Re: MXDL
Got your PM... And read the thread..
So, first of all, you DO need a driver. There are MANY reasons for this..
Direct drive: (battery + resistor)
- Current starts high on a full battery, and then drops all the time, the power is never the same twice, when you turn it on
- As the diode heats up, it's internal resistance decreases, so the current actually climbs at first. If it's not limited low enough, it can climb too high, and kill the diode. More current means more heat, more heat more current, sometimes resulting in a thermal runaway.
- If you put a better battery in, or a rechargable instead of primary, with a higher voltage, your diode is dead
This means, that while it is possible to safelly drive a diode from a battery through a resistor, you have to set the current much lower, so that you are safe for sure. On a full battery, the current will have to be below the max safe current, because it will climb first (with heat). Then it will drop, and the power will be lower and lower - the battery voltage drops as it discharges.
If your diode dies, and you put another one in, you have to adjust it again, because the current will not be the same, even if it is the same diode!
Because of the properties, you have to set the current through a DMM. But as you then remove the DMM, the current actually goes up, because you remove the resistance, the DMM introduced! Unless you have a 1 Ohm 1% resistor permanently in the circuit and measure the voltage drop across it.. But this 1 Ohm resistor will become part of the limiting resistance and influence the current.
Constant voltage: (7805, Cdanjo's boost circuit, or any other CV source....)
- here the power will start at the same level every time you turn the laser on, untill the battery is empty.. BUT!
- again, as the diode heats up, the internal resistance decreases. Since the voltage stays the same, and resistance goes down, the current goes up with heat.
Again, this means you have to set the current BELOW the max safe current, because it will climb with heat. How far it will climb, depends on heatsinking, but it WILL climb in any case, even with the largest heatsink. To prevent thermal runaway, you should limit the current 20mA below the usual current, and hope, that the current will not increase more than 10mA.
It's still much better, than just a resistor, because the power doesn't drop with the battery voltage - you get the same power, every time you turn the laser on. Also, if you put a better battery in, the current will not jump up and kill your diode, like it would with a resistor. But the stronger the diode, the less suitable this option becomes, because it will heat up more..
If you put another diode into the circuit, you have to adjust it again, because the current will not be the same, even if it is the same diode!
Because of the properties, you have to set the current through a DMM. But as you then remove the DMM, the current actually goes up, because you remove the resistance, the DMM introduced! Unless you have a 1 Ohm 1% resistor permanently in the circuit and measure the voltage drop across it.. But this 1 Ohm resistor will become a part of the limiting resistance, and influence the current, so you have to take this into account.
Driver: (317, Rkcstr's micro driver, LavaDrive, any constant current source)
- The laser will again come on at the same power every time you turn it on, even when the battery voltage drops, right untill the battery is empty.
- But with a driver, the current will remain the same, no matter how much the diode heats up.
- This means, the diode will heat up, the internal resistance will decrease, and the driver will automatically LOWER the voltage just enough, so that current stays the same all the time.
So, your power is not dropping with the battery voltage, when you use a driver. And there is no risk of a thermal runaway. You can actually set the diode to the highest safe current, because you know for sure, that it won't change.
You get the same power every time you turn the laser on, right until the battery is empty.
If you put another diode in, the current will be the same, even if it is a completelly different diode!
If you set the current through the DMM, and then remove it, the change in the resistance does NOT influence the current. If you put a 1 Ohm resistor in, to measure the current, it also does not influence the current. The driver makes sure the current is always the same, regardless of the load.
With low power diodes, that don't heat up, you can use a constant voltage source if you adjust the current correctly.. The current won't change much, but it should still be set a few mA lower. (with low power diodes a few mA too much is enough to kill them)
With high power diodes, that create a lot of heat, you should always use a driver.
It can be done without a driver. But there are the above mentioned drawbacks - less power, more risk.
With a driver, you can actually set the power higher, and be safer at the same time!
Constant voltage is better, than a resistor, but again, the current will climb with heat, use a series resistor large enough, so the current ALWAYS stays below the max safe current.
With only a resistor, you have to limit the current so, that it stays quite a bit below the max safe power when the battery is COMPLETELLY FULL, and the diode hot.
Oh, and a constant voltage circuit (boost or not) is not a driver. But it is still much better, than a resistor!
A real driver is a constant current source. With a constant current source, the resistor actually SETS the current. If you know the regulator's feedback voltage, you can just calculate the resistance needed for a certain current. After you set it, the current will ALWAYS be the same, regardless of the battery voltage or small differences in the forward voltage of the diode, or even with a completelly different diode - the driver simply adjusts the voltage so that the load is always getting just enough for the current you set to flow through it (as long as thedriver is also getting what it needs for this).
With a constant voltage source, the current will be different with every load, even if it's two identical diodes from the same batch. There are always small differences in the Vf (forward voltage) of the diode, and a small difference in voltage results in a BIG difference in current.
So with constant voltage, you can't set the current, you can only limit it somewhat, with a series resistor. And you have to adjust is for each load separatelly, because of the small changes. It's actually harder to adjust correctly, and after that, the rest depends on your heatsinking..
And a resistor + capacitor is the worst of all options.
To drive a red from one Li-Ion, you could use an AMC7135. It's cheap from DX. But then you have to insulate the diode heatsink from the host body. If you can insulate the heatsink, then the AMC is very good. Otherwise you need two Li-Ions and a linear driver or one Li-Ion and a buck/boost driver.
For a blu ray, you need three Li-Ions and a linear driver, or one Li-Ion a boost driver.
A red could work from a single Li-Ion and a resistor, but you would have a lower power laser with an always changing, mostly dropping power.
A blu ray could work from two Li-Ions, with the same drawbacks.
If you want to save money on the driver, you will end up losing it on diodes. And if you don't lose them, your laser will not be what it could be with a proper driver.
