Xhp50.3 Datasheet
Rosella Brain
UnbeLEAFable! This is a tiny leaf growing from a callus culture, produced from a tiny cutting harvested from the wild. This is exactly what I expected (and hoped) would happen with this project. In a few more months, I hope to have rooted plantlets that I can transfer to normal soil media for hardening! Once the plants are rooted in soil, I will write up a detailed procedure on the whole process.
Reading a thermistor seems like a pretty straightforward task, and there are a lot of guides on how to do it, either using just the beta value, or using the full set of coefficients for the Steinhart-Hart equation. I wanted and needed to do something a little different, because I wanted to measure temperatures somewhat accurately over a wide range (25-200C) with a single thermistor, for an upgraded version of the PCB hotplate.
Measuring this range is difficult because the resistance of the thermistor changes a lot over the range I care about. at 25C, this particular model is 100k ohms, and at 200C it is about 1k ohms. It is very not linear, changing very rapidly at first. Thermistors are often used in a voltage divider, where the output voltage is related to the temperature of the thermistor:
This causes a problems for my implementation because choosing other resistor in the divider causes the most sensitivity near the value where Rt=R2. That is, the temperature sensing will be easiest near where the thermistor and the other divider resistor have the same value.
This is a chart showing the expected output of a voltage divider with three different resistor values, computed from the temperature-resistance chart plotted above. The steeper the curve, the better, since a small change in temperature will create an easily measurable output. But none of these curves are linear across the whole range that I care about, from 25-200C. Instead, the 100k looks good from 0-50C the 10k looks good from 50-120 and the 1k looks good from 120-200. At the center of each of these ranges is where Rt=R2, and the output of the voltage divider is about half the input voltage.
My solution was to use several different resistor dividers instead of a single resistor divider. The final implementation uses an analog mux instead of discrete FETS, and included a 100k resistor divider as well.
Looking at the two remaining 10k and 1k curves, the question is where to swap from one to the other. The goal is to keep the amount of ADC counts per degree C as high as possible over the whole range. This can be found by inspecting the derivatives of the fitted curves and finding where they meet (in this case, just around 120C).
This chart also shows why exactly a single value for R2 in the resistor divider would be bad. For example, below 40C, the 1k resistor shows less than 1 tick per degree C, so a 1 degree change would be hard to measure there.
This was done with a single set of data, so it may not be accurate for all time. Taking more data in the future would be neat, especially because the python script for analyzing the data will just spit out numbers. It would be interesting to compare the calibration coefficients for various data sets to figure out how much they change, run to run. Hopefully it would be a small amount.
Instead of using a purpose-made plant growing container, or even a polypropylene takeout container, I am using some condiment containers I have left over from the original cell culture experiments I did a few years ago. These are half polypropylene, which is autoclavable, and half polystyrene, which is not. To use these, I autoclave the bottom half and soak the other half in bleach while the autoclave is running- when the parts come out, I pour media and fish a lid out of the bleach. it seems to work well enough against contamination.
These cultures are growing relatively slowly (months between re-plating), possibly due to the totally uncontrolled and unoptimized conditions they are in, aka an funky basement. Surprisingly, contamination has not been a huge issue (transfers have been done in a pcr hood).
The next step will be to take some larger cell masses and try to get them to grow a bud, shoot, root or leaf- to start turning back into a plant. My plan is to grow them on a deeper media in a different container, undera lighting, without 2ip on something like WPM with zeatin (although zeatin is expensive, so maybe just WPM!).
I think in the future, it would make sense to avoid using the actual boot and reset buttons and use something like the ESP-PROG to toggle all the lines. Needing to push buttons to upload code is a drag!
I also consistently had issues soldering the QFN package the ESP comes in- I think this was due to too much paste on the center thermal pad. This causes the chip to float, and sometimes this causes one side of the chip (or a few pins) to lift. Next time I will reduce the amount of paste on the center pad to fix this, but I resolved it here by removing the chip, solder-wicking the thermal pad, and then re-soldering the part. Using low temp solder paste made this really easy.
