Magnetic Switches for RC Aircraft

Three versions of a simple on/off power switch activated by a magnet.

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This project is an offshoot of the RC battery backup project. It is a tiny circuit board that switches power to the electronics of a radio controlled aircraft. The switch is toggled by placing a magnet near the circuit and then removing it.

This project arose because an engineering friend asked the question, "How does this compare to the competition?" I responded that it would be much easier and cheaper to just build a magnetic switch without any of the battery backup aspects. So here it is.

Power to the load can be switched on or off by momentarily placing a magnet near the circuit. This allows the circuit to be completely enclosed within the aircraft fuselage.  The current drawn from the batteries while in the off-state mode should be as low as possible. It should take more than one year to discharge the battery. The voltage drop across the switches should be as low as possible to keep power dissipation, heat, and losses to a minimum. 

The twist on this application is that it should remember the state of the switch if the input power is disrupted or disconnected for up to 10 seconds. It would be disastrous if the power glitched during a flight and the switch permanently disconnected all of the electronics controlling the operation of the aircraft.

I've decided to pursue three types of switches. One for low current/low voltage applications such as RC gliders without motors. Another with higher current and voltage for motorized RC aircraft. A third with an integrated battery eliminator circuit (BEC).

Low Current Switch Specs:

Input Voltage Range: 3 - 30V (1S to 6S LiPo/Li-Ion battery)

Max current to load: 7A continuous, 10A for <30 seconds

Insertion Loss across switch at 7A: < 30mV

Off-state Current:  5µA

On-state Current: < 15mA

Power State Memory Hold Time: > 10 seconds

Size: ~ 10 x 18mm (smaller is better)

Cost:  ~$3.75

Latest Schematic (2018-03-28):

High Current Switch Specs:

Input Voltage Range: 6 - 20V (2S to 4S LiPo/Li-Ion battery)

Max current to load: 30-40A

Insertion Loss across switch at 30A: <40mV

Off-state Current:  5µA

On-state Current: < 15mA

Power State Memory Hold Time: > 10 seconds

Size:  13.3 x 20mm

Cost:  ~$5.33

Latest Schematic (2018-03-28):

Switch + BEC Specs:

Input Voltage Range: 6 - 18V (2S to 4S LiPo/Li-Ion battery)

Output Voltage: 5V ± 5%

Max continuous current to load: 3A or 5A versions.

Max burst current to load: 5A or 6A versions.

Efficiency: >90% at max load. (500kHz switcher)

Off-state Current:  10µA

Power State Memory Hold Time: > 10 seconds

Size:  13 x 35mm

Cost:  ~$6.00

Latest Schematic (2018-03-28):

Gerber files for BEC mag switch.

Zip Archive - 27.57 kB - 04/26/2018 at 20:27


Gerber files for 30A mag switch PCB.

Zip Archive - 21.61 kB - 04/26/2018 at 20:26


Gerber files for 7A mag switch PCB.

Zip Archive - 22.23 kB - 04/26/2018 at 20:25



Bill of Materials for BEC mag switch.

ms-excel - 9.00 kB - 04/26/2018 at 20:25



Bill of Materials for 30A mag switch.

ms-excel - 7.50 kB - 04/26/2018 at 20:24


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  • All's well that ends.

    Bud Bennett04/29/2018 at 22:07 0 comments

    At this point I consider the project completed. I have been flying my AXN Floater for a while now without any issues. The 30A mag switch works without any problems. It is nice to connect the battery and place the cover then just touch the magnet to the fuse to turn on all of the electronics. After landing I just pick up the plane and disable the power with the magnet. I don't have to open the cockpit and disconnect the battery until I get it back to my workbench. Very nice. 

    This summer I plan to install the 30A switch into two other powered gliders -- I don't expect any problems. I also want to install the BEC version into at least one unpowered glider.

  • 3rd Pass Low Current Switch Results

    Bud Bennett04/24/2018 at 21:09 0 comments

    The third pass low current switch design was to lower the switch resistance from 12mΩ to around 4mΩ by replacing the 3x AO3400 FETs with a single TSM038N03PQ33 FET. I built 5 prototypes. The measured switch resistance at 1A load current was between 4.2Ω and 4.8Ω, as expected. 

    All of the other measured parameters were consistent with previous measurements.

