can provide up to 500mA without negotiation. The 700mA charger I have is actually an older Samsung with a proprietary connector. I chopped that end off, connected the +5V and the ground/negative wire to a spare USB connector, and shorted the data pins. (I've charged a phone off this without issues)
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#11 User is offline   waldojim 

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Posted 22 November 2012 - 03:16 PM

To be honest, for testing, I would suggest using a 6V battery, and drop the voltage accordingly. The battery will be very consistent while testing.
"There is a cult of ignorance in the United States, and there always has been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.'" -- Isaac Asimov

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#12 User is offline   LiveBrianD 

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Posted 22 November 2012 - 03:51 PM

Wouldn't 4.5V (no regulation on the source voltage) be better?
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#13 User is offline   waldojim 

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Posted 22 November 2012 - 04:55 PM

View PostLiveBrianD, on 22 November 2012 - 03:51 PM, said:

Wouldn't 4.5V (no regulation on the source voltage) be better?

It is far easier to find decent 6V batteries - and why I mentioned it. I use a 6V sealed lead acid battery for project testing.\

Similar to one of these here.

This post has been edited by waldojim: 22 November 2012 - 05:01 PM

"There is a cult of ignorance in the United States, and there always has been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.'" -- Isaac Asimov

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#14 User is offline   LiveBrianD 

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Posted 22 November 2012 - 06:15 PM

So I'm assuming those can provide more current than 3 D cells?
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#15 User is offline   waldojim 

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Posted 22 November 2012 - 07:54 PM

View PostLiveBrianD, on 22 November 2012 - 06:15 PM, said:

So I'm assuming those can provide more current than 3 D cells?

You know of any 4500mA D cell batteries? I don't. For that matter, you ever find a $6 set of rechargeable D cell batteries?

This post has been edited by waldojim: 22 November 2012 - 07:55 PM

"There is a cult of ignorance in the United States, and there always has been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.'" -- Isaac Asimov

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#16 User is offline   LiveBrianD 

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Posted 22 November 2012 - 08:09 PM

OK, I see your point. That said, I figured that D cells might be easier, as I already have several lying around, the voltage doesn't need adjustment, and I shouldn't be anywhere near the limits.
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#17 User is offline   pavel87 

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Posted 10 December 2012 - 08:50 AM

The basic question in itself is pretty simple; you need to run 20 LEDs with the given ratings. They require at least 1.92W. Whereas a USB port can only provide up to 0.5W. When you connect these in parallel the current adds up to 100mA slowly because that’s what a USB port or most of the phone chargers provide generally. Either you need to use a better charger which can provide you with as much current as your circuit wants or you can also try to establish a USB protocol which negotiates up to 500mA current from some USB ports.
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#18 User is offline   LiveBrianD 

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Posted 10 December 2012 - 09:26 AM

The thing is, I shouldn't have to negotiate with the charger - after all, that's rated for 700mA, and it only has a positive and a negative lead coming out of it, so how would I do that anyway? (It had some proprietary Samsung connector I cut off)
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#19 User is offline   waldojim 

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Posted 10 December 2012 - 01:01 PM

You know, right now my brain isn't working, what is your total resistance on the circuit?
"There is a cult of ignorance in the United States, and there always has been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.'" -- Isaac Asimov

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#20 User is offline   LiveBrianD 

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Posted 10 December 2012 - 02:18 PM

3.5V 25mA per LED, so I guess 140ohms each and since that's 300mA total, is that 11.6 ohms total? (V=IR)
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