# Limiting max voltage in an LED series string using a PTC

I have three color LED ‘Pucks’. They were found in a junkyard, taken from under vehicle lighting (ROCK LITES). I have more then thirty of these ruggedized cast aluminum pucks. I want to repurpose them to do landscape lighting around my home. The LED elements are of unknown specification. I have driven them to 700mA without failure and note a ~3.25 voltage drop. I decided to series 8 pucks, each having RGB elements. So I purchased three driver boards (MORNSUN KC24JT-700R3). Because I have 8 seriesed I need 26 volts (3.25*8). So I bought a 30 volt supply, powered the driver board (it’s max is 36VDC input). I am adjusting brightness using an Arduino NANO. But now I have fried LED’s (4). I believe it is a >max volts thing on the LED’s. Mornsun recommends a PTC in series with the LED’s, what does that do and which ones do I need? Not knowing the specification of the LED’s makes this so difficult.

Hi johnk31415927 ,

Thank you for reaching TechForum. Unfortunately, without the exact LED specifications I would be unable to answer that question. I am not aware a PTC would be something to place in the circuit, but one of our engineers would have more knowledge and may be able to comment further on this.

It is best to use a constant current supply for driving series connected LEDs because each LED has a different voltage drop.

A constant current power supply will automatically adjust its output voltage to match the sum of the individual LED voltage drops.

With a constant voltage power supply, some of the series connected LEDs will have too much voltage and some will have too little voltage.

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Yea I kinda tried to do that, constant current supply. But unfortunately I oversized it for my LED’s, LED’s of unknown rating and unfortunately-unfortunately the buck style driver I selected has inverse control signal (ON/OFF) logic, 0V = full current, 5V= zero current. So until my controller wakes up (Ardiuno NANO), I am frying LED’s. I ordered some NTC’s (or PTC’s??) per DIGI-KEY support. I am also going to pull the ON/OFF signal high and use an Arduino output to pull them low when it begins running.

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