Zeners? We don't need no stinking Zeners!

The title is just for comic effect. Zeners have a valid place in at least clickie drivers. They, however, present cell damage risk in momentary switch lights due to large parasitic drain. Forgetting to lock out can result in your cells' voltages going into the danger zone in short order. I've turned many of my buck drivers (that don't have zeners but use diodes to regulate voltage and provide reverse polarity protection) in to momentary switch drivers and they all have horrendous parasitic drain.

If your shopping for a new driver with low parasitic drain RMM and led4power have some great options.

For existing drivers, a very nice solution is readily available and somewhat easy to implement.

It's the LM2936. It marketed as an "Ultra-Low Quiescent Current LDO Voltage Regulator". It has less than 15-μA quiescent current at a 100-μA load. It was mentioned in here by texaspyro, but no one seems to have reported trying it out. The data sheet looks real good to me. It also has built in reverse polarity protection.

Started playing with one. Man, it has rock solid voltage output. Voltage only varied 10mV from input of 9V to input of 18V. Very nice. None of my drivers come even remotely near that. It can take up to 40V input in the normal model! Oh yeah, this baby is going in all my >1S drivers.

Here is the recommend circuit to implement with it.

The 10uF caps already on our drivers should be way over kill in covering the input requirement. It's advisable to got to higher voltage rated caps though. I have like 500 10uF 50V X7R's on the way. I will probably try to sell off a good chunk of them. I intend to solder the + and - pins to each side of the cap on most of my drivers.

The datasheet says it is essential that the output capacitor meet the capacitance and ESR requirements, or oscillations can result. Ceramic capacitors can be used only if a series resistor is added to simulate the ESR requirement. Typically, a 500-mΩ to 1-Ω series resistor is used for this purpose. The suggested resistor in series with the output cap is not shown in the above diagram because you can also buy caps with the proper ESR.

It comes in several package options including:


I ordered two different package types of the 5 volt version. I purchased the T0-92 package here. I will use the TO-92 package for converting existing drivers as the long pins will provide flexibility built in air wires. I found that I could easily solder the SMD caps and resistor to the top of the pins if needed. I purchased the SOT-223 package here. I haven't used these yet. I intend to design some boards that will use them. They are much bigger than 7135's.

Pilotdog68 rendered this drawing to show me how much bigger the SOT-223 is than the AMC7135 SOT-89. Probably will be limited to bigger boards.

I'll be documenting the various drivers I put it in here. How to and results.

  • First up was a HX-1175b1 in Post 16. Will be used with 4S cells to drive an MT-G2 with a momentary switch. LVP resistors are 100K for R1 and 5.6K for R2.
  • Wight's 22mm DD+7135 in Post 35. Will be used with 2S3P cells driving 2S6P emitters.

That thread is a few years old but I think the Zener won out due to fewer extra parts needed and with the first FET mods using the larger FET there wasn’t board space for a single sided version. I like them even so and agree they need to see more use.

I thought the zeners just won out because most people used them in tail switch lights instead of momentary. Doesn’t RMM have LDO drivers? Thought I saw mention of them.

^ Yes, RMM has some nice LDO drivers and I highly recommend them. I'm a tinker and find great joy in tinkering. So playing my existing drivers that I have acquired over years is my cup of tea.

RBD wrote:

That thread is a few years old but I think the Zener won out due to fewer extra parts needed and with the first FET mods using the larger FET there wasn’t board space for a single sided version. I like them even so and agree they need to see more use.

You both mention the zener "won". Can you point me to where the LM2936 was actually used and compared to the zener? I was recently poking for more info in a thread, but no one provided such info. I'm far from any kind of expert, but the zener clearly loses (pun intended) in my book.

As far as I know the the thread linked was started well before the influx of most of the Zener modded driver threads which took off mainly due to the work of Comfychair and some others that were piggybacking Attiny 13a mcu’s onto other FET boards. The mtg was just coming out and a 6V option was needed. I was doing this LDO Vreg method and CC advocated the Zener and I think mainly due to the high volume of work he was doing and the simple fact that a Zener is easier to piggyback on a nanjg 13a board the Zener found favor. Remember, this was well before Mattaus and others proliferated the Oshpark options we now enjoy so it was 105C or 101-AK01 and little else to choose from. With Eagle design software in more use more options are becoming available and the problem of parasitic drain is pushing things back towards LDO VRegs again. Before that the change to LFPAK 56 and the LFPAK 33 fets made possible single sided FET boards for smaller lights. It’s been fun to see the progress over the last several years.

