Convoy M21B, M21E, M21F w/ GT-FC40 4500k: 3 CRI-95 324m throwers w/ Nichia-like tint -->3 home runs. Rec w/out reservation :-).

I was comparing Wurkkos with Convoy L8 and it was a touch choice. Any larger host like L8 will have better throw (but possibly/likely smaller hotspot) and thermal mass advantage. Wurkkos won because of its much lower price, smaller size, and lower weight (the last 2 factors significant consideration for me). No regret at all as the Wurkkos actually fits in my khaki pants’ side pocket (NOT tight pants, loose) and therefore gets used a lot. I don’t like “too large” lights but obviously YMMV.

Both SBT90.2 lights are great and you won’t go wrong with either. Size will provide certain advantage so you just have to draw a line on how large of a light you’d like (and of course how much you want to pay). Check out this post comparing runtime of L8 vs Wurkkos - Wurkkos did surprisingly well: Wurkkos TS30S + monster LED SBT90.2 = $60 4750lm 1km SUPER-Thrower. Comparison w/ TS30S Pro & other lights incl. the :-) insane 13000 lm Nightwatch Valkyrie. (Summary & measurements on P. 1) - #126 by cannga

^ Looks good. I’d seen TacGriz say that the L8 has some driver issues that put him off, so he recommends the L21B if going for Convoy. The T30S came down to $65 but is now back up to $70. I’m thinking if Wurkkos makes a price break on 11/11, I may go for it. Would really broaden my perspective seeing SBT90.2 in person.

M21E gtfc40 is using boost driver.

S21E 519A is using what kind of driver? Boost/buck or linear driver that burns of excess energy as heat?

@xevious: TS30S’s cost does go up and down so I’d wait for low $60. Pls post here Wurkkos TS30S + monster LED SBT90.2 = $60 4750lm 1km SUPER-Thrower. Comparison w/ TS30S Pro, Olight, Nitecore, & other monster lights. :) if you find unbeatable deal. :+1: :slight_smile:

@Tendou: I’m not an expert at all so anyone pls correct as needed, but since it’s not boost, and doesn’t seem to be buck from random unconfirmed web discussion, that leaves CC? Of course Simon on Aliexpress or here will give official answer.

I’m not well versed in driver technology. So constant current driver are able to gives constant current but it is not as efficients as boost and buck? Is the efficiency same as FET?

Forgives my noob question.

^No problem it is a good question, but asking me would result in a situation of the blind leading the blind. :innocent: I came across the discussion below from google search. It came out on top of the search and I believe this means it’s read by a lot of people (?). I bold faced the parts that seem relevant to your question.

BTW may I ask you why you’re asking this question (in case someone more knowledgeable than me could help)? Are you choosing lights based on the driver technology used? Hope this helps.

Reddit Post HERE

Answer by poster “dingwat”:

I’ll try to answer this is reasonable concise post but this is a huge subject (I’ll probably have to add to edit a few times so bear with me). If any of the terms/abbreviations/initialisms aren’t clear, let me know. I’d love to provide more schematics, but many “driver people” do not release schematics for reasons I don’t fully understand.

First off, what are we trying to do with a “driver”? For LEDs, a flashlight driver is a constant-current (CC) source: LEDs are happiest when driven CC, as opposed to CV (constant-voltage) or something else. Here’s a comment of mine attempting to give a short explanation of why. Most white LEDs have a forward voltage (Vf) around 3.0V, but some LEDs have multiple dies connected in series (e.g. Cree XHP50, or Nichia 144A) which increases the voltage. Also note that with LED Vf and battery voltages below, I’m referring to nominal, the min and max may be significantly different: a Li-ion battery has a nominal voltage of 3.7V, but is typically operated over a range from 4.2V (full charge) to 3.0V (mostly discharged).

Switching converters

Both buck and boost are switching converter architectures (there are others, but they are not as commonly encountered in flashlights). A well designed switching converter is very efficient, but they are complex and may be expensive. Most buck and boost converters in flashlights have a MOSFET (operating as a switch), an inductor, a diode, and a capacitor. The diode can sometimes be replaced with another FET to improve efficiency, and the capacitor can be left out in some situations (particularly in smaller LED drivers), but the FET and inductor will always be present. Switching converters are true regulators, and (if well designed) will drive an LED at a fixed current regardless of the battery voltage until the battery cannot provide enough current, resulting in very flat runtimes (no brightness variance over time).