  • Second Pass Results for Mag Switch + BEC

    Bud Bennett04/24/2018 at 20:53 0 comments

    The second pass modifications were only to improve the input transient response to prevent catastrophic failure when connecting 4S (16.8V) batteries. Here's the scope trace of the input transient from a Multistar 3S battery (with a 25mΩ resistance):

    This was a HUGE improvement from the first pass:

    It appears that the input transient problem is solved. The only downside is the additional power dissipation from the 75mΩ input resistor.

    All of the other parameters that I measured were consistent with first pass results, as expected. There  won't be a third pass. I still need to evaluate the circuit using a 6A switcher IC, but that is pretty low priority.

  • Value Engineering

    Bud Bennett03/29/2018 at 00:14 0 comments

    I did not build any of the 2nd pass low current mag switch prototypes. They just didn't measure up against the Zepsus product (and I was too cheap when designing them). My decision to use three AO3400 FETs instead of a more capable power FET seemed wrongheaded. Spending $0.50 more for a better power FET would  add capability and robustness. So why not?

    I found a relatively cheap ($0.61/each in QTY=10) FET in a 3.3mm x 3.3mm DFL-8 package -- TSM038N03PQ33. Its RDSon = 4mΩ typ. @ VDS=4.5V, which is 3x lower than the combined AO3400 FETs, with the same 30V VDS max rating. But it cost $0.61 (Digikey) vs. $0.026 (eBay) for the AO3400.

    I bit the bullet and scrapped the second pass low current switch boards. The single big FET will increase the current handling to at least 7A, while allowing a short term burst current capability of 10A. Another consideration in making the switch is that these DFL packages, while seemingly difficult to solder no-lead packages, were surprisingly easy to solder (if you know the trick) with consistent results.

    The latest schematic for the low current version is posted in the details section. I've ordered 9 PCBs from OSH Park in 2 oz. copper. They should arrive in a few weeks.

  • 2nd Pass 30A Mag Switch Test Results

    Bud Bennett03/28/2018 at 23:42 0 comments

    I assembled one of the second pass 30A prototypes using different FET switches than the first pass -- substituting a  Vishay SiSS28DN for the IRLHM620. The Vishay part claimed slightly lower RDSon (1.6mΩ vs. 1.8mΩ @ VGS = 4.5V) and a higher max VDS of 25V instead of 20V. I thought, erroneously,  that the higher voltage rating might improve the survivability of input transients when batteries are plugged into the input. The addition of R1 between the input and the LDO solves the problem by eliminating the capacitor (C2) that caused the ringing so a higher voltage rated FET is not required to support 2S-4S LiPo batteries.

    I was able to use a different FET because the 3.3mm x 3.3mm DFL-8 package is apparently a standard. 


    The switch no longer is sensitive to disconnection by high currents. I ran more than 35A through it and it remained closed. 

    It is now tolerant of the input transient caused by plugging a 4S LiPo battery into the inputs -- there is no measurable ringing. 

    Those were the two major items that had to be fixed on this pass.

    The new Hall-effect device trips at 45 Gauss and requires a stronger magnet. I ordered some 6mm x 10 mm N52 magnets and now the switch trips when the magnet is about 5/8 inch (16mm) when directly over the Hall-effect device. Since the Hall-effect device is active high, the switch activates and deactivates when the magnet is applied, rather than removed, which makes this measurement simpler (and the operation of the switch more straightforward).


    Off-state current :

    5.3µA @ Vin = 6V

    5.7µA @ Vin = 21V

    On-state Current: 7.2mA @ Vin = 16.8V

    Switch resistance: 

    1.22mΩ @ 8.2A

    1.23mΩ @ 14A

    1.3mΩ @ 33A

    1.3mΩ @ 35A (45mV across switch)

    EDIT 2018-03-29:
    The switch resistance measured above was taken from the two pads on the board. While testing the other 5 boards I noticed that the resistance was 25% lower when measured from the copper landing that I had placed near the sources of M1 and M2. That's significantly lower, which I did not expect. I connected up two more boards, using the landing pad for the B- lead. Here's what I measured with that configuration:
    1.0mΩ @ 1A for the board using SS28 FETs.
    1.2mΩ @ 1A for the board using IRLHM620 FETs.
    In the end, I did get a pretty good improvement in switch resistance from the SS28 devices.

