I had originally said October (based on the DigiKey backorder estimate) but the caps came early. Haven’t had too many Batch2 orders yet so I thought I’d get a head start on building them…
Got all of them soldered together, and 10 sets potted…
I’ll be traveling for the rest of the month, so I am pushing to finish these tomorrow so orders placed before 9/15 will get in the mail on Monday. If you haven’t ordered yet, yours will ship out before October 15th.
Started packing. Each order gets a whole bunch of stuff that needs to be put together…
By 3PM I had the pre-orders packed and off to the post office. Not sure if they will get processed today or tomorrow, but they are in the mail! Finished the Batch1 orders when I got home and will take them in to the regional hub tomorrow.
Then I took a nap. And then wrote this post. Mic drop.
PS. There are only two lonely SKBOWE kits left that are in need of a good home… Can anyone take them in?
Epoxy Encapsulation (potting) serves to waterproof the SKBOWEs, insulate them from vibration, prevent corrosion, and conduct heat away from the components to the enclosure. It’s the messiest and trickiest part of the build process, as there is no turning back once you’ve mixed a batch of compound – and it’s $260/gallon.
Long day of work (8AM – 7PM) but Batch 1 is nearly complete! Enjoy the pics…
Last night, I was able to get all the resistors and the flyback diodes (R1 and D3) installed and leads trimmed. I also got the wires soldered on, and ended with this lovely ball.
This AM I got the primary wires trimmed, and capacitors stuffed, soldered, and trimmed… This is a lot of capacitors – 2,132,000uF aka 2.132 Farads to be precise.
Next came the diodes. As I had suspected laying out the new PCB, and confirmed when building #002, the heat sink (anode) of D2 does overlap the leads of D1. This shouldn’t be an issue if it’s assembled properly, but once the potting goes it it’s impossible to verify or adjust so I added some UHMWPE tape over the leads of D1 just to be safe.
By 3PM everything was soldered, trimmed, and ready to go. I have a lot of respect for the poor souls at Foxcon who do this all day, every day, with much smaller components. Wow.
Next I cut the wire pen into the enclosures. Took a little extra time to jig it up but it was well worth it.
Finally, and this was the most physically exhausting part, assembling the PCB assembly, heatsink, and insulator into the enclosure. But 2 hours later, sweet victory:
Also jigged up a workstation for potting so spills are easier to contain. Just a piece of plywood with the right shape cut out.
As much as I want to jump in to the potting, I’m mentally wiped and will almost certainly make a mistake. Better to wait till tomorrow.
The 11 hours I was working today I left SKBOWE #002 running on the testbench, powering Morimoto XB55 at worst case DRL duty cycle (50%). The temp never broke 90°F (ambient +20°) anywhere on the enclosure. That’s really cool (excuse the pun), especially compared to the only other error eliminator that seems to (barely) work on 82Hz PWM, which had a +80°F rise after an hour at 80% duty. For comparison, the XB55 was hanging around 140°F
After much ado, the production PCBs are here! Worked like crazy today to get everything ready for cranking up production this weekend.
And unit #002 put together for testing
Also made some heatsinks – enough for 50 pairs. Didn’t take as long as I thought it would, once I got the system figured out.
Also decided to go for cutting the wire, since I was on a kick. Each zip tie holds the wires for 5 pairs. The in/out wires will be soldered to the boards as loops, then cut to length and terminated after encapsulation to protect the ends. Here’s 50 pairs-worth of wire cut out:
Tried out the potting system, worked pretty well, but there will be a learning curve. Lots of wasted goop on the first try. Vacuum system worked well though.
So here we go, the first official pair off the line… Drumroll please:
Don’t worry, the real ones will be prettier! I mixed up the wire lengths on the first one, and then crimped the connectors backwards (out-in and vise versa) so had to cut off the connectors and ended up with short stubs. These will most likely be destructively tested.
Emboldened by this success I decided to just jump in and bang out the remaining 25 or so units left to build for the preorders and batch 1.
An easy way to speed up production is to split a complex process into small steps and do all of the same step at the same time, probably as a combination of deveohpling muscle memory as well as reducing the number of different tools required. Downside is, should you make a mistake, it is likely to be repeated many times. After going through all steps serially with unit #002, and stuffing #003, I decided to do the next 5 in parallel.