A boost driver is a CC switching converter , with a lower voltage on the input than the output. For example:

1.5V to ~3.0V (for a alkaline/NiMH to single emitter driver, like)

3.7V to ~6.0V (for a Li-ion to two series emitter driver)

3.7V to ~12.0V (for a Li-ion to four series emitter driver)

A boost converter (both CC and CV) is, at it’s core, a switch that, when turned on, pulls current through an inductor, and when the switch is turned off, the current flowing through the inductor doesn’t want to stop (that’s the defining characteristic of the inductor) so the voltage increases, and a diode then prevents the current from turning around again. There is generally a capacitor at the output as well, but that’s not strictly necessary depending on the load. This Wikipedia image illustrates it very well.


A buck driver is a CC switching converter (again, has a FET and inductor), but the voltage in is higher than the voltage out. For example:

3.7V to ~3.0V (for a Li-ion to single emitter driver, although this has some complications)

7.4V to ~3.0V (for two Li-ion to a single emitter)

A buck converter (both CC and CV) is essentially a switch which is turned on or off by a control loop that looks at desired voltage or current versus the actual voltage or current, with a diode to allow current to flow when the switch is off, followed by an LC (inductor-capacitor) filter to smooth the alternating “all current”/”no current” into “some current”. Again, the Wikipedia image is an excellent explanation.


In the context of a flashlight driver, linear refers to a constant-current linear regulator. The most commonly used part is the AMC7135, a 350mA current regulator. These devices are very much like a linear voltage regulator (e.g. LM7805), except they regulate current. They generally use a bandgap reference controlling a pass element. They are called linear because the pass element is a transistor (usually a BJT or a MOSFET) operating in the linear region (meaning it’s not all the way on or off). They are very simple to design in, but cannot handle very much current per device (so larger drivers have many e.g. Convoy S2+ driver has 8x7135 paralleled together). They are technically much less efficient than switching converters because they dissipate the extra voltage across the pass element, but when used with a low voltage drop (going from 3.7V to 3.0V, for example) and at low currents, they can be quite efficient and may be more efficient than some switching designs due to the high quiescent current of the switcher.

Here’s a schematic of the “Nanjg 105c” driver from many Convoys, from this thread on EEVBlog.


Calling a FET a driver is a stretch. A “FET driver” is really just a big MOSFET in series with the emitter, operating as a switch (i.e. the MOSFET is either in saturation/active or cutoff). When it’s on, the full voltage of the battery minus the voltage drop of the wires and components is put across the emitter. This is typically used when the input voltage is close to the LED forward voltage (3.7V to 3.0V), and the current in a FET driver is generally high enough that the internal resistance of the battery plus the resistance of the wires, springs, PCB and the Rds(on) of the FET greatly affects the voltage at the emitter. Consider: at 20A, 50mΩ results in a drop of 1V, so the Rds(on) and the springs are very important in how much current the emitter sees. Unlike the above regulators (switching and linear), a FET is not regulated, so as the battery voltage decreases, the LED brightness will also decrease.

The reason I say a “FET driver” isn’t really a driver is because the only mechanism “controlling” current through the emitter is the resistances mentioned above, a FET driver is literally just a switch that goes “hey there emitter, I’m gonna give you as much current as the system can provide whether you like it or not”.

Linear and FET are commonly combined in one driver, the linear element(s) for high efficiency and simplicity on low output, and the FET for driving the LED as hard as it can on high modes. These are frequently called “FET+1” or “FET+N” meaning a FET plus some number of linear regulators (generally AMC7135). These drivers have a restriction that voltage in must be greater than voltage out by a small amount, determined by the dropout voltage of the linear regulator (for instance, the 7135 has a dropout voltage of 120mV, so the driver’s input voltage must be at least 120mV greater than the Vf of the LED.


There are other types of DC-DC converters, but they are less commonly seen and/or rarely used for flashlights or constant current applications. These include buck-boost (inverting and four-switch), SEPIC, Ćuk, flyback, and AC-DC SMPS designs.

