# Signs of the Times - March 2014 - (Page 34)

Lig hti n g T e c hn i ques Marcus Thielen is a physicist and lighting-industry consultant from Duisburg, Germany By Marcus Thielen LED circuits Constant current or constant voltage? L ED products currently (pun intended) dominate the electricsign market. However, different principles govern LED electric circuits: the low-voltage parallel circuit and the high-voltage series circuit. Here is a short comparison. As a basic component, an LED needs, for proper operation, a low, electrical DC voltage in the 1.54.8V range at a constant, direct current between 20mA and 1,000mA (depending on type). By itself, the LED has a positive, voltage-current slope, but a negative, thermal-voltage characteristic. Consequently, you can't operate multiple LEDs directly in parallel, because the "weakest" LED will draw almost infinite current and die first; the others will follow in a chain reaction. To prevent this scenario, the current through the LED must be kept more or less constant for proper operation. In cheap LED modules, only a resistor in series with the LED(s) limits the current. This resistor dissipates considerable energy into heat - the higher the resistance, the better the current's stability in various operating conditions, but the higher the losses. Thus, module manufacturers always design the resistor "just" high enough to prevent self-destruction in the appropriate operating-temperature range. It's walking a knife's edge between economy and self destruction. For common, low-voltage LED systems, often two or three LEDs (which may be on the same LED component) are connected in series, and the remaining supply voltage (from a total of 12 or 24V) is consumed by the limiting resistor or electronic regulator. Regarding the losses, here's a sample calculation: a typical white LED takes 3V at 0.05A. Three LEDs connected in series makes 3 x 3 = 9V (four LEDs would be exactly 12V, thus there's no chance to regulate the current); thus, another 3V at 0.05A are dissipated into heat in the resistor. If, for a sign, we need 210 LEDs (or 70 modules), the total consumption is 70 * 12V * 0.05A = 42W, while 70 * 3V * 0.05 = 10.5W are dissipated into heat. The 12VDC are obtained from a switching power supply, which, at this power level, has a typical efficiency of 90%. Thus, the total power consumption is 42/0.9 = 46.6W. Many high-power LED modules now keep the current constant by means of a switching power supply on the module board itself. With a A parallel circuit 110 volts A series circuit 220 volts Photo 1: A channel letter with series-connected LED modules and an integrated power supply. Total height: 8 in. 34 SIGNS OF THE TIMES March 2014 Different wiring principles: parallel and in series

Signs of the Times - March 2014
Contents
ST Update
Technology Update
Designing for Large Format
Vinyl Apps
Strictly Commercial
Lighting Techniques
The Moving Message
Technology Review: Summa’s DC5 and DC5sx printers
Technology Review: PrinterEvolution Neo Series printers
Design Matters
New Products
Happiness Squared
They Speak the Language
The 2013 Lighting Survey
Ganassi Racing’s Green Light
Dynamic Digital Signage Made Simple, Part 3
Industry News
Editorially speaking

# Signs of the Times - March 2014

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