Only 4 pairs fit in my vice at a time, but by the time these were finished I found it faster to skip the vice all together except for soldering the wires, where they have to go in one at a time anyway.
Crimped connectors on #003A to check the process, and it tested good, so off to do the rest at once.
Hopefully be done with assembly today and can focus on potting this week. I just hope I can pot each one in less than the 2 hours it took me to do the first one (above).
With the first prototype complete, it was time to build a test bench to evaluate the real-world performance of the SKBOWE. I wanted this simulator to allow instrumentation, but also to be as accurate as possible without physically installing it in the car.
The CEM basically follows the datasheet verbatim, down to the 1k R_IS, so I did the same. Rather than try to mess with the tiny BCP54C SMT transistor I used an old school 2n2222A, which doesn’t affect the performance of the circuit at all, but other than that the circuit is identical to that on the CEM, down to the 7.5A fuse and the 1000uF capacitor.
The MCU that emulates the CEM’s PWM is a Rugged Circuits Rugged MEGA which is pretty much bullet proof – a very good thing considering what’s coming for it! For those who are curious, there’s an 8-position rotary switch connected to pins 30-37 (for the duty cycle).
Interestingly, there is no way to make the Arduino hardware PWM run at 82 Hz without messing up all the timing libraries, so I had to get creative with a 122uS ISR. Pardon the ugly code…. This was like 7 minutes:
Anyway… The lower deck has a 12V 30A power supply (set to 14.2V) and a 55W dummy load (aka an old H7 bulb stuck in the end of a mason jar):
On the upper deck I added a ground bolt (simulating the chassis) and 5 feet of 16AWG wire (simulating the wiring harness), the selector knob, and a terminal block to make it easy to change out connectors.
I will post detailed data later, but Prototype #1A is performing flawlessly, exactly as designed! This was obviously expected but always nice to see theory turned into a hefty feeling block of capacitors 🙂
Here’s a video showing why you want to have a SKBOWE in a P1 car:
In the video, the yellow trace is the headlight voltage (PWM), and the red trace is the current flowing out of the CEM. You can see this ballast is definitely not happy about even 95% duty cycle PWM when hooked up directly. In the car, this may or may not trigger a short-circuit fault code before it takes out the WMM!
The test bench doesn’t emulate the open circuit (bulb fail) or over current (current fault) detection in the CEM – these levels are handled by an ADC in software, so I’d have to test on my car to see where they are at. Maybe another day…
Update 6/29/17: Test Bench Upgrade
Today I upgraded the SKBOWE test bench with a second can for an H11 HID bulb and mounted my flux meter.
The Dr. Meter LX1330B is designed for photography applications, so this is operating at the top end of it’s range (200000 lux). The absolute measurement won’t be meaningful (in terms of lumens, for instance) but it will do nicely as a relative measurement to ensure that the ballast is working at full capacity.
This is with a new bulb, I’ll have to burn it in for a few hours before using it to compare HID ballasts in the upcoming Ballast Review Page.
Appendix A: Data
With the SBKOWE installed and driving a 55W ballast (HID50), I measured current draw for each duty cycle by monitoring the feedback pin (#4) on the BTS443P. The current to ground through that pin is proportional to the current flowing through the switch, with around an 8200:1 ratio. Since I used IS = 1kΩ, the current is about 121.95 times the voltage (mV) on that pin. The Input and Output voltages are measuring the Mean RMS, the “battery voltage” is a constant 14.2V. Pk-Pk ripple was measured with AC coupling on the highest sensitivity.
PWM Duty Cycle (%)
RMS Input (V)
RMS Output (V)
Peak Current (A)
Min Current (A)
Ripple P-P (mV)
As you can see, at the DRL-level duty cycles (50% and 60%), the peak currents are extremely high (8-9A) and while the minimum current is below 7.5A (the fuse), it might be above what the CEM is willing to provide before soft-shutdown. Also, the SKBOWE is working 2x-3x harder (both caps and diodes), which means more heat and shorter lifespan.
That said, running with DRLs will not instantly kill the SKBOWE, and depending on your ballasts it may actually work fine. Just don’t complain if they don’t!