    Memory hold time is between 30 seconds and 45 seconds. The lower off-state current drain has caused this to lengthen. C6 should be reduced to get this back into the 15-20 second range.


    Meets both functional and parametric requirements. No circuit changes necessary. I'm going to build a total of 6 units -- 3 or 4 for my use and the rest will be sold. I'll report back on how they perform in my electric powered planes.

  • Power Corrupts (Absolutely)

    Bud Bennett03/27/2018 at 22:31 0 comments

    I've been traveling a bit the last few weeks and came home to a bunch of PCBs and a few components that finally arrived from China. I had to prioritize. The BEC version was a first pass effort, so that got the highest priority. I knew that the BEC was going to need a second pass to fix the input transient problem so i only assembled a single unit. It was larger than I expected. I got accustomed to dealing with very small boards and this one is huge by comparison.

    Crappy Input Caps:

    To keep my options open, I had ordered several ceramic SMD capacitors from Chinese eBay vendors that I hoped would work as input caps:  

    • 10µF 50V 1210 (no temp rating)
    • 22µF 25V 1206 X7R (not really)
    • 22uF 50V 1210 X7R (unbelievable!!!)

    I tested the capacitance change vs. voltage and was disappointed with the result. The 10µF/50V caps decreased to less than 1µF @ 17V. The 22µF/25V measured <3µF at 10V. The only hope was the 22µF/50V, which measured 6µF @ 17V. I could have ordered some X7R 1210 from Digikey, but it was going to cost $3.00/each in my quantities -- not an option. 

    The Data:

    I measured a few parameters to make sure that things were working:

    • Off-state current draw was pretty good -- 7.7µA @6V increasing to 10.3µA @16.8V.
    • On-state current maxed out at 14.2mA @ 16.8V.
    • Line regulation is so-so: 5.05V @Vin=6V, to 5.17V @Vin=16.8V.
    • Load regulation is hard to measure without an electronic load, but seems to be better than the line regulation.

    I also measured input ripple voltage and output ripple voltage:

    No Load Output Ripple Voltage:

    Output Ripple Voltage with 5A load:

    Input ripple voltage (Vin ~ 3S LiPo Battery):

    This all looked pretty good. I decided to see what the input transient voltage waveform looked like for a couple of different battery types:

    Input voltage transient for 2S LiPo battery:

    The battery had a resistance of about 100mΩ so the resulting transient doesn't ring much.

    Input voltage transient for 3S LiPo battery:

    This 3S battery had a much lower internal resistance and there is a bit more ring as a result (And there was quite a bit of wire length between the battery and the board). Even so, the peak is about 17V, which is not a problem for the switching regulator (which has a 20V abs. max. input voltage rating). Though it is doubtful that the board will survive a 4S LiPo battery.

    Load Tests:

    I used a 22Ω 1W resistor for an easy load test. The BEC did not have a problem with this load and the output decreased only about 20mV over the entire input voltage range. 

    Things got more interesting when I attached a 1Ω/20W resistor to the output. The output voltage dropped to 4.97V and then steadily dropped over the next minute or so until the output voltage dropped below 4.9V where the buck converter hit its thermal limit and it began to disconnect the load to limit its junction temperature. 

    At this point I was pretty bummed out. It was obvious that the BEC was not capable of handling 5A as a continuous load. This result was not unexpected. I was boning up on thermal resistance and power dissipation of PCBs while traveling and had determined that a PCB this size could only handle about 0.5W of power dissipation to keep the junction temperature of the switcher IC less than 150°C.

    So what load current was it capable of handling continuously? I kluged together three 4Ω resistors in parallel to yield a load resistance of 1.33Ω - 1.35Ω, which would draw 3.75A at Vout = 5V. When I connected this reduced load current the BEC was able to keep the output voltage steady at 4.95V until the battery dropped below 6.5V and the input voltage crashed pretty quickly. My calibrated thumb determined...

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  • Preventing Poof!

    Bud Bennett03/04/2018 at 02:18 0 comments

    After destroying the LDO on the 30A mag switch board I decided to revisit the protection circuitry for all of the other boards. The 4A circuit is a copy of the 30A board so it should be OK. The problem is with the BEC version. The switching buck converter requires low ESR ceramic caps at the input. This causes ringing when the battery is connected to the board -- potentially damaging or destroying the circuits that are connected to the battery terminal.

