My head is spinning after reading so many datasheets of different types of regulators and so on… It’s over my head… as is using FETs and/or MOSFETs in the linear region.
The 7135s don’t seem to be that bad really. Vf over them is not that much higher from than many regulators out there, and spreading the heat over so many chips isn’t such a bad thing. They’re easy to use in parallel and don’t require any resistors.
I’ve ordered a bunch of those cheap D882 transistors (from djozz thread) to play around with. If they don’t work out I think I’ll just stick with 7135s… for now at least.
I ain’t gonna argue, having not even reached novice level with this things. Probably not constant current, but should at least be linear? And if they can be hooked up parallel I just have to have more on the MCU pin that I want the higher mode on. They are used in flashlights so at least I should be able to get some light out of the darn things.
I agree that you should be able to get light at least.
I don’t know any reasons to consider using them though… Using transistors in the way that driver does seems to achieve the same thing as using banks of current limiting resistors, like the very cheapest drivers do. Using the transistor probably saves some space, but otherwise I see no advantage.
I’m not sure exactly what you mean by “should at least be linear”. I suspect linear does not mean what you think it means! Linear regulator - Wikipedia I do not expect the D882 to “regulate” anything, only “limit”. Someone may correct me?
The use of D882 just seems like a bad idea for a high powered light.
The real disadvantage is that there is no regulation. (7135’s are regulated.) Considering you wanted constant current, that seems like a major disadvantage…
Another thing, that’s not really a disadvantage, is that while it may appear that ‘part count’ could be reduced by switching to D882’s, I suspect that they may act badly when used together in a high powered light at ~700mA or ~1400mA each.
All the drivers we use are “constant on” (PWM free) in their highest mode, including FET drivers. PWM only shows up in the output when the driver is getting PWM from the MCU. The D882’s will be the same way: if you attempt to dim them with PWM you’ll get PWM on the output. You’ve already discussed the only way to avoid that: run them in groups from separate MCU pins without PWM. That will get you several PWM free modes, up to 8 depending on how things are configured, but probably less than 8 useful modes.
A worse idea than using 7135s?
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Not lots and lots worse, but sure… at least somewhat worse.
