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Doubling the protection of laser driver

17 Apr 2013  | Tai-Shan Liao

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An excessive level of light from a laser pointer, even if only for a short duration, can be dangerous if it enters the human eye either directly or through reflection from a shiny object. Most countries, therefore, have laser safety requirements that limit the maximum emission level. This article describes a laser driver that works even with a single 1.5V cell discharged to 1V, and uses dual current-control transistors to improve reliability against shorting and allowing excessive laser current and light emission.

In the figure, the transistors Q1, Q2, and Q3 compose a negative impedance, which can be described approximately as Zā‰ˆāˆ’ β (VDDāˆ’VBE)/RF. Assume that all of the transistors have the same current gain (β), and VBE is the base-to-emitter voltage of all transistors. Feedback is provided through RF, and R1 bias controls the collector current of Q1. Inductor L1 and parasitic capacitance form a resonant circuit that oscillates due to the negative impedance, resulting in about 3.5V pk-pk at Q1's collector, with the battery at 1V. Schottky diode D1 and C1 form a half-wave rectifier that provides about āˆ’3V for the laser cathode; with VDD at 1V, this provides a 4V working range to overcome the laser threshold.

Figure: You can use this dc-dc step-up circuit and dual current-control transistors to safely power this laser from an almost-discharged battery.

Q5 and Q6 control the laser current. The photodiode built into the laser assembly monitors the light intensity and sends negative feedback through Q4 to bias Q5 and Q6 to the proper collector current for the constant desired laser intensity. The Q5 and Q6 pair is series connected so that if one should fail shorted, the other will still maintain the laser current at a safe level. The probability of failure of two transistors at the same time is far lower than the probability of failure of a single transistor.

Note: Due to variations in laser and photodiode efficiency, R7 might need to be adjusted to ensure the laser output is within safety-regulation limits.

About the author
Tai-Shan Liao is from the National Applied Research Laboratories, Instrument Technology Research Center in Hsinchu, Taiwan.

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