    I investigated protection devices like TVS diodes or Zener diodes to clamp the inputs, but they did not have close tolerances that would prevent the input voltage from exceeding the 20V abs. max. VIN value for the RT6255 switcher. I decided to dust off LTSpice to see what was happening and look for possibly a simpler, less expensive solution. Here's the test schematic that I used:

    The inductance is just the wire leads from the battery to its connecter and then from the BEC connector to the PCB. I measured a fairly constant battery wire length of 4 inches for all of my batteries. If I add 1.5 inches for the mag switch leads then I get a total length of wire connected to the input that is approximately 11 inches (doubling the wire lengths to accommodate both the positive and negative leads.) I then visited this website to calculate the approximate inductance of the wires leading from the battery to the PCB -- which is about 350nH, represented by L1 above. Of course, there is some variation in these values, so I varied it over a reasonably wide range (300-500nH) to accommodate reasonable scenarios

    I also measured the resistance of nearly all of my stock of LiPo batteries. I found that it ranged from about 25mΩ to over 100mΩ. If you're interested, the Multistar 1.6A 3S 40C LiPo batteries had the lowest resistance and the cheap Chinese NoName batteries tended to be at the top end of the resistance range. I figure the middle of the pack is about 50-60mΩ. This is represented by a portion of R2.

    C1-C3 are the input capacitors required by the RT2255 buck switching regulator. I gave each of them 5mΩ ESR ( variation around this value did not have much effect).

    This is what the input transient looks like without any circuit changes made for protection and R2 = 25mΩ for the battery resistance:

    That first peak of nearly 30V would smoke the switcher IC. The peak transient waveform ranged up to 24V-27V, depending upon the values of C1-C2 and the associated ESR, the inductance in the lead wires, and the battery resistance.

    My first thoughts were to add a snubber -- a series R and C across the inputs. This could be just a tantalum capacitor with an appropriate ESR value . The problem I found was that the capacitor had to be greater than 100µF and the series resistance (or ESR) around 100mΩ. I could not find a reasonable size ceramic 100µF capacitor with a 25-50V rating, or a tantalum with <500mΩ ESR. The solution for the snubber to work was to put 15x 10µF ceramic caps in parallel -- so I abandoned this idea in favor of adding resistance in series with the input.

    R2 now represents the battery resistance and also a discrete resistance added in the circuit, R10, for the specific purpose of transient suppression. I won't bore you with the details, but I found that the value of R2 needed to be between 100mΩ and 150mΩ to keep the peak of the transient waveform below 20V (the absolute maximum VIN value for the RT2255 IC). C1 and C2 need to be increased as well -- even with a 50V rating the capacitor value decreases by 40% with 17V across it -- so the simplest solution is to stack two 10uF/50V caps in the place of one. This would not be a solution for high volume production but I can accommodate it easily when populating the board manually.

    Here's a typical simulation waveform result showing the improvement:

    R10 needs to be a power resistor. When you insert 100mΩ of resistance into the input...

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  • 30A Mag Switch Problems

    Bud Bennett03/01/2018 at 23:29 0 comments

      I assembled the 30A mag switch PCBs without issues. I expected some problems with the M1 & M2 PowerPak packages, but for some reason they always solder up just fine. I measured about 22µA of off-state current with 6V < VIN < 20V. Pretty much the same as the 4A boards. With 5A applied load current I measured 1.1mΩ switch resistance -- spot on the design target. 

      The bad stuff happened when I hooked the switch up to a 30A_ESC/Radio/Motor. The switch disconnected itself when the current exceeded 16A. Bummer. I remembered a warning from a colleague at LTC/ADI about "scary currents" that could affect the hall-effect switch, so I went online and found this website calculator for magnetic field strength vs. current through a wire. It turns out that the magnetic field strength 1mm distant from a wire carrying 16A is about 32 Gauss -- very nearly the trip point of the hall effect switch. Stupid me.

      In order to test out this theory, I desoldered the hall-effect switch from the circuit and located it onto a separate board with 2 inches of wire. Problem solved. The switch is now apparently immune to current flow.

      But I could not reproduce the same problem as before by holding the hall-effect sensor near the current carrying wire as high currents flow. That was somewhat disconcerting, but par for the course.