You also mentioned a series resistor, which is, in a way, an LED driver. A series resistor can be used to drive an LED when there is a constant Vin (or nearly constant) greater than the LED Vf. The resistor is chosen such that the resulting voltage drop through the resistor equals the difference in voltage between Vf and Vin at the desired brightness (desired current). Depending on how big the voltage difference is, and the amount of current through the emitter, this can be a very large amount of power, and therefore this method isn’t really usable for anything but very low output emitters and a suitable battery. Also note that as Vin decreases, so does the LED brightness.

…my fingers are tired, I’m going to go make potatoes.

^ Thanks for the recap, cannga. I have to admit I frequently forget the precise meanings of these driver setups. I hope one day someone puts out an informative video that targets the layperson a bit more.

Well, I’m mostly interested in efficient driver and good voltage brightness stabilization in the same light.

Especially in budget light.

@xevious you’re welcome. I looked up this topic many times before but thanks to Tendou’s question came across this one, to me the most succinct yet.

1 Thank

Some quick readings with my newly acquired toy, TA lumen tube – highest values (subjected to change with repeat re-measurements):

All 4 lights with GT-FC40 4500k:
M21B 2250 lm
M21F #1 2150 lm
M21F #2 2220 lm
M21E 2140 lm

One light with GT-FC40 5500k:
M21E 2220 lm

Simon’s specs for these lights are ~2000-2500 lm .

Simon’s has a nice summary of specs for different LEDS in same host, M21B with 12-Group driver\_randl_shipto=US:

Max current output - Max output
Nichia B35AM 2400 mA - 1400 lm (my measurement ~1100 lm)
SFT40 8000 mA - 2000 lm
XHP70.2 5000 mA - 4000 lm
Getian GT-FC40 2500 mA - 2500 lm (my measurement ~2200 lm)
Nichia 519A 6000 mA - 1400 lm

And to make your shopping easy (grin) here are relevant throws.

A very interesting comparison is between the two M21B lights. Even though throws for the 2 lights are the same, the higher lumen FC40 light has larger hotspot size and much brighter spill. Whereas the B35AM light has an equally bright hotspot, but of much smaller size, and the spill is not nearly as bright. Both specs (brightness and throw) are important in determining the beam character.

I love both lights, probably B35AM even a little more because its Ra and R9 are off the chart :slight_smile: , but for an all around every day light or walk light, FC40 is much more useful IMHO, especially in the M21E host (my most used light). Hope this helps.

M21B B35AM 4500k
Throw 263m~1100 lm
M21B FC40 4500k
Throw 262m~2200 lm

M21F FC40 4500k
Throw 273m
M21F FC40 4500k (second light)
Throw 276m
M21E FC 4500k
Throw 324m

Smaller size and de-domed, so it likely would be higher than 263 m - exact number I don’t know. But… it will be a smaller hotpot than B35AM, and it may be on the dim side.

Although Simon lists output for 519a in M21B host as 1500 lm (same as my B35AM), after subtracting some amount for “generous” Convoy listing and the de-doming, we might be looking at something less than 1100 lm.

What I’ve learned so far with flashlight design is like everything else, there is a trade-off: there are so many lumens from any particular LED, the more throw you get from a particular optic design used, say reflector, the smaller the hotspot will become. With a lowish output like that of BAM and 519a LED’s, as beautiful the beam color is, my opinion is if you really want a throw light in the order of 300m and more and want area of illumination to be “good” (wide hotspot, bright spill), get something with higher output like FC40. Just MvHO - meaning YMMV depending on use.

Here is a beamshot comparison of BAM vs FC40 in same host M21B. Note the larger hotspot and brighter and larger spill of FC40 - a result of twice the output and a larger LED. 519a de-domed will likely look similar to BAM with still smaller hotspot. Possibly dimmer if de-domed 519a’s output is less than the 1100 lm I measured for BAM.

Interesting note: the spill is also larger for FC40 vs BAM. Thanks to Simon for releasing these LEDs in same host, really a fascinating look at how LED size and output affect beam character. Both domeless, FC40 larger.

If that’s the case then M21B w/ 519a de-domed might just be a great light for you - go for it. Not sure if the throw will be 400m though. M21B has OP reflector.

I had been eyeing an M21B with this setup. I’m sold now!

thank you for your data measurement.
My m21f is about 2300lmvalues ​​almost the same as yours.

Glad I could help in spending your hard earned $ :wink: . Have fun w/ the toy.

You’re welcome. Guess this means your integrating sphere is working!? Congrats.

Thank you Sir,
yeah it works!

Fantastic! Congrats.