Battery Pack Load

Powering device via USB has its advantages even if device doesn’t need connectivity. Most notable of those advantages is a wide availability of USB battery packs. That makes powering device really easy. However, what if your device uses so little power that batter pack keeps turning off?

All three battery packs I had available had the same issue. Put something that pulls just a trickle of current and they would turn off. One of them did have a trickle-charge functionality and that sorta worked but required manual intervention. And yes, I forgot to turn it on more times than I care to remember.

So I went looking for solution and found Dorkbot PDX article by Paul. The way how he solved this issue was quite a nice one. Just put intermediate load on the bus and you’re golden. Instead of going with his solution, I decided to use my favorite hammer - Microchip PIC microcontroller - to make my own board using the same concept.

Realistically, there’s no reason to use microcontroller for this. Having a programmable part just introduces assembly complexity and you cannot use it out of box, without programming it first. But there are advantages too. The major one being adjustability and that came in really handy when figuring out the timings needed. Instead of dealing with large value capacitances, one can use inherit time keeping mechanisms of microcontrollers. Furthermore, it also allowed for easy placing of blinky light. And who doesn’t love those?

The whole solution is based on a small 6-pin PIC10F200 microcontroller. When you discount programming pins, that leaves you one pin to work with. And yes, you can pretty much use programming pins too (3 of them) if you use a few extra passives but I generally leave them alone if I can. Fortunately, application I had in mind was simple enough to solve with a single pin.

The device logic is as simple as it gets. To keep the battery pack turned on, it uses MOSFET to pull 200 mA over two resistors for about 20 milliseconds. Once pulse is sent, wait for 10 seconds. Rinse-repeat. On all packs I’ve tried this combination worked flawlessly. Of course, if there’s a pack where this doesn’t work, adjustment is easy enough (yes, I will update code with some constants later :)).

The heavy lifting is done by MOSFET driven directly by microcontroller. Once turned on, it will allow two resistors to dissipate a bit over 200 mA of current. While this seems like a significant amount of power to pass over puny 0805 resistors, short duration means they will never get hot enough to present a problem.

One could also take an offense to how MOSFET is driven too. Due to the nature of the beast a lot of current will flow out of the gate pin and by common knowledge one has to have a resistor to limit it. However, this MOSFET is small enough that current limiters built-in to Microchip PIC microcontrollers are fine. I had even bigger MOSFETs work in circuit for years without gate resistor so I am not too worried. That said, if I ever change to different microcontroller, some reevaluation will be needed.

And yes, there is no bleed-off resistor on MOSFET gate either. If circuit is powered on, PIC will be driving it either low or high so pulling the MOSFET gate down is not really needed. If circuit is not powered on, who cares - there’s no power to go around anyhow. That leaves only the short amount of time when circuit is powered on and PIC is not yet driving the gate. And that is the interval we can safely ignore as it’s shorter than 20 milliseconds and even erroneously active MOSFET will cause no damage to the circuit in that time.

Thanks to Microchip’s semi-standard pinout, this device can use any number of PIC microcontrollers. As long as footprint fits (SOT23-6), you’re golden. I have tried it with already mentioned PIC10F200 and PIC10F320 but any variant should work too. Yes, the firmware did need a bit of adjusting between methuselah PIC10F200 and newish PIC10F320 but those were simple enough. In these times of chip shortages, having multiple parts fit is always a nice thing.

How much battery this uses, you might ask. It would be a bit annoying to have your battery drained by a device whose only purpose is to keep it awake. Well, to start with, we need to account for all power usage. The main one is of course our 220 mA pulse (5V/(47Ω÷2)) lasting 20 milliseconds. Since it repeats every 10 seconds, that leads us to a duty cycle of 0.2%. Multiply one by another and we can see that average power usage is about 0.5 mA.

Second power user is our LED that blinks with the pulse. Due to large value resistor, it will require slightly less than 1 mA for each pulse. Using our duty cycle, that us average consumption of 2 µA (0.002 mA). Pretty much a noise compared with our load.

