Controller Part 4

I had a busy day today on the project. I made up a single sided PCB to hold my controller capacitors.

3 x 10mF 63V electrolytic capacitors. Quite a bit of capacitance for such a small volume.

3 x 4.7uF 200V metallized polypropylene capacitors, with a low ESR and more importantly a high ripple current rating. One of the three is not installed, it's on my driver board for the moment (see a previous entry).

Making the traces for this board was a huge pain. I don't have PCB etchant, or PCB tinning solution. I tried cutting out traces with a box cutter, but they didn't peel off the PCB well, and heating the thin strip with an iron didn't release the strip as nicely as it does to small pads (when you don't want it to!)

So I resorted to using a small grinding wheel in the drill press. I then tried tinning the board with my iron. Even my trusty Weller isn't up to soldering pads this big.. so I used a blow torch. This board can't repel firepower of that magnitude!  (hehe Admiral Ackbar)

Flux for copper pipes worked well in allowing the solder to flow, I got all sorts of nasty fingerprints on this board that not even alcohol could remove. (the triple distilled kind, not the isopropyl kind lol)

And as you can see in the first picture I tinned the buss bars as well. Just the top where the components will be mounted. I used a *lot* of propane today.

Today I also used my "silver epoxy" to mount one diode and one mosfet to the buss bars. That epoxy is remarkably hard to work with, it does not flow - at all. Nasty looking joints but they should be solid. The capacitor PCB is also epoxied to the buss bars, though that looks respectable.

The kapton tape is under the Gate lead (there are 5 source leads) and I wired that to the mosfet driver from before. The 'scope trace is below.

10ns rise and fall times with this mosfet is pretty respectable, I was expecting a lot more. And this is with haphazardly strung wires that are waay too long.

Rehashing my dV/dt calculation from before, we have 12V rise in 10 billionths of a second.

12/10 billionths = 1.2 billion volts per second (1,200,000,000 V/s)

I recently realized that I lost a few zeroes last time, whoops!

Testing:

Safety note: When you have large capacitors in a circuit like this controller, you have to remember that it now poses a safety risk even when unplugged from the power source. Currently this is 12V, so it is totally safe. Even with wet fingers it's not going to do much to you. In the end it will be a 48V controller, which is apparently still safe to touch (according to a random person in telecom I read on the internet - take with a grain of salt). But if you are working on a 60V+ system, it is a recipe for death.

I precharged the controller with some wire wound resistors I bought for the 48V system.

3 x 125 ohm 13W resistors. So in parallel, that's 41.2 ohms, 39W.

Power dissipation at 48V is V^2 / R or 48*48/41.2 = 55.9W

Yes, they are undersized, but this is a very intermittent load. I could have gotten away with a smaller wattage rating but I've read a great way to test is running your motor with the precharge resistors *not* bypassed as you would in typical use. This limits the maximum current to a sane value. They may be too small to work in this application, we shall see.

So I attached the small test motor to the buss bars, and it worked first try! yay!

One thing I noticed immediately was that the motor squealed much more than with the junky mosfet. I am still not sure why. I've changed from Phase Correct PWM , 10 bit to the 8 bit equivalent. So I have 256 duty cycle options now, at 16kHz (rather than 1024 at 4kHz). I can still hear it, but it is extremely faint.

The best news so far: The copper buss bars haven't heated up at all (that my fingers are capable of resolving, anyways).

To do:

Interface current sensor

Make leads for the starter motor and battery

Speed control a starter motor!!!!!