      I decided to use a 4S battery and a 60A ESC to generate a current above 30A to test the circuit. When I connected a fully charged (16.8V) 4S LiPo to the input the circuit lets some of its magic smoke out. After all the cussing ended I was able to determine that the TPS70950 LDO had apparently latched up -- the current was enough to fuse the VIN lead open:

      It is apparent that a 30V rating is not enough to prevent inductive transients damaging the LDO. The second pass, and all the other mag switch circuits, will have a protection resistor to prevent this in the future. After replacing the LDO and the hall-effect device, which was also damaged by the event, I jury-rigged a 330Ω protection resistor in between the LDO and the battery. I plugged/unplugged the battery many times to see if there was any remaining tendency to smoke, but the circuit doesn't have that problem anymore. 

      High Current measurements:

      With load current 35A, the voltage across the switch was 40mV (that's 1.1mΩ). Power dissipation is only 1.3W, which should not be a problem for a board even this small. I repeated the measurement with 20A load current and got 22mV across the switch, and the same switch resistance. No changes required.

      So here's the plan:

      1. Add a 330Ω protection resistor between B+ and the LDO input.
      2. Change the layout to move U2 as far from the current carrying traces and wires as possible. Here's the proposed topside layout (the bottom side did not change much):
        See that U2 (the hall-effect switch) moved to the bottom right side of the PCB. I measured 4mm from the high current traces. A wire carrying 50A generates 25 Gauss at a distance of 4mm.  The board area increased, but not materially.
      3. Change the hall-effect switch to increase the trip threshold. The S-5716ACDH2 component is specified with a typical threshold of 45 Gauss vs. 35 Gauss for the AH180 part. The minimum specified threshold is 25 Gauss.
      4. Use a magnet with higher field strength to compensate for the increased hall-effect threshold. I ordered a few 6mm dia. x 10mm length N52 Neodymium magnets to use in the application going forward. These new magnets have significantly higher B-field to compensate for the increased threshold. Another website yields useful calculations.
      5. There may be some users that want to locate the hall-effect sensor away from the main circuit board. I added pads to allow leads to be soldered instead of mounting the sensor on the board.

      The change from the AH180 hall-effect sensor to the S-5716 sensor changes the way that the switch works. Previously, the switch would change state when the magnet was removed. Now...

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  • Working Second Pass 4A Boards

    Bud Bennett02/28/2018 at 03:33 0 comments

      Today I received PCBs of the 4A and 30A mag switch circuits. These boards are 2oz. copper and designed to keep PCB trace resistance to a minimum. I built all three of the 4A version with the following results:

      1. Verified 20V operation.
      2. Off-state current 22µA for 2 boards, 60µA on one board. The high off-state current did not increase with input voltage, which leads me to believe it is from a component on the 5V rail. Even this higher current draw is not really a big deal in the scheme of things. It doesn't seem to affect operation in any way.
      3. Memory hold time is greater than 1 minute. This is way too long. I will decrease C4 to 100uF (1206 10V X5R ceramic) on future boards. I also increased R6 to 220Ω to get a bit more isolation for the LDO -- the larger resistor only causes a voltage drop of about 10mV.
      4. Switch resistance measured at 11-12mΩ with a load current of 1.5-2.5A. I was expecting better, but it should be good enough. The power dissipation of the switch will be less than 300mW with 5A of load current. That's only 100mW for each output FET so temperatures should not get too hot.

      A couple of days ago Digikey notified me that the AH180N hall-effect switch was going to be obsolete. I found a drop-in replacement -- S5716ACD1M3T1 by ABLIC. This new part has a CMOS output, 4uA typical supply current, and slightly better sensitivity with a 30Gauss trip point. The footprint is identical. The cost is $0.72 in lots of 25. I will use this hall-effect device after using up current inventory.

      So this latest 4A switch seems to meet all of its objectives. I just have to install it into one of my planes and test it out in the real world.

  • Another Fork in the Road

    Bud Bennett02/21/2018 at 06:14 0 comments

    I've been thinking about integrating a Battery Eliminator Circuit (BEC) with the magnetic switch electronics for a few days. Yesterday, at the monthly RC club meeting I floated a Mag Switch BEC board after demonstrating two mag switch prototypes. There seemed to be more interest in a MagSwBEC than just a switch, so off in the weeds we go again.

    I posted the feature set and specs in the project details. It makes a lot of sense to integrate the BEC with the switch. Most users just want to hook up a battery and get the proper voltage and current at the outputs to drive the receiver and servos. 

