ok so i know we dont run leds from fixed voltages.

however i need to find a way to drive 64 of these:

https://ampul.eu/it/led-cob/3994-diodo-cob-led-40mm-4w-bianco-caldo

with one of these:

https://www.ledstripstudio.com/shop/product/powered-bulb-dimmer/

obviously the dimmer is quite a specialised piece of kit so its not as simple as "get a constant current driver"

the leds are quite specific too, although i would entertain other options if its really impractical to use these two together.

the led datasheet gives no further info past "12v, 400mA"

given there are 64 of them, specialised circuitry beyond some simple off-the-shelf chinesium board per led is not practical.

id be willing to entertain using chunky resistors, even if its not ideal. however without the forward voltage of the leds im not sure how to work out the correct value for a 12v led running from a 12v supply.

id also entertain the idea of undervolting them enough to avoid thermal runaway -i likely dont need the full 4w output.. however the dimmer outputs minimum 12v (ive asked if it can run on slightly less) also i have to consider these will be used in summer, outdoors (protected from rain by a roof) it can arrive at 35 degrees c at night here.

can anyone suggest a) my options and b) a workflow for calculating the correct resistor value given i dont have a specified forward voltage.

i have one of the leds arriving to test on my bench supply in the next day or so.

many thanks.

 

First, LED current is dependent on the voltage applied, BUT the relationship is not at all linear.
Is that 12 volts, 400 MA for each LED?? or for some assembly with a lot of LEDs??
When a voltage AND current are specified then there is already something to limit or control the current already present. So you could just use a 12 volt supply and set it to 10 or 11 volts, and have slightly less light.

 

I assume given it's a cob led it will be a series\parallel arrangement of lots of tiny emitters. 12v 400ma total rating. I'm not 100% sure it doesn't have some current limiting built in, but from what I've read online about similar LEDs, probably not. the datasheet linked above is not very helpful. I understand these type of ring LEDs are used quite often in automotive projects, but I'm not sure if that changes anything.

I'll have to test one on my bench supply and see how it behaves at different voltages.

 

see what voltage makes it just start to illuminate! And let us know how much current at that lower voltage. THAT will be educational!

 

see what voltage makes it just start to illuminate! And let us know how much current at that lower voltage. THAT will be educational!

as soon as it arrives (damn couriers) I'll do just that

 

I don't see anything in the dimmer spec about the current rating per channel, or the total current rating. Can the dimmer really handle 64 x 4W = 256W?

 

actually there is some confusion re. that aspect. it supports 500ma per channel, but in the specs it seems to suggest max total 12 amps which would not be sufficient. however I have already found an alternative (stil constant voltage) which definitely will drive those LEDs. 30 channels at max 8 amps each , and 70 amps total:

https://www.nledshop.com/30chan/

 

Really, in those first two links I saw NO USEFUL INFORMATION, other than a specified voltage. And the information I saw about the control box was vague enough to assure that I would look elsewhere.
For LEDs with a common voltage listed and some believable current associated with it,, there is either an active regulator circuit or series resistors already in place. And certainly you can adjust the brightness by reducing the voltage, because LED current draw is certainly a function of applied voltage up to the point of failure. It IS a VERY NON-LINEAR relationship, and so attention to the limits is important.

 

unfortunately I'm still waiting for the sample led. couriers delivering to the countryside in south Italy have an amazing range of excuses for not delivering on time.

as soon as I have it I'll hopefully be able to share some better information.

thanks

 

One point to understand is that when a VOLTAGE is specified, especially a standard one like "12 volts", generally there is some sort of current limiting scheme already present. I have 12 volt automotive small wedge based lights rated "12 volts" and I can see that there are resistors and other tiny parts present. Also strings RBG that include resistors so that they work with 12 volts DC. Certainly they can be run at a higher voltage, and also at a lower voltage. And even with resistors in series, there is a very non-linear voltage/current relationship.

 

ok so i know we dont run leds from fixed voltages.

however i need to find a way to drive 64 of these:

https://ampul.eu/it/led-cob/3994-diodo-cob-led-40mm-4w-bianco-caldo

with one of these:

https://www.ledstripstudio.com/shop/product/powered-bulb-dimmer/

obviously the dimmer is quite a specialised piece of kit so its not as simple as "get a constant current driver"

the leds are quite specific too, although i would entertain other options if its really impractical to use these two together.

the led datasheet gives no further info past "12v, 400mA"

given there are 64 of them, specialised circuitry beyond some simple off-the-shelf chinesium board per led is not practical.

id be willing to entertain using chunky resistors, even if its not ideal. however without the forward voltage of the leds im not sure how to work out the correct value for a 12v led running from a 12v supply.

id also entertain the idea of undervolting them enough to avoid thermal runaway -i likely dont need the full 4w output.. however the dimmer outputs minimum 12v (ive asked if it can run on slightly less) also i have to consider these will be used in summer, outdoors (protected from rain by a roof) it can arrive at 35 degrees c at night here.

can anyone suggest a) my options and b) a workflow for calculating the correct resistor value given i dont have a specified forward voltage.

i have one of the leds arriving to test on my bench supply in the next day or so.

many thanks.

"
An LED rated for 12V can almost certainly run at 13.2 volts; if the LED is rated for 12 volts then it's probably made for an RV or some other vehicle and all 12 volt vehicles run around 14 volts when the engine is running; I'd say that you're worrying about nothing. But if you're really worried then just use two diodes in series; a diode drops 0.6 volts so two diodes will drop 1.2 volts and as long as you use a diode that can handle the current you never need to worry about the voltage drop changing if or when the load current changes like you would if you used a resistor. You can buy 20 for $5.99 that are rated for 10 amps and that's far more current than any string of LEDs will ever draw and you don't need to worry about making sure you have the correct resistor value - each diode drops 0.6 volts:
https://www.amazon.com/20-Pieces-10...721047677&sprefix=10+amp+diode,aps,438&sr=8-3"

 

From the controller link:

Special 64 channel SPI dimmer with built-in power supply designed for dimming 12-24V constant voltage (CV) fixtures such as dimmable LED bulbs or short LED strips.

This is not the controller you need.

It is designed for LED assemblies that are current limited and designed to run at a constant voltage. COB LEDs are not that. They are a series / parallel arrangement of raw LEDs with no additional circuitry for current limiting. They need to be driven just like raw LED chips. The 12V rating tells me that they consist of paralleled groups of 3 LEDs in series. The Vf of white LEDs is typically 3.2 to 3.6V. They will run safely at 12V fixed, but likely well below the max power. Using a resistor to control the current would drop the voltage by at least 1V, so 11V in all.

Are you aware that the products you linked are not a complete solution? It needs a multi-channel SPI controller to operate it.

Also, do you need individual control of the 64 LEDs? Because that is what this controller does. If you want to dim them all together, a much simpler and cheaper driver would work.

 

Guys, the OP hasn't been heard from since May.

 

Today I clicked on that link in the initial post.
To be polite about it, my new opinion of the seller website is totally negative in every aspect.