# LED Driver with “Common cathode”

#### spacyal

Joined Mar 10, 2021
3
I'm in the process of building a lamp. It is about a Pendant lamp in which a coaxial cable with an outer braid is used for the power supply in order to swing the light objects (5 pendulums). In the light objects there are LEDs that require CC with 700mA. This connects the GND of all constant current sources in the canopy of the lamp. I am therefore looking for an LED driver (or an LED driver IC) that can handle common cathodes. The luminous objects should each have their own KSQ. Do you know something suitable? Maybe even a circuit diagram?
greetings

#### dl324

Joined Mar 30, 2015
16,166
Welcome to AAC!

Post a diagram showing how the LEDs are connected and how much current each "string" of LEDs requires. What is the nominal voltage for each "string"? What supply voltage are you working with?

#### spacyal

Joined Mar 10, 2021
3
If you want to understand more this is the concrete setup:
It is a pendant lamp, the pendulums are on one suspended steel cable. In addition, a coaxial cable with a copper braid (unisolarted and having connection to the canopy) on the outside and an insulated conductor on the inside leads to each pendulum. The pendulums each have built-in LED modules which require a constant current of 700mA / approx. 14.8V. Each pendulum should have its own KSQ. But since there is a connection to the canopy due to the suspension of the pendulum by the coaxial cable and its screen (which should act as a negative pole), I need a KSQ which works with a common cathode. Because ultimately all Vout- the individual KSQ's have a connection to each other via the metal canopy.

#### dl324

Joined Mar 30, 2015
16,166
Do you have room for a commercial power supply to generate a DC voltage from the line voltage?

#### spacyal

Joined Mar 10, 2021
3
Yes the DC Voltage comes from a power supply also sitting in the canopy. So I would like to use a DC/DC constant current source

#### ElectricSpidey

Joined Dec 2, 2017
2,636
Did you want dimming?

#### dl324

Joined Mar 30, 2015
16,166
Yes the DC Voltage comes from a power supply also sitting in the canopy.
What is the output voltage of that power supply?

To be clear, all of the cathodes of the pendulums are connected so you're driving the anodes?

#### ElectricSpidey

Joined Dec 2, 2017
2,636
The designer of the lamp should have left the LEDs isolated, this would have made powering them much simpler, and more efficient.

The problem I'm having looking for drivers is...the manufacturers don't post schematics, so it's difficult to know if you can connect the grounds together or not.

That leaves the option for me to go to a Chinese supplier on amazon or some such dealer and get something there, but I won't go there for other people.

#### sghioto

Joined Dec 31, 2017
5,119
Maybe even a circuit diagram?
You can use a linear voltage regulator as a constant current source.
Drive it with a switching power supply with an adjustable output.

#### Phil-S

Joined Dec 4, 2015
238
Should be really straightforward to do this with off the shelf drivers and PSU.
I know you need 700mA, but people like Maxim do constant current drivers e.g. CL25N, a two or three pin CC driver in TO92 that operate over a wide voltage range and I believe they can be high side or low side.
By the way, what's a KSQ?
As mentioned by anniel47, a pukka supplier is always worth the extra money, and in any case, in a domestic setting, safety, particularly overheating, should be a number one priority.