The LM3409 bringup went relatively well, but thermal issues started to crop up pretty much immediately. There are some surprising deficiencies in the layout and parts that I will be modifying for future revisions. The thermal camera really helped to visualize the issues quickly, and given the number of thermal/power projects I have worked on recently I am sure I will get to use it quite a bit.
One hair raising issue was that during bringup, the board started to smoke. This is almost always a bad sign. In my original estimation, this was caused by the buck PFET getting hot due to having a high gate charge. The odd thing is that the freewheel diode also got really hot, and stayed hot even if the led was very dim.
Gate charge was not the entire story. With the FET replaced with a lower gate charge/higher RDSon model, there was still an issue! With the LED aggressively cooled (to allow it to live), the device would still go into some kind of limp mode after a few minutes of operation at high currents (3.6A avg, 12V, about 42W).
This seems to be due to the driver itself overheating, which is due to my poor thermal design. The driver is marked U in the layout above- this is supposed to have a thermal pad connected with vias to ground. I omitted the vias, essentially insulating the chip- no good.
The reason the diode gets hot is because usually, the diode only free wheels for a short amount of time- when the FET is off. When the driver goes into limp mode, it has to dissipate all the energy stored in the inductor, many times a second.
So the full story is- this driver will self destruct if the driver overheats and goes into hiccup mode. Better thermal design (and eventually cooling it in the ocean) will probably improve the situation, by preventing driver overheating.
This is going to be very hard to keep cool, even with the die almost being in contact with the seawater. The good news is at lower settings (around 2A) I should still get a very respectable 3000 lm out of the LED- more than the sola light!
The light head has two rings of LEDS- one for spot lighting and one for flood lighting. the small inner ring of three LEDs is the spot light, and the outer LEDS are the flood light. Given the package size and general shape the LEDS, I assume the inner ones are CREE XP-E2 LEDS, and the outer ones look like CREE XM-L2 LEDs.
Having multiple LEDs is very smart, because LEDs run more efficiently (in terms of light per heat) at lower currents. It also spreads out the heat loading of the PFB- CREE says to estimate 75% of LED power to turn into heat instead of light- that means that a 4W LED needs to dissipate 3W of heat.
Given the stated lumen output of the light, I would guess the flood lighting runs around 9-10W, or 1A at about 9V. The ring of 6 flood LEDs probably run at about the same current (1A) but at about 18V, for an output of 18W. This means that about 7-14W needs to be dissipated. This seems to be done through good contact of the aluminum PCB with the metal ring that goes on the front of the light. If you look at the PCB photo above, you will see a smear of thermal grease along the edge.
Running multiple LEDS efficiently means spreading the heat around, especially for the radial LEDs, which are closer to the heat-sinking bezel. Its also important because the light is powered by a relatively small battery pack.
The optics look a lot like they are made by carclo (wild conjecture). There are two styles- reflectors for the spot lights, and a total-internal reflection style optic for the spot light. I do wonder if there is any attempt to collimate the spot beam to make it extra tight, by biasing the three spot beams inwards.
The LEDs are driven by an LT3755 wide-input rage LED driver. This driver is probably operating in boost mode all the time, since the battery voltage is too low to drive any of the LEDs. Interestingly, there is only one driver (just like in my design) but there appear to be two current sense resistors. I suspect these resistors are switched in for the spot and flood modes (on the high side, with a PFET), and then the overall brightness is controlled by PWM. This makes sense because these LEDS are probably operating at reasonable efficiency, so there is no advantage to turning down the average operating current.
As you can see, most of the corrosion/damage happened near the boost converter. It is close to where water can come in, and it is also where the highest voltages exist on the board. Seawater can cause a short between the current driver outputs, which would then tend to increase the output voltage until the current set point was reached, or until the driver maxes out or reaches some thermal limit- in other words, its a vulnerable circuit.
This board has something pretty unusual on it- a big floppy ribbon with conductors on one side. At first, I thought it might be some kind of temperature sensor, but I think it is a flood detection circuit- if it gets wet, it will alert the micro to shut down the led driver. This makes a lot of sense, both to protect the battery and the PCB. If the lamp is dried out after being protected from a flood, I imagine it would be just fine.