LED lighting

Hello all,

Can anyone please help? I have a ring of LED lights, sold as being 12v and 42 in number. Can I simply connect the leads to a 12v DC supply or do I need to have a resistance in series? If yes, what size resistance. Grateful for any advice as my electronics knowledge is lacking.

John

My LED's are just connected to a 12v DC power supply from an old cordless phone.

Hi John,
I suspect that the array is made up of 14 paralleled groups of 3 LEDs connected is series and each of the 14 groups will have a SMD resistor connected in series with the LEDs If you can take a photograph of both sides of the board it should be possible to see if it does have resistors fitted. They may have double sided adhesive on the back which you may have to peal back to see these resistors.

Les.

Depending on the Diode Forward Voltage (The manufacture LED voltage of each “lamp” ) - This is usually stated as 1.8 to 2.2 V (Not always the case)

So If we work on 2 as the operating voltage then for 42 units it would work out at 7 strings of 6 “lamps” in series with a 1 ohm resister in each string.

Each string would be in parallel in the 12v system.

Extra point to remember is that the LED is voltage flow sensitive - they will only work with the + and - correctly connected

If you can see the wiring board then it is “usual” practice to put the resister in the negative end of the string

Edited By HomeUse on 11/05/2015 09:03:00

Edited By HomeUse on 11/05/2015 09:06:07

Hi HomeUse,
The voltage you quoted is for red, green, yellow and orange LEDs I assume these units use white LEDs which have a forward voltage of about 3,3 volts. I don't know how you arrived at a value of 1 ohm for each string. As these units are probably designed to be used in cars the battery voltage when charging can reach 14.5 volts. This would give a current of 2.5 amps in each string. A total of 17.5 amps.

Les.

I'm no electronics guy either but for r/c night flying we tend to use this stuff **LINK** connected directly through a 3-cell lipo - so around 12.5 down to 11.5v and a huge current possible with the high C rated lipos. They work fine without extra restistances and are cheap enough to replace if they ever do fail. HK also do cheap good heatshrink and silicone covered wires.

Should be fine with a regulated 12v DC supply

Hi Les - The sample I used were based on Bright Light White which were 1.8v - 2.2, I know there are various other voltages based on use and colour

You can check the calculations but It may be easier to use this link which will give a lot of details about LED’s

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

Cheers

Hi HomeUse,
I have never seen any white LEDs with a forward voltage of around 2 volts. The site that your link goes to lists white LEDs as having a forward voltage of 3.3 volts.

Les.

Hi Les - Looks as thought you could be right - Looking through suppliers of LED’s they all seem to list White as 3 to 3.8 v. But the supplier I used Assisi Electric UK ( which supplies goods from Hong Kong ) list them as 1.7 to ? 3v - I have checked the packets I have and they are stamped 1.8 to 2.2 ( All the colours bar Blue )

Have checked the voltage on a couple of them and they need 1.83v to start and tend to “blow” (fail) at 3v.

I have to agree that the normal LED’s would be in the 3 to 3.5 range

The site I linked was mainly for info. and calculation of resistors.

MikeB

Hi all,

Thank you all for your replies.

The ring came from Ebay, described as being intended for car use and called "Angel Eyes." They come in various diameters, the number of White LEDs increasing with the dia. increase. There are 14 pairs of solder blobs on the back and they appear to be connected in 14 batches of 3, exactly as described by Les. However, I cannot see any extra bulges on the back where there might be a resistor unless item is unbelievably microscopic, cannot see any sign of double sided tape to unpeal and I currently cannot photograph the object. Between each pair of solder blobs on the back, there is a black portion with white numbers, 191, reading from outside of ring, same orientation as the lettering - WX-130MM, so presumably 191 not 161. Les, would these be the resistors referred to by you?

Thanks for any further advice.

John

Hi John,
Without seeing pictures of the item it is difficult to make any suggestions on how to confirm that there are current limiting resistors. As they are designed for use in cars I would EXPECT that they could just be connected to 12 volts. This can vary between less than 11 volts (with a discharged battery.) and about 14.5 volts (When the engine is running and charging the battery. Do you have the ebay item number so I could see the picture on ebay ? If you can see the individual LEDs and get to the connections you could measure the resistance between the power connections and each of the six connections on one group of three LEDs I would expect one power lead to show almost zero resistance to one of the connections and the other power lead to show a reading of between about 50 and 200 ohms to one of the other connections. This assumes that the resistor (If present.) is at one end of the group of three LEDs. (It could be between two of the three LEDs in the group of three.) Another possibility is that only one current limiting resistor is used for the whole array of LEDs (It is bad practice to do it this way.)

Les.

Angel Eyes are the ring lights seen predominately on BMW cars, early DLR or as side lights.

The answer is yes, they are designed to fit directly on cars and just require a 12v supply. I buy quite a few in for an industrial application and power these from a 12v DC wall wart.