Re: 215mW 6X Key Chain Blu-ray! - With FlexDrive!
Hehe, no problem Jay, anytime..
Since we talked about this through email, here is a "shortened" version..
Still quite long tho.. :
The current draw will be the lowest, when the keychain build is closed with the tailcap. The tailcap is a massive piece of metal, and if the threads are clean (this is important!), it will provide very little resistance. So whatever you measure, the actual current draw is always a bit lower in a switching driver powered laser...
This happens for the same reason, as the reason why a switching driver will draw more current when the battery voltage drops.
DMMs measure current by measuring voltage across an internal resistor (called "shunt"). Just like we sometimes put a 1 ohm resistor and measure voltage across it.
On top of that, the long wires of the DMM itself introduce a resistane, which the current has to flow through. And all that requires some extra voltage, which is lost, by the time current gets from one probe, through the DMM, to the other probe. This is called a voltage drop, and depends on HOW MUCH current is flowing through a resistance (the higher the current, the higher the voltage drop across a resistance -> U = R x I). If you use a second DMM, you could measure this voltage drop across the first DMM.
So, the DMM introduces a resistance into the system, the resistance causes a voltage drop - the driver gets a bit less voltage than the battery actually has. If a switching driver gets less voltage it will compensate by drawing more current, to keep power (P = U x I) the same. It's the only way it can supply a constant output...
If you would measure the voltage drop across the first DMM with another DMM, you could then calculate how much less the actual current draw in the laser is. But since the 10A (or 20A - whatever the highest) range uses a thick metal rod as a curent measuring shunt, the resistance introduced is small, so the measurement is quite close to the actual current in the build.
The reason this effect is not noticable with linear drivers is, that they draw a constant current (not constant power), as long as they get enough voltage. Putting a DMM in between reduces the voltage the driver gets a bit. But a long as it is still enough, the driver will keep drawing a constant current.
If however, a linear sytem would be barelly above the minimum voltage required for regulation, adding a voltage drop could reduce the voltage to bellow the minimum, and cause the driver to drop out of regulation, while it could still regulate for a while, if there was no DMM in it's way.
At a certain point, a linear driver might still get just enough voltage to regulate when measured through the 20A range, but drop out when measured in 200mA range.
Due to the different behavior of linear drivers, the effects of the voltage drop are harder to notice there, and since this voltage is low, the above situations are unlikelly to happen when testing a linear driver setup.
A situation where the difference in DMM's resistance between the two ranges is very noticable is in the many cheap chinese unregulated green lasers. There, the drivers are out of regulation most of the time, and act as complicated resistors. Measuring the current with the high range on the DMM will show a much higher current than the low range on the DMM!
Another situation is, if you were to make a laser with nothing but a pot between the diode and the driver, set the current with the DMM in-line and then remove it. The actual current in the laser could be so much higher, that it woud kill the diode!
