Author Topic: 36 leds in series through usb ? how is that possible ?  (Read 1589 times)

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Offline respoda

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36 leds in series through usb ? how is that possible ?
« on: March 05, 2018, 10:14:28 am »
Hi, I'm a complete noob so bare with me...I have 'some' understanding of the bare-knuckle basics but it is extremely limited.
Just so you know, because what's coming next may be a really stupid question, though I just want to understand better.

I have a usb desk lamp, and it has 36 (!!!) leds (in plain series as it seems). First off, I do know this is not a good idea to do since the provided voltage through usb is going to be too low. White led's have a forward voltage of about 3.4 (?) volts, so in series that would require 3.4x36= 122.4v

Despite this it used to shine pretty bright none the less. I tore it apart a while ago, and remember there was a small brown ceramic capacitor (I think ?) between the plus and the switch, but I didn't look at its values. I'm trying to make it work again, but haven't added a resistor yet and thus the leds all shine pretty weak. I may have broke them already perhaps and they're simply dying, that or they're running on way too low voltage.

My question:

a) how is it possible, if these leds need 3.4v each, that a usb 'low' voltage source could make these 36 leds shine bright (in SERIES ?) with just a tiny ceramic capacitor ? The voltage is always going to be too low, so I don't get it... they really seem to be mounted just in series. They're all on a round piece of pcb, and they seem to be all interconnected on the pcb. Nothing else on it except those leds, and a plus/minus wire connected to the plus/minus of the pcb...

b) How can I determine which resistor would make them shine brighter, perhaps figuring out how it was configured originally ?
    There really was nothing in this lamp except the leds, a tiny wire connecting them to a switch and some tiny capacitor in between
    plus wire and switch.

 

Online Brumby

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #1 on: March 05, 2018, 10:27:38 am »
IF the LEDs are, in fact, in series, then there will have to be a boost circuit somewhere.

If you have access to the points of the LED string where each LED connects to the next, apply your power, pick a reference point and take voltage readings.

Pictures would be good - but make sure they are well lit and in sharp focus.  (Getting close is useless if all we can see is fuzz and/or more shadow than detail.)
 
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Offline jim_griff

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #2 on: March 05, 2018, 01:12:24 pm »
If there is only a capacitor and LEDs then they will most likely be wired in parallel with the capacitor acting to smooth any potential ripple that could cause flickering.

If they are in series then a boost converter is required.

One thing you can do to check is to get it on a power supply (3.2V non current-limited) and connect it across one of the LEDs. If all light up then they're in parallel. If only one lights up and the rest dont, then it's in series.
 
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Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #3 on: March 06, 2018, 02:10:45 am »
Ok, thanks for clarifying that.

Here's a picture of the pcb :




Also, the upload feature for images seems to not work.
I can upload it but can't add it to the post ?
 

Offline paulca

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #4 on: March 06, 2018, 02:16:22 am »
They appear to be in parallel to me.

How do they prevent the current flowing through only a few of them?  Last time I put two LEDs in parallel on a breadboard only one of them lit, the other flickered.
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Offline plazma

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #5 on: March 06, 2018, 02:23:52 am »
There are leds with built in series resistors.
 
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Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #6 on: March 06, 2018, 02:25:37 am »
correction, it is automatically added to the post it seems. Didn't show up in the preview though.
 

Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #7 on: March 06, 2018, 02:27:26 am »
There are leds with built in series resistors.

You think these are such leds ?
In any case, I should be able to measure it somehow ?

This pcb and the resistor were all inside the lamp housing...
 

Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #8 on: March 06, 2018, 02:29:54 am »
They appear to be in parallel to me.

How do they prevent the current flowing through only a few of them?  Last time I put two LEDs in parallel on a breadboard only one of them lit, the other flickered.

So this is what I do not get: since they are all interconnected on the pcb's plus/minus, I'd think they are in series ?
Parallel they would all have their separate connection and 'not' be interconnected like this...or am I seeing this completely backwards ?
 

Offline Audioguru

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #9 on: March 06, 2018, 02:32:00 am »
My cheap Chinese flashlight has 24 white LEDs all in parallel like this one. The internal resistance of the "super heavy duty" leaky battery limits the current and a kid in China was paid a bowl of rice to test thousands of LEDs and group them into piles that have the same forward voltage.
 
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Offline plazma

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #10 on: March 06, 2018, 02:35:50 am »
They appear to be in parallel to me.

How do they prevent the current flowing through only a few of them?  Last time I put two LEDs in parallel on a breadboard only one of them lit, the other flickered.

So this is what I do not get: since they are all interconnected on the pcb's plus/minus, I'd think they are in series ?
Parallel they would all have their separate connection and 'not' be interconnected like this...or am I seeing this completely backwards ?
You got it backwards. In parallel all leds share the same anode and cathode node.
 
