Two possibly silly questions that despite me being pretty sure of the answers, I just need confirmation….just in case.
Anyway, firstly from a 3V source (CR2, CR123) is there any advantage to using a linear regulator over direct drive? Single mode only obviously. I know the regulator will ensure the same current output provided the input voltage is high enough, but let’s just imagine the output current is identical in both cases – basically the current consumed by the emitter at the given forward voltage (which closely matches Vin for DD applications). The voltage supplied by the battery is no where near high enough to blow the emitter…what I’m getting at is unless there is actually a beneficial reason to using the linear regulator in this application, then why bother? Remove a component and keep the whole set-up simple.
Secondly, is there any harm in connecting the ground connection for the LEDs, straight to the thermal pad of the LED on the emitter PCB? The reason I ask is because every single MCPCB and emitter PCB has all 3 paths isolated from each other (LED+ and LED- is obvious) but when these PCBs are installed in lights where the ground path is also the flashlight host, from an electrical perspective LED- may as well be connected straight to the thermal pad. Does that make sense?
Again – super simple questions but I just want to make sure I’m thinking correctly here.
I know there is no harm in having the ground be connected to the thermal pad, since the thermal pad is already electrically isolated from everything else.
As for the driver, I am inclined to think you are correct but some people would argue that a driver is best due to a more constant output. However, that wouldn't happen since the voltage of the CR123 would most likely always be under the vf of the 219 at 1.5 amps (or whatever you plan on driving it at); especially with the voltage drop due to the driver. So no driver would be brighter, and most likely have a longer run time.
If input voltage is guaranteed to be less than the Vf of the emitter (3V almost surely is), then the driver is a complete waste. Not only won’t it do anything productive, but no driver is 100% efficient, and therefor it will steal voltage that you could be using to drive the LED with.
Thermal pad can be connected to anything (within reason - do not connect to one million volts). It’s electrically isolated from all other connections.
and now I can’t tell you that you beat me to it because it’s been less than 1 minute since you beat me to it ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………
I think I had the same question, which seems to have been answered but I wanted to verify.
Basically the center of an LED can be electrically connected to the LED- and be ok? As in if you direct bond an emitter to a copper pill but the copper pill is part of the current’s pathway.
Yeha that's right. Theoretically you could do the same for LED+ but because of LED thermal requirements and the way the ground path normally functions in LED applications, it's just easier to go LED- and thermal path together rather than LED+ and thermal path.
Ok cool. But I assume you can’t just connect the LED center ground and the - portion of the LED (as in the - “electrode” of the LED needs to be isolated still) or else the current would just bypass the driver and the LED would just be run as direct drive?
The center LED (on MOST LEDs, and ALL Cree LEDs) pad isn’t ground. It isn’t anything. It can be connected to ANYTHING and it will not affect operation of the LED.
Thanks, good to know. I’m planning on direct bonding some LEDs to homemade copper stars (OL made a thread about this) if the 16mm sinkpad thing takes too long to come to fruition.
Yes I was referring specifically to linear regulators in this example. I'm looking at modifying a light to be as small as possible had the brain wave last night that for my specific needs, direct drive was perfect.
skyrider1 - out of curiosity what is this 16mm sinkpad thingy you are refering to?
I did exactly this with an xre in a Lumatic special. Ground off the positive pad on the underside if the led, reflowed both center and negative to the sink, and ran a positive wire from an isolated B+ pad to led plus.
If you direct bond the center pad to copper or sth else with no dielectric inbetween, you should not connect LED- and the center pad. Some drivers regulate the negative path, some the positive..
Might be more accurate to say "If you think that one day you might decide to use a rare driver that does things in a nonstandard way you should not do it, otherwise it's fine."
I would venture to say that the AMC7135 is the most commonly used driver on this forum. It regulates in the return path. This means you probably won’t want to connect LED- to the pill since the pill is directly connected (in most hosts) to battery-
So, “might be more accurate to say if you are using the single most popular driver, or one like it, you probably should not do it, otherwise, it’s fine”
Connecting the Center slug to “Whatever” isn’t a problem at all… Connecting LED- to the pill will be a problem for a low side regulated driver (AMC7135 and many others).
I figured that just had to be wrong, no sensible driver would be designed that way, but I just tested it, and dammit, you're right. Jumping the LED - to the pill body completely bypasses the driver.
I've been mucking around with building my own drivers recently. It wasn't until I started doing this that I realised there was actually a different way to hook up LEDs than what is commonly practiced with the AMC7135 chips. I never knew it was any different because ever since I got into flash lights (about 2 years ago) I barely used anything but 7135 based drivers! I actually think it was Pilot that pointed this out to me when I asked him about a different linear regulator not too long ago...so thanks haha.