And lastly there’s the PIC itself. Good news is that this is well under 1 mA. How much under? I have no idea as I didn’t have anything capable of measuring it. I will definitely need to create a board for that too. However, I did use USB power meter to get a long term reading of the usage and it was slightly under 0.5 mA (averaged over an hour) if using PIC10F320. For older PIC10F200 usage is a smidgen above it.

Those reading more carefully might wonder, if a power pulse needs about 0.5 mA, and total consumption is under 0.5 mA, we have the free energy as microcontroller is actually using the negative power from a far dimension to run itself. Sadly, it’s not so. Due to how MOSFET is driven, it won’t turn on nor it will turn off instantly. So our 200 mA pulse average will actually be lower. And PIC consumption is low enough to “hide” in those numbers.

Code actually runs constantly in the background due to a quirk of PIC10F200. If you put it in sleep, it resets itself (by design) making it annoying to keep track of time longer than 2.3 seconds. I was toying with idea of just using the newer PIC10F320 but power usage of constantly running PIC was low enough that I decided not to care.

Either way, if you have 10000 mAh battery, this device could theoretically run for 20000 hours. Suffice it to say that it shouldn’t reduce battery life too much. If you really want to squeeze the last possible mAh out of it, you could adjust timings. For example, my Anker power bank was quite happy with more than a minute between pulses (0.033% duty cycle). However the default 0.2% duty cycle is probably a good starting point when it comes to compatibility.

Lastly, there are actually two versions of the device. The “normal” one just plugs in USB port while pass-through has a female USB connector on it allowing it to be used on battery packs with a single type A output. You can find gerbers for both on GitHub alongside with the source files.

Dealing with part shortage - Microchip edition

For a small electronics project of mine, I needed literally only one output pin. My go-to part for these situations is PIC10F200. It’s a 6-pin SOT-23 device offering internal oscillator and not much more. Microcontrollers don’t really get smaller/simpler than this.

Due to the hamster habits I have, I actually had enough enough parts to finish up the prototype so the only thing left was to order a few more parts of DigiKey. Well, as many components lately, my favorite PIC was out of stock.

However, when one door closes, another one opens. Microchip is really good at keeping pinout similar over multiple microcontrollers. And DigiKey had PIC10F202, PIC10F206, PIC10F220, PIC10F222, and PIC10LF322 available in stock. While all these PICs are slightly different, they share the same basic pinout. And for my project any of them would do. Even if I used some less common feature, Microchip often has multiple products differing only in memory amount.

While hardware might be similar enough, firmware does have significant differences - especially between older PIC10F206 and newer PIC10LF322 setup. Even turning LED on/off uses different registers between them. Instead of having different firmware for each, one can make use of compiler directives and check which PIC is actually being used. Something like this:

#if defined(_10F200) || defined(_10F202) || defined(_10F204) || defined(_10F206)
    GP2 = 0;                // turn off GP2
    TRISGPIO = 0b11111011;  // GP2 is output
#else
    LATAbits.LATA2 = 0;    // turn off RA2
    TRISAbits.TRISA2 = 0;  // RA2 is output
#endif

While out-of-stock syndrome has hit Microchip too, with a bit of care, they do make transition feasible if not always trivial.

Adding USB to A-BFastiron SS-305MP

I needed a cheap power supply for a project and it was easy to find a nice one in A-BFastiron SS-305MP. It was small enough, looked good, and had shiny display. What could man want more?

Well, when I got it and saw cutout for USB, I know what more I wanted. An USB port.

And strangely enough once you open the power supply, you’ll find connector providing about 8.2 V already there without anything to plug into. It’s almost as if somebody placed it there to be an input for 5V linear voltage converter and then later figured electronics and heatsinking would cost too much and covered the hole. And yes, it’s a proper hole cover that you can remove - no drilling necessary.

If you open power supply you will even find standoffs already in place. It’s simply begging to have PCB mounted in.

First thing to figure out was which USB connector will fit. Searching on DigiKey found quite a few of them roughly matching the dimensions. So I just selected the cheapest one that matched standard footprint. And yes, looking on side you might find it protruding a bit too much but not criminally so. It might be original designers were fine with this or the had a custom length connector in mind. For me this was as good as it gets.

