With the new boards in, we are finalizing what will be “in the box”. We’ve decided to leave relatively short wires hanging off the PDU and use Weatherpack terminations for everything but the main battery connections. Weatherpacks are reasonably priced and of reasonable quality. It will allow you the greatest flexibility in your installations without me having to leave enormous lengths of wires attached. I believe it will make it easier to retrofit the PDU into existing installations as well.
Also on the agenda this weekend is the building of a test rig that I can use to conveniently test the PDU. The one I am building can sink 80A though it will be easy to increase that to 100A. This one will also have the ability to generate destructive transients of >200V in order to confirm the system is safe in automotive environments. I am only about half way through the build of this but the “honey do’s” are piling up so it is time to call it a day. The last few parts should show up next week and I’ll finish the build next weekend.
The new power board has been built. I am waiting on the mil-spec wire to be delivered to finish it up though. That copper bar across it provides the necessary electron path to move 100A across the board. The resistance across the board is ludicrously low. In fact, it is beyond my ability to measure (and I have good equipment). That said, theoretically is should be 3.645×10-6Ω or 0.000003645Ω.
We ordered these boards about 17 days ago. That is a great turn around time. It did take a week of negotiating to settle the details of the order, but I suppose you have to take the good with the bad.
The main difference between this version and the one I’ve been using is that I’ve significantly modified the constant current driver circuitry, rerouted a couple sense lines, and attempted to fully protect all the inputs and outputs from large transient spikes that could grenade the delicate silicon on the board.
If I am correct, this is ready for the road or trail!
I’ve shown the signal inputs in a previous video but the cause and effect were statically programmed into the firmware of the chip. I’ve finally got the inputs so that you, the user, can fully program them.
The new power supply has arrived that I will use for testing. It is a 60A 13.8V voltage source. I’ve added a bunch of 100A diodes in series with it so that I can fake an ignition signal. The diodes should drop about 1.4V when the current goes through them so that it appears the device is on a battery and when I turn that red key I short the diodes out and it delivers the full 13.8V simulating the voltage delivered from the alternator.
This is helpful because the PDU can trigger actions based on this transition from 12V to 13.8V and back.
I’ve added PWM modulation to each output channel. You might use this for daytime running lights as vehicles are more visible when their headlight is oscillating in brightness during the day. I’m sure there are tons of other uses as well, that is just one idea.
The process to use this function is to go to the output setup screen for a given channel and turn on the modulation by selecting the “Slow” or “Fast” button and then adjusting the lower bound of the PWM modulation. The upper bound is the main PWM slider at the top of the setup screen.
With the new access to temperature sensor data from the PDU, I embarked on a study of the PDU’s temperature characteristics. This is important to understand because the temperature of the silicon affects how the circuits behave. It is especially critical for the ADCs (analog to digital converters) which are used to measure current flow. Essentially all the bits and bobs placed on the PCB (printed circuit board) will behave differently based on their temperature though. Characterizing the temperature movements therefore will help inform the maximum currents we can work with and how the final enclosure will have to be designed to handle the expected temperatures.
The PDU was designed to keep the heat creating parts away from the logic circuitry. You may recall that the design is two PCBs stacked together but separated by about 17mm (.65″). The power FETs (field effect transistors) are on the bottom board and all the logic is on the top board. The FETs are what directly control the current flow and are where the majority of the heat is expected to be “created”. The FETs are sensitive to temperature as well but I selected rather expensive FETs that can each handle up to 100A continuous current and 1400A peak current and I am limiting them to 15A each (some 85A below their maximum continuous figure). In this configuration they are dropping a maximum of ~225mW of power @ 15A. If all the FETs were dropping this amount of power (not possible because I’ve set the total current to 100A and 8*15A = 120A) it still represents only 1.8W. You should all be relatively well familiar with that it feels like to hold something burning ~2W in your hand as that is about the same as the amount of power used to transmit voice over with a cell phone. It gets a little warm but that’s about it. There is another source of heat on the power PCB, the resistance of the copper on the board to the currents flowing. The current goes in on one side and flows across the board to the get to the farthest FETs. This is not necessarily a small amount of heat and must be managed. I solve this problem by using thick copper on the PCB as well as busbar (a big chuck of copper) to further reduce the resistance across the board. The thickness of the PCB is equal to 9 AWG wire and the busbar is equal to another 4 AWG wire. Combined they are just over 26mm² of copper conductor moving current across the board which is roughly equal to 3 AWG wire. The power conversion there is (0.06465mΩ/meter * 2″) * 100A² and equals ~320mW. That adds to the 1.8W above and still places us comfortably with cell phones in the 2W realm. By the numbers, things are looking fine but nothing replaces a good lab test so lets see how that went.
I performed a “controlled” test so I could get a measure of the heat transferred from the power PCB to the MCU (micro controller unit) where the temp sensor resides and the heat generated on the logic board itself where the MCU is. The first test I performed was to run 30A (about the most I can do with my current lab power supply) through the board until we achieved homeostasis of temperature. It took about 30 minutes for the temp stop climbing but I let it sit for an hour to confirm it was done warming. It raised 10°C above room temp (the starting point). That figure is affected by the test conditions somewhat. A relatively significant issue is that the loads are within 10″ the PDU and they are radiating a lot of heat, ~375W. The power supply is also blowing all of its heat in the general direction of the PDU via it’s cooling fan. I used two sheets of copier paper folded over the PDU to protect it from directly radiating heat of the bulbs (I made shade :P) but that does little to ameliorate the heat moving around the PDU from the two cooling fans blowing heat around from the power supply and the giant .6Ω load and it’s cooling fan. Taking all that into account, the numbers are still quite encouraging. Of course, a test cannot be “controlled” without a control test so lets discuss that one. The control test was running the PDU with no load until we achieved homeostasis of temperature. I again let is sit running for an hour but turned all the outputs off. The power supply was still blowing it’s warm air but as it was doing less it would have been running cooler than the previous test, there was no radiant heat coming from the bulbs though as they were not on. In this test the it raised 5°C above room temp. To me, this means that the 30A from the previous test accounted for only 5°C of the 10°C shift. So, what is responsible for the other 5°C? Well, I suspect most of that is the switch mode power supply which is converting the 12V DC to 3.3V DC to run the electronics. All the logic electronics do consumer power too so they all contribute a little to that total as well. I logic board is burning about 330mW when on but not driving any loads and 375mW when driving the 30A so that by itself contributes about .66°C of the 5°C. The heat contribution of the power PCB while sourcing 30A therefore is responsible for 4.33°C. Assuming this is a linear function (a huge assumption that SHOULD NOT BE MADE) at 100A the power PCB will contribute 14.5°C to the system at 100A while the logic board will contribute about 6.5°C to the mix for a total of 21°C at 100A (with the poor assumption that the power board’s contribution is linear). These numbers are all relative to the board sitting on my bench in relatively still air and not encapsulated. The epoxy used for encapsulation can actually improve the thermal characteristics. The actual numbers at 100A will require further testing which is waiting on the new boards and a new power supply (a Jeep in this case).
I added a system temperature sensor to the gizmo last night (notice the 34°C below system voltage). It will tell us the current temperature of the PDU. This is useful (for testing purposes at least) as it will let us know if we are cooking the electronics by running too much power through the device. I do not expect any issues, but I really want to make sure the device is robust and these sorts of details help ensure we get many years of reliable service.