Setting the Laser Driver Current
This section shows:
- How to choose a dummy load to use in place of a laser diode when setting up a driver board
- How to measure the current to set-up the driver using a dummy load
- How to measure the total driver board current (which allows the current to be set if the characteristics of the driver are known)
1. Laser Diode Driver Test Loads / Dummy Load
The safest way to set the output current of variable output laser driver board to suit a particular diode is to use a "dummy load" or "test load".
In the set-up process the dummy load is used instead of the laser diode to allow the current to be set accurately and then replaced with the laser diode. Unlike the laser diode, the dummy load will not burn out if the current is too high during the setup process.
[It is not possible to directly measure the output of a driver circuit without an appropriate load, or at least if it is attempted the output will change when the diode is connected.]
Test Loads:
The test load is designed to behave in the same way as a laser diode in the driver circuit, it uses diodes to drop the voltage and a resistor to provide a load.
Since different laser diodes have different operating voltages, the number of voltage-drop diodes varies according to the type laser diode used. The diodes need to be connected the correct way with the band connected to the -ve of the driver circuit, the resistor can be at either end of the diode chain:
Infrared (IR) Laser Diode Test Loads
The IN4001 diodes each drop around 0.7V so 3 diodes is appropriate for IR diodes operating at around 2.1V, the 1 Ohm resistor and the diodes need to have sufficient current rating for the intended drive current, typically <1W.
Red Laser Diode Test Loads
The IN4001 diodes each drop around 0.7V so 4 diodes is appropriate for red diodes operating at around 2.8V, the 1 Ohm resistor and the diodes need to have sufficient current rating for the intended drive current, typically <1W.
Blue-violet (405nm) Laser Diode Test Loads
The IN4001 diodes each drop around 0.7V so 6+ diodes are appropriate for 405nm diodes operating at around 4.2V, the 1 Ohm resistor and the diodes need to have sufficient current rating for the intended drive current, typically <1W.
Assembling a Test Load
The diodes and resistor can conveniently be mounted on a piece of 'Veroboard' or similar prototyping circuit board and connections made to allow the board to be used for several diode types, (in this example the resistor is at the other end of the diode chain):
2. Using a Dummy Load to Set Drive Current
If a multimeter (set to read mV) is connected across the resistor the reading in mV will be the same as the current (in mA) that is flowing through the dummy load. The current can be adjusted to the required level on the driver board and monitored by the multimeter. (The example shown uses a load for a blue-violet diode).
3. Measuring the Driver Board Current Consumption to Set the Laser Diode Drive Current
Caution! This method cannot be used with Boost Drivers!
Some driver boards such as OFL144 and OFL184 consume an almost constant amount of current to work (around 30mA for these boards) so the laser diode drive current can be estimated by using a multimeter in the current measuring mode to directly measure the total current consumed by the board plus laser diode (or dummy load) and subtracting the power consumed by the board from the total. e.g. if the total consumption is 380mA then the load is being driven at (380 - 30) = 350mA .
Again, it is safest to use a dummy load in case the driver board is supplied set at a current that is too high for the laser diode.
With care (and some risk!) it is often possible to turn down the drive current on the driver board and then connect the laser diode directly, gradually turning up the current on the driver board until the correct operating current is obtained.
Using a laser diode directly connected to a pre-set driver board is risky because:
- It is not always obvious what is the lowest current setting on the driver board and the correct way to turn it to gradually increase the current.
- Some potentiometers (used to adjust the current on the driver board) may give intermittent contact when they are being adjusted which can lead to spikes in the output current of the driver which could destroy a laser diode.
4. Using a Variable Current Power Supply to Set the Driver Current
Caution! This method cannot be used with Boost Drivers!If a power supply is available which has a facility to limit the maximum current this can also be used to set the driver current directly using the laser diode with a reduced risk:
- Set the voltage of the power supply to the required operating voltage (usually the voltage of the battery to be used to power the laser).
- Set the current output of the power supply to zero and connect to the driver board (with test load or laser diode connected).
- Gradually increase the current from the power supply until either (i) the current reaches the required current or (ii) the current stops increasing (and is below the required current).
- In case (i) adjust the pot on the driver board until it reduces the current slightly, now the driver is limiting the maximum current and increasing the current setting on the power supply should not increase the actual current shown on the meter.
- In case (ii) the driver is limiting the current; increase the current setting slightly on the power supply (the actual current will not increase) then increase the actual current using the potentiometer on the driver board. If the required current setting is not reached on the first time adjustment of the driver, increase the current setting slightly on the power supply and repeat the process until the driver board rather than the power supply is limiting the current at the required value.
Power Supply Set At 3.9V In Current Limited Mode (CC) Limited To 0.51A
