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A Way of Basic Laser Diode Power Supplies
A very basic power supply can be used to safely drive low and medium power laser diodes.Battery power for laser diodes is a way of very basic laser diode power supplies.
Batteries are in fact a relatively safe alternative to sophisticated laser power supplies if their characteristics are well understood. Since a properly connected battery can never put out more than its rated voltage when new or fully charged, and can't produce reverse polarity, all that is needed is current limiting via a high power resistor. A 0.1 uF capacitor, 1N4148 reverse protection diode, and 100 ohm resistor directly across the diode though are still recommended.
Here are some guidelines:
A new or fully charged battery can have substantially more voltage than the nominal rating. For example, a new Alkaline is around 1.57 V, not 1.5 V. A NiCd may start out at 1.3 V or more when fully charged.
Don't get too greedy and use a battery voltage close to the diode voltage, include a reasonable size current limiting resistor and use a higher battery voltage. The internal resistance of NiCd and NiMH batteries is quite low and should never be depended upon for a significant part of the current limiting.
Caution: There must NOT be any filter capacitance in the power supply after the current limiting resistor. This is to minimize the chance that a bad connection to the diode will result in excessive current should such a capacitor charge to a much higher voltage and then discharge through the diode without current limiting.
It's fine to trickle charge a battery while it's being used since regardless of line voltage fluctuations and spikes, not much will happen to the battery voltage. However, due to the internal resistance of the battery, fast charging may not isolate the output enough. Better to implement a double buffering scheme where one battery is being charged while the other is in use, switching using a relay with an electrolytic capacitor to hold the voltage for the millisecond or so when the output is disconnected from either battery.
The voltage of Alkaline batteries drops steadily as they are used while that of NiCd and NiMH batteries is nearly constant until they are fully discharged. Without an active regulator, this must be taken into account.
To vary the current with no active components, a high power rheostat or selector switch must be used. Make sure it's wired so that intermittent contact can't result in current spikes.
For example, to drive a typical IR laser diode, a pair of D-size Alkaline cells can be used in series with a power resistor. For a 1 W (rated) laser diode which has a threshold of 350 mA, voltage drop of 1.8 V, and slope efficiency of 0.8, an output power from near 0 mW to 1 W can be selected.
Double check polarity and take appropriate safety precautions!
This article introduces a way of basic laser diode power supplies.
Keywords
Diode Laser, Laser Power Supply
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