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Offline Audioguru

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #11 on: March 06, 2018, 02:37:02 am »
Why not look in Google?:
 
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Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #12 on: March 06, 2018, 02:38:42 am »
My cheap Chinese flashlight has 24 white LEDs all in parallel like this one. The internal resistance of the "super heavy duty" leaky battery limits the current and a kid in China was paid a bowl of rice to test thousands of LEDs and group them into piles that have the same forward voltage.

 :-DD

Well, this one was cheap as f* as well. I think it cost me 4 or 5 euro's (that's about 6 bucks)...

So the psb 'does' indicate a 'parallel' connection here ? Seems I am seeing it completely wrong then...
 

Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #13 on: March 06, 2018, 02:40:02 am »
Why not look in Google?:

Thanks! Hadn't thought of that yet :/

Seems pretty obvious here indeed :)
 

Offline respoda

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #14 on: March 06, 2018, 02:44:02 am »
Now that I know they're in parallel I can start some testing as to why they are shining too dim.
I guess it's simply the ceramic capacitor then which I need to add. Just a matter of calculating the correct value...

Many thanks for the responses, you guys are awesome.
 

Offline Audioguru

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #15 on: March 06, 2018, 06:30:33 am »
USB has 4 wires. Some of them require resistors across the two extra wires of the jack to tell it how much current to produce.
Maybe your USB before always provided high current for high brightness but now maybe you are using a different USB that provides low current for low brightness.
 
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Offline paulca

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #16 on: March 06, 2018, 07:15:12 am »
USB has 4 wires. Some of them require resistors across the two extra wires of the jack to tell it how much current to produce.
Maybe your USB before always provided high current for high brightness but now maybe you are using a different USB that provides low current for low brightness.

The resistors across the D+ D- wires tell the device how much current to pull.  You can put a 2.5Ohm resistor across Vcc/GND of a USB port and pull 2A and the device supplying the current will just drop it's voltage ... or go on fire.

Are all of the LEDs lit at all?  Are any of them "out", dead?  If a single one fails short circuit the whole setup is fried and needs to be put in the bin before you fry the device powering it.

In fact putting so many cheap LEDs in parallel is asking for trouble, even if they have current limiting resistors which they will need or one single LED will take most of the current and explode showering little bits of burning plastic into your face if you are unlucky.

I'd bin it and move on.
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Online Brumby

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #17 on: March 06, 2018, 09:22:37 am »
I'd bin it ...

Those LEDs have probably already been binned.

Parallelling LEDs like this seems to be becoming more common - and it will work if the Vf of all of them are close enough.  Manufacturing processes have no doubt improved over the years and to believe they are tight enough to allow this sort of array construction is not outside credibility.

I'm inclined to agree with this:
Maybe your USB before always provided high current for high brightness but now maybe you are using a different USB that provides low current for low brightness.
 
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Online tooki

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #18 on: March 06, 2018, 02:48:22 pm »
Assuming they’re normal (not high-power) white LEDs, assume 20-30mA per LED. (When loads are in parallel, you add their current.) 36*20 = 720mA, 36*30 = 1080mA.

Many cheaper USB chargers — as well as the standard USB ports on computers — provide only 500mA.

When lit, what voltage do you measure across an LED?
 
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Online mariush

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #19 on: March 06, 2018, 03:09:57 pm »
you also have to account for pcb track resistance which could be 50-100mOhm maybe even more
 
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Offline Jwillis

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #20 on: March 06, 2018, 07:15:19 pm »
Not sure what led he has on that thing but a standard 5mm has an forward current (If) of 12ma. That's 432 ma for 36 in parallel.
 
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Online kjr18

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #21 on: March 06, 2018, 08:06:09 pm »
Don't forget about wire resistance. Thin wires act as a current limiting resistor. Led's don't need 20 mA to shine brightly. Even 10mA per diode is enough to shine brightly, as modern led's are very efficient, even 1mA is sometimes enough to use as a indicator.
 
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Online tooki

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #22 on: March 07, 2018, 01:44:34 am »
Not sure what led he has on that thing but a standard 5mm has an forward current (If) of 12ma. That's 432 ma for 36 in parallel.
You can’t make a proclamation like that. Typical is 20 mA, but white LEDs are often driven harder.
 
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Online tooki

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #23 on: March 07, 2018, 01:45:37 am »
Don't forget about wire resistance. Thin wires act as a current limiting resistor. Led's don't need 20 mA to shine brightly. Even 10mA per diode is enough to shine brightly, as modern led's are very efficient, even 1mA is sometimes enough to use as a indicator.
Yes, for indicators. But in LED lighting they’re often driven hard, 30 or even 40mA.
 
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Offline Audioguru

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Re: 36 leds in series through usb ? how is that possible ?
« Reply #24 on: March 07, 2018, 05:58:57 am »
But in LED lighting they’re often driven hard, 30 or even 40mA.
I looked at the datasheets in Google for a few 5mm white LEDs. 30mA absolute maximum (not enclosed but in in a cool ambient) and 20mA is typical for their brightness rating.
I think 40mA would burn out a white LED out soon.
 
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