They weren’t ever put head to head. A couple years ago a group of us put our heads together to come up with a BLF SRK drive. Mattaus did the board design, CC did virtually all the testing and Warhawk-avg and I stood around pointing and making suggestions. I suggested both the LDO Vreg and the LFPAK 56 as options but was overruled in favor of the Zener and larger Dpak FET mainly because at the time CC had lots of those fets and favored the Zener. Anyone who remembers him can probably recall that he followed his own lead. I don’t recall anyone else using the phrase ” I can’t understand why anyone would do…” quite as often as he. A ton of work contributed but not big on seeing a different point of view.

^ True.

It was your thread here that opened my eyes to using small voltage regulators and using 7135's in higher voltage apps. Great ground breaking and thought provoking thread. Last time we talked in it you seemed preoccuppied with something else that night. Anyway, the increase in momentary light mods and series cell lights for higher Vf emitters has increased my need for solutions like this voltage regulator. No rocket science to using them, but I haven't seen much real world application of them around here. This LM2936 is a beauty to watch in action to me.

Regulators are definitely better imo unless you’re low on room.

Be aware that the upper end of the voltage range for regulators may require a package that can get rid of extra heat. Effectively some packages give you a lower voltage range, else it can burn up.

^ Thanks for the heads up Halo.... I only plan on taking mine up to 16.8V (they have a 40V limit). I will be watching (actually feeling) for that though. Especially in the bucks were the LM2936 will be driving an MCU and a Buck Converter. Now that you mention that, I think I should implement a buck first to get a more demanding situation out of the way first.

Opps, I missed that they went up to 40v :smiley:


Ok, I don’t understand. Are you suggesting using the LDO regulator on a driver in place of a FET?

The LDO seems to have the advantage on a e-switch style light, replacing the Zener and yes… saving the cells.

Nice to see this being worked on again. Not electronically oriented here, I just like to apply power and see what gives… :wink:

Ohaya wrote:


Ok, I don’t understand. Are you suggesting using the LDO regulator on a driver in place of a FET?

Great to see you back bro! In a DD style driver, the LM2936 would go in place of the zener diode and resistor. It would be used to regulate voltage to the MCU. In a buck driver like the HX-1175b1, it would replace 3 diodes, a capacitor, and maybe one resistor.

Nice photoshop Jubeldum You made me just realize I have been misspelling Zener.

:smiley: I thought you were just mis-spelling zener for added comic effect!
We don’t need no stinking Zeners!

I’m often preoccupied but some times just occupied and still others just tired. I’ve enjoyed learning new things from others on this forum and very glad some of the threads prove useful.

Here is the plan to convert the HX-1175b1. Well remove 8 components and replace with the following:

  • LM2936
  • 10uF ceramic capacitor (kind typically used on a 105C)
    • Since my current caps are cheapos, I will use two parallel caps.
  • Resistor (Probably around 4 ohms)
    • The resistor, in series with the above capacitor, will simulate a capacitor with the proper ESR.

Just need to pick one of my existing lights to do the above to.

Anyone know what data sheet belongs to the tantalum capacitor I'm pulling? It's a 22uF 6.3V. I want to determine what it's ESR is. It is labeled "226J" and "40LN3".


Parts needed. LM2936, Output Cap (only one needed, but opted for 2 as there is not limit on capacitance that can be added, and a resistor to simulate the proper ESR.

Doing an Attiny25 upgrade at the same time (Parts on the right). That 10K resistor should be a 100K resistor. It and the 5.6K next to it are for the LVP.

Pulled off unneeded components. All that copper makes it tougher as it sucks heat from the soldering iron.

Installed parts, but accidentally connected LM2936 backwards. Also installed wrong resistor for R1 of the LVP. Didn't notice either mistake until I tested.

Put wire sheath on output pin to avoid same mistake on reinstall.

There, properly installed and tested.

Installed new MCU.

(To be completed)

Yes Tantalum 226J is 22uF 6.3V. See: http://i00.i.aliimg.com/img/pb/866/944/341/341944866_155.jpg taken from http://www.avx.com/docs/masterpubs/tantniob.pdf

^ Thanks ledoman. That is the document that I had looked at to determine that it was a 22uF 6.3V tant cap, but I couldn't find this specific cap in there. So that I could determine its ESR.

Halo... wrote:

I thought you were just mis-spelling zener for added comic effect! We don’t need no stinking Zeners!

I only wish I was that intelligent. Zeneeeer! would definitely added to the comic effect.