    This device is a whole different beast than the previous circuits. Switch-mode regulators are generally a pain to get working correctly. I trolled Digikey and found a series of synchronous buck switchers in SOT23-6 packages made by Richtek. There are three parts that look promising: RT6254B, RT6255B and RT6257B. These parts have a VIN range from 5-18V, integrated high-side and low side FET switches, 500kHz switching frequency, and have an enable pin that can shutdown the converter with only 3µA of supply current.

    I decided to target a 5A BEC, but a 3A and 6A circuit can be made with the same PCB since the 3 regulators ICs have identical pinout, and I'll need to find some inductors with similar footprints. The inductors should all have 6.5mm to 9mm nearly square outside dimensions. I settled on a range of inductance between 3.3µH and 6.8µH, with the higher values needed to keep the ripple current to a reasonable level with higher input voltages. It is difficult to find a reasonably priced inductor in this size that will handle 5A to 6A.

    I found a ECS MPIL0630-5R6MC 5.6µH for about $0.70 in low quantities. It's rated for 5.5A, with Isat = 6A. I calculate ripple current should be around 1A, or 25% at VIN=16.8V. I have a PG0083.332 3.3µH in inventory that should handle the 6A switcher. The inductor for the 3A BEC should be the easiest to find, but I haven't looked for it yet.

    Adding the switcher was just an extension to the basic mag switch. I have a pretty solid layout already. Here's the top side of the board:

    All of the components are on the top side this time. It's easier to assemble and I wanted the bottom side to be just a large ground plane. All of the low current circuitry is located on the left side and the high current switcher circuit is on the right. I moved the pads for the B+ and B- leads far into the board to make the traces shorter. The leads will run across the bottom side of the board where there are no components. The big GND plane and 2 oz. copper should help dissipate heat.

    I'm not too concerned about board size with this one. You're getting the BEC and the mag switch for not much more than the BEC alone. And more area means lower temperatures. This layout is 0.52" x 1.36" (13mm x 35mm).

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Martin wrote 02/12/2018 at 09:51 point

The symbol for the FlipFlop is not recognizable. A schematic symbol of a simple component, like logic gate or flip flop should represent it's function, not just be a box. With power pin at best at the top or at least upper left corner. For the 13V Version the use of 4000 series CMOS (with a 15V Z diode and a series resistor could eliminate the voltage regulator alltogether. Otherwise a TI TPS70933 or TPS70950 (can also be LM..., but the number is correct) could be a good option.

In your design most of the on-state current is wasted by the LED. With a high efficiency blue or green LED 0,5mA would be enough.

  Are you sure? yes | no

Bud Bennett wrote 02/12/2018 at 19:07 point

I used to care about how the symbols looked, but I'm using Diptrace for schematic capture and it is not possible (at least I don't know of a way) to get the pins on top or bottom to have the proper pin name rotation (i.e. horizontal). I took your suggestion to heart though and made a new symbol. But I left the CLRB pin at the lower right side because it made more sense to keep the RC components on the output side.

Thanks a bunch for suggesting the TPS70950. I put it in all of the designs (it was a drop in replacement for the LT1761). It is a bit more expensive than the MCP1702 ($1.04 vs. $051), but has lower Iq, will tolerate input voltage up to 30V, and doesn't require a blocking diode or protection resistor. I could not find it during a long search on Digikey. That was a golden tip. I'm going to have to study the data sheet a bit more and probably change the output capacitors, since it is only stable with caps in the range 1.5µF - 47µF, but at least it will handle ceramic caps!

The Zener + pass transistor approach is one that I considered an dropped. Most Zeners need a bit of current to get up into their knee region, where they're stable. After you add the resistor and another SOT NPN it's a larger solution. The LDO is certainly more expensive, but simple and reliable. Or it just could be my prejudices from a long time ago and things have changed now.

I've tried the low current LEDs, but the one I used previously wasn't very bright. Plus I have an inventory of 0603 LEDs in a bunch of colors that cost me a penny/each. They seem plenty bright to me at 5mA -- the request that I got was to make the LED visible in bright sunlight while out at the flying field. Compared to the 4A or 30A load the 3mA of LED current is a nit.

And lastly, thanks for the comment in general. It's great to get this kind of feedback and really helps me out.

  Are you sure? yes | no

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