# Constant voltage costant current for cob led

#### Jamshi

##### New Member
Hi Everyone..
I need a circuit for driving my cob led..
It is rated at 9 vdc at 1500 mA.. I need a circuit with constant voltage of 9 vdc at 1500 mA constant current...
The power supply is from my car (12vdc-14vdc)..

#### Nigel Goodwin

##### Super Moderator
It's impossible, you can't have both constant current and constant voltage. All you need is a constant current source.

#### Jamshi

##### New Member
So what about the led's power supply (9vdc)...?
If we regulate current only the led will receive 12 volt... That could damage my cob led...
Is that right..???

#### Nigel Goodwin

##### Super Moderator
So what about the led's power supply (9vdc)...?
If we regulate current only the led will receive 12 volt... That could damage my cob led...
Is that right..???
No it's utterly 100% WRONG - a constant current source works by adjusting the voltage to maintain that current - the level of that voltage is set by the requirements of the LED.

#### Jamshi

##### New Member
So you are saying that when i regulate the current at 1500 mA the voltage is set according to the led's need(9vdc)..?

Purchased...

#### Jamshi

##### New Member
Can you please provide me a circuit diagram suitable for my project..?

#### audioguru

##### Well-Known Member
We do not know if the LED already has current limiting. It might have two 3V chips in series and a current limiting resistor with 3V across it.
Or it might have three 3V chips in series with nothing to limit the current.
Who knows? The datasheet of the COB LED will show all the details that are missing in this thread.

AliExpress has a Cob LED for illegally modifying the lighting on a car. It shows two parallel strips of 3 LEDs in series. They say, "Use voltage of 9V - 12V" and say, "use current of 1000mA to 1500mA". It is rated at 15W to 18W.
They say, "Cooling temperature is not more than sixty degrees." They also say, "They may burn out if they receive enough power with no cooling , And if you use it for a long time, please give them colling!!!" That is their Chinlglish with no details.

#### audioguru

##### Well-Known Member
Can you please provide me a circuit diagram suitable for my project..?
We do not know how much current you want. 1500mA might be the absolute maximum allowed current at the maximum allowed temperature with poor reliability. The datasheet will show how hot it gets at various currents so you can select a reduced current and a proper heatsink for it.

It might limit its voltage to 9V if you feed it 1000mA or it might limit its current to 1000mA if you feed it 9V. And 12V for 1500mA or 1500mA for 12V.
who knows? The datasheet will show all the missing details. But it is Chinese so a detailed English datasheet might not be available.

#### Nigel Goodwin

##### Super Moderator
So you are saying that when i regulate the current at 1500 mA the voltage is set according to the led's need(9vdc)..?
Yes, it's impossible to have both constant current and constant voltage at the same time - power supplies which do both (such as bench power supplies) switch between the two modes as needed.

#### Jamshi

##### New Member
This is the details of the cob am using,

Parameters ===

Watt: 15w
Votage: DC 9-10V
Max Current: 1500mA
Ra: >70%
Lumen: 100-110LM/W
Emitting Colors: Cold white (6000-6500K)
Material: Aluminum-alloy + LED chips +Silicon rubber

#### audioguru

##### Well-Known Member
The Cob LED sets its own voltage somewhere between 9V and 10V maybe if you limit its current to 1500mA.
Will you operate it at its absolute maximum allowed 1500mA current and therefore its maximum allowed temperature for less reliability even if you cool it perfectly?
How will you cool it?

#### ronsimpson

##### Well-Known Member
2A automotive LED current source. A2614

#### Jamshi

##### New Member
The Cob LED sets its own voltage somewhere between 9V and 10V maybe if you limit its current to 1500mA.
Will you operate it at its absolute maximum allowed 1500mA current and therefore its maximum allowed temperature for less reliability even if you cool it perfectly?
How will you cool it?
It has a heatsink with fan..how about 1300mA..is it okay...?

#### Pommie

##### Well-Known Member
I would start with a 3ohm 5Watt resistor in series and see how hot things run. Maybe start at 5 ohm and work down.

Mike.

#### Jamshi

##### New Member
I have looked for resistor then i found 3 ohm at 7 watt resistor...
Can I apply this circuit... Is it okay

#### Pommie

##### Well-Known Member
That should work. Are you able to measure the voltage across the resistor when it's on?

Mike.
Edit, I'm assuming you know what "in series" means.

#### Jamshi

##### New Member
Ya
That should work. Are you able to measure the voltage across the resistor when it's on?

Mike.
Edit, I'm assuming you know what "in series" means.
Ya,i know...

#### rjenkinsgb

##### Well-Known Member
The voltage from a car battery can be anything from 11V to 15V.

If you use just a resistor, you must be sure the current through the LEDs will never be too high at the maximum voltage.

However, with a 9V LED assembly, that means the resistor may be dropping anything between 6V and 2V; the current will vary by a factor of three times over the battery voltage range, from maximum at 15V down to one third at 11V.

The three ohm resistor would give around 2A at 15V (engine running and battery at full charge) down to ~0.67A with the battery near flat.

That's why an electronic regulator is better, if you want constant brightness.