With connector found, it was time to figure PCB. And I decided to keep it really simple. The whole setup would revolve around VXO7805-1000. It’s a nice DC-DC switching regulator that will take any input higher than 8 V and drop it down to 5 V with some efficiency. In its pinout it emulates beloved LM7805 but at 90% efficiency and without all the heat.

Regulator itself requires just two capacitors and I decided to go just with them. I was tempted to add a smaller 1 µF capacitor to output and maybe even a 100 nF one for decoupling purposes but decided against it. Due to wide variety of cables and outputs USB device might face, all of them already have more than sufficient decoupling and adding more wouldn’t really do anything. So why waste a component.

The only really unneeded components would be an LED and its accompanying resistor. While they serve no function, I really love to have an indicator of output. If there was ever an issue, looking at LED would at least tell me if power is going out. And quite often that’s quite a big help.

Speaking of power going out, I don’t consider a fuse optional. It’s a minimum you need in this setup. Another thing I would consider bare minimum for power supply would be a short-circuit detection but that’s fortunately already a part of voltage regulator. And yes, I could have gone further, especially by adding reverse polarity protection to the input and I was tempted but in reality you’ll just connect this thing once and leave it connected. As long as you connect it correctly the first time, you’re good.

Connecting all this to the power is entrusted to any JST-XH 2-pin cable - 10 cm in length. Just make sure that the negative wire is going next to COM marking on the power supply motherboard. If in doubt, just double-check with voltmeter.

And that’s it. For a few bucks more and some extra soldering, we have a nice 500 mA USB port at the front of the power supply. Just in case we need it.

On GitHub you’ll find source files and releases with gerbers and part list.

Changing A-BFastiron SS-305MP Binding Posts

I needed a cheap power supply for a project and it was easy to find a nice one in A-BFastiron SS-305MP. It was small enough, looked good, and had shiny display. What could man want more?

Well, how about proper binding posts?

And no, I am not only talking about quality albeit one coming with it are quite flimsy and it already arrived with one cracked. I am talking about spacing. I simply hate when binding posts don’t observe standard ¾" distance between them.

And this power supply almost had it right. I measured spacing to be a smidgen over 20 mm while standard would call for 19.05 mm. With such a small difference, there was literally no reason to go non-standard. But non-standard they went.

If you open the power supply, you’ll see that binding posts are held by the PCB in the back. Thick wires are soldered onto it and nuts are used to connect to posts themselves. So the whole operation can be done with a simple PCB update with correct spacing. Only thing needed extra is a bit of filing action and you can reassemble it all.

However, since my binding post was already cracked, I decided to swap them for Pomona 3760 (black and red) set. But that brought another issue - panel cutout for them is completely different. And yes, a patient man might shape it enough, but for those with 3D printer there’s an easier solution.

To mount it all, I used some nice red MH Build PLA to print really tight mounting base and spacer for posts.

After filing plastic a bit to expand holes toward each other, I placed binding posts into the printed base, pushed it through the hole, used another 3D printed spacer on inside and added some more height to set using spacers that came with binding posts themselves. Then in goes the custom PCB and finally all can be fastened using lock washer and nut that cane with posts.

Result are nice binding posts at proper spacing. :)

On GitHub you’ll find source files and releases with gerbers and part list. 3D model can be found on TinkerCAD.

PS: The only downside of Pomona is that it uses ¼" imperial nuts while the power supply originally had 7 mm nuts. So, in addition to metric socket set you already have, you’ll need a witchcraft-sized set too.

Prepping Image for ESXi

I like using vboxmanage for disk conversions. When dealing with major formats it often can do everything I need. For example, if I wanted to convert raw disk image to .vmdk, it’s easy:

vboxmanage convertdd in.raw --format VMDK out.vmdk

However, sometime this simple tool is too simple. For example, using that image with ESXi, any modern version will just give you “Not a supported disk format (sparse VMDK version too old)”.

But it’s not like vboxmanage is the only game in town. For example one can use qemu-img.

qemu-img convert -f raw -O vmdk in.raw out.vmdk

Different tool, same error.

For ESXi to work, we need to tweak options a bit.

qemu-img convert -f raw -O vmdk \\ -o adapter\_type=lsilogic,subformat=streamOptimized,compat6 \\ in.raw out.vmdk

And this one does the trick.