A lack of understanding about LED drivers....

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
I wanted to set up half a dozen Hi-Brite LEDs under a ledge to provide illumination, and I had a convenient connection to a UK mains line (240v AC single phase).

The LEDs are 3v, 30ma. And size was an issue. So I bought this LED driver from ebay. https://www.ebay.co.uk/itm/Led-driver-power-supply-transformer-240v-dc-12v-adapter-1-36W-300ma-8-12w-5v/362673175057?ssPageName=STRK:MEBIDX:IT&var=631973633249&_trksid=p2057872.m2749.l2649
It fits nicely in the space available.

The spec says 3-9v output, but the heading mentioned 12v, so I rigged it up with a test LED and put a 680 ohm1/8W resistor in series - the LED calculation for 12v says 700. That worked nicely with around 2.8v across the diode, so I wired up the LED string - two parallel wires with 6 LEDs and Diodes across the wires, and connected the other end to the mains.

What happened was:

1 - the leds lit nicely
2 - the resistors got hot - about 100Cdeg.
3 - the voltage across the wires measured at about 40v with a digital multimeter
4 - the voltage across each diode measured about 25v with the same multimeter
5 - the driver got warm

I do not understand the voltage measurements. I was expecting around 5-10v across the parallel wires, and 2-3v across each LED. The LEDs have not blown, so the voltage MUST be within limits, but my meter suggests that it is not. I don't understand what is happening, and don't want to go to the next stage, which is putting in a PWM switch to dim the LEDs without knowing a bit more about what is going on. Has anyone got any ideas? Or suggestions for further measurement which would give more information?

DG
 
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djsfantasi

Joined Apr 11, 2010
7,769
Can you show your calculations? You may have made a mistake.

UPDATE: Nevermind. Confused Wattage and current ratings. 0.5W is well within the 1-3W rating of the module.

Just calculating a reasonability test, if you have six 30mA LEDs in parallel, you are drawing 6 x 0.3 amps or 0.18A. . At 3V, this is 540mW. Umm, your module is rated up to 300mw. You may be drawing too much current and the fact that everything is running hot (for now) supports this.
 
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BobTPH

Joined Jun 5, 2013
3,506
Please post a schematic. We cannot tell how you have hooked this up.

That said, the LED driver you bought is not the right thing. It appears to be a 300mA constant current driver.

Bob
 

BobTPH

Joined Jun 5, 2013
3,506
Can you show your calculations? You may have made a mistake.

Just calculating a reasonability test, if you have six 30mA LEDs in parallel, you are drawing 6 x 0.3 amps. At 3V, this is 540mA. Umm, your module is rated up to 300mA. You may be drawing too much current and the fact that everything is running hot (for now) supports this.
6 x 30mA is 180mA.

Bob
 

eetech00

Joined Jun 8, 2013
2,498
Maybe you ordered/received the wrong power supply. There is one with and output of 36-65vdc.

You haven't stated how many LEDs your planning to connect. It may be better to connect them in a series/parallel configuration.
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
Wow! I hadn't expected such a response so rapidly! Thanks, guys.

I assumed, before the components arrived, that the output from the drive unit would be a max of 12v. So the LED calculators suggested 700 ohms, and I used 680 to drop this down to around 3v. When I rigged up a test led the meter read 2.8v, so I assumed that I was pretty close.

I assumed that I would draw 30ma at the designated voltage, which would be 180ma for 6 leds. The unit was meant to put out 300ma at 5v, so I assumed I was well within limits. I was not sure what a 'constant current' driver was, but if it was an LED driver I assumed it would power LEDs.

The unit which I ordered was the smallest of the ones on sale. When it turned up it matched the Ebay picture, so I assume that it is the correct one. I was connecting half a dozen LEDs - 6 - and this is shown on the simple schematic I enclose. Rather small, I'm afraid - but it should expand
 

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Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
Perhaps I should point out that I do not know what a 'constant current' LED driver is, and just assumed that it was a power supply. Looking at the spec again, I can't understand why it specifies an output of 3-9V.

Is it saying that I don't need any LED dropping resistors, that I should just connect 3 LEDs in series and the unit will provide 9v to power them? I am most interested in BobTPH's comment that I have 'bought the wrong thing' - if it is an LED driver then how should I use it?
 

Dodgydave

Joined Jun 22, 2012
10,040
A Constant Current led driver puts out a Fixed current and the Voltage varies depending on how many LEDs that are in series on it.

So if says that the output goes from say 12 to 70V the current is 30mA,,it will Always be the same Current but the Voltage across all the output will increase as you put more LEDs on to maintain the current.

The led drivers you bought are 250mA Constant Current output , 3 to 9V.

Work out what each led Wattage is add them all up and that is the total wattage you need to drive them, then you can select the led driver .


UK mains is 230V +/- 6%.
 
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Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
A Constant Current led driver puts out a Fixed current and the Voltage varies depending on how many LEDs that are in series on it.

So if says that the output goes from say 12 to 70V the current is 30mA,,it will Always be the same Current but the Voltage across all the output will increase as you put more LEDs on to maintain the current.

The led drivers you bought are 250mA Constant Current output , 3 to 9V.

Work out what each led Wattage is add them all up and that is the total wattage you need to drive them, then you can select the led driver .


UK mains is 230V +/- 6%.
The LEDs I have are hi-brite 5mm ones - 3v and about 30ma. Does this mean that I should connect 8 or 9 of them in series, using about 270ma, and the driver will provide 24- 27v with no dropping resistors needed? That would be about 0.8W - well withing the smallest driver capability.

UK mains used to be 240v until a few years ago, and most people over here still refer to it as 240.... I believe that it is actually nominally 230v (-6% +10%), and the actual supply has remained the same, onl;y the label has changed.
 
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Dodgydave

Joined Jun 22, 2012
10,040
No, you have ordinary LEDs , you just need a 30mA constant current driver, or use an ordinary 5V PSU and a 68ohms series resistor @250mW.
 

dendad

Joined Feb 20, 2016
3,891
I should connect 8 or 9 of them in series, using about 270ma,
Hooking 8 or 9 30mA LEDs in series does not increase the current. They still need the 30mA. All that goes up is the power supply output voltage needed to maintain the 30mA.
Basically, devices in series share the same current. Those in parallel share the same voltage.
Unless you are using a constant current supply of 30mA for these LEDs, you MUST include a current limiting resistor. LEDs are current operated devices, not voltage as such. They just happen to have a certain voltage drop across them when in operation. You take this voltage drop off the supply voltage to get the value needed to calculate the series resistor for the required current.
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
So, if I want to drive 6-8 of these LEDs, this 'constant current' driver will be of no use? From what you say it looks as if I could connect 8 of these LEDs in parallel, and the driver would then provide 3v at 240ma?

At the moment I have the leds connected in parallel, but with a 680 ohm resistor in series with each one. Could I simply remove the resistors?
 

dendad

Joined Feb 20, 2016
3,891
The driver will adjust its output voltage to that required to supply 300mA. It has an adjustment range of 3 to 9V according to the posts above. So, 3V may supply the 300mA.
One problem of LEDs in parallel is they do not always share current evenly. Quite a few systems do run LEDs this way, but one would hope they are fairly well matched and thermally connected.
If you have an LED fail open circuit, the current in all the other LEDs will increase as the supply still drives the 300mA. Even if you use one LED, the supply will drive the 300mA through it and pop it.
To help balance the current, low value resistors can be wired in series with each LED, and that helps to damp out the small variations. But, it may be better to just run the LEDs in maybe groups of 4 in series with a suitable resistor, like 150R, from a 12V plug pack.
Or just go and get a 12V stick on LED strip and use that.
Do not run directly from the mains as it looks like you are doing. That sounds very dangerous to me.
EDIT: Oh, that supply is probably isolated so may be ok. But some cheap ones leave a lot to be desired.
 
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dendad

Joined Feb 20, 2016
3,891
At the moment I have the leds connected in parallel, but with a 680 ohm resistor in series with each one. Could I simply remove the resistors?
Maybe you could get away with it but I would say no.

If you have each LED with a resistor, use an isolated plug pack to drive them. But to save power, go to a lower voltage supply and suitable resistors.
Like an old 5V phone supply, and a 120R for each LED.
 

dendad

Joined Feb 20, 2016
3,891
Just out of interest, I recently purchased a couple of circular LED arrays with power supplies (Ebay) to replace the circular florescent tubes in our kitchen.
When I saw the "quality" of the power supplies, they went straight back into their boxes and on the shelf in the shed until I get around to making a real supply. They look really poorly designed and I would not trust them at all.
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
So your advice is to use a standard PSU and dropping resistors? The reason that I obtained an LED driver was that it was small enough to fit in the space I had available, and most PSUs are going to be too large.

Am I trying to use an LED driver incorrectly? Now that you have helped me to understand how it works, it seems to be an appropriate power supply for this task. What is this driver for if not for driving a string of LEDs?
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
169
Ah - thank you. Now I see where I am not understanding the compatibility issues. LED technology has moved on a lot since I touched it last... :(
I wonder if these can be dimmed using a PWM unit? Probably not....
 
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