Universal bus precharge & fan speed controller

[futurist]

Hi everyone

welcome to my latest project thread!

Universal bus precharge & fan speed controller is a board designed to be used with UD2.x or UD+ DRSSTC drivers

When building my first DRSSTC I needed 12V power source for fans and 24V for universal driver. Also I needed some way to precharge and discharge bus capacitors. Solution was to use two separate power supplies for 12V and 24V, and use 555 in monostable mode for precharge relay delay. After testing everything I noticed that fans were too loud so I decided to swap out 555 board for arduino nano and use it as fan speed controller and precharge timer. It all turned out messy and difficult to debug so for my small DRSSTC build I decided to design a board inspired by loneoceans UD2 precharge board (http://loneoceans.com/labs/sales/ud2precharge/) which would eliminate messy wiring and additional 12V PSU

Features are
- same footprint as UD2.x and UD+ which allows it to be mounted on top of or bellow the universal driver using M3 stand-offs
- 24V DC input shared with UD with built in 24V to 12V DC-DC converter
- two PWM fan speed controllers adjustable with pot with selectable 24V/12V voltage and fan type (4-wire fan or normal fan)
- precharge relay with adjustable delay and selectable 24V/12V voltage
- discharge relay with selectable 24V/12V voltage
- optional external 10k thermistor input with optocoupler which can disable UD when it detects overtemperature (selectable with pot)

Board is based on 32-bit STM32F030F4P6 MCU and is designed to be easy to assemble, program and use. Firmware is based on STM32 HAL libraries and only thing that isn't implemented are fan tachometer inputs which probably needs to be done without using HAL libraries. HAL takes a lot of memory and firmware already uses 15 out of 16 KiB of flash

Formula for precharge relay delay
 Needed delay/s = 3*chargeResistor(resistance)*busCapacitance(F)
 
 example: 150 ohm resistor charges 500 uF bus capacitor in ~0.25 seconds
 
Formula for calculating precharge relay delay (1 to 10 seconds)
 Delay/s = CHRG_T(voltage)*2.73 + 1
 
Formula for calculating overtemperature cut-off (30 to 100°C, 0V to disable)
 Temperature/°C = OTEMP_SET(voltage)*22.73 + 25
 
Setup diagram


Schematics


Almost complete board


Kicad project with gerbers
ubpfsc.zip

Firmware (needs HAL libraries)
ubpfsc_prog.zip

That's it for now, I'll update this thread in the following weeks when I assemble my small DRSSTC and fully test the board

Happy coiling

[Mads Barnkob]

Thank you very much for making this both open hardware and software, great spirit shown right there!

Would it be an idea to add another relay output for fans? A output that is controlled by the optional thermistor. I would need something like that in my large DRSSTC as the fan is a 3 phase fan being run on a single phase with a run capacitor, but the idea would be to first switch it on once the heat sink is above f.ex. 40 degree Celsius

What is the function of the beeper? Can you even hear it if you are throwing sparks?

[futurist]

Yes, there is an option to turn on a fan when thermistor exceeds certain temperature, it could be easily programmed
In your case since the fan is controlled by MOSFET external relay could be connected to fan+ and fan- terminals

Beeper indicates when the bus precharge is finished and if the temperature exceeds set over-temperature. Since it can be wired to disable UD it should be heard easily

I consider using STM32F103C8T6 (64 KiB of flash) for next revision of the board which would allow for greater flexibility using STM32 HAL libraries which take a lot of memory
Only thing I'm still worried about is EMI which could interfere with MCU and cause restarts. I had those problems with arduino on my bigger DRSSTC, but apparently using MCU's internal RC oscillator reduces this problem

[acobaugh]

This is exactly what I've been looking for, and means I can do away with the DIN rail mounted timers I was planning on using. Do you have a mouser or digikey BOM?

Anxious to see how noise-immune this is in your coil.

Any interest in making this closed-loop by reading the DC bus voltage? One of the things this could provide is visual feedback for when the system is safe to handle after a run.

[Mads Barnkob]

Any interest in making this closed-loop by reading the DC bus voltage? One of the things this could provide is visual feedback for when the system is safe to handle after a run.

If this is added, please make it so that it is at least two different types of visulisation. LED bar graph and a neon bulb f.ex. so make it more fail safe.

[futurist]

Do you have a mouser or digikey BOM?

I can make csv bom. Parts that I used are from lcsc.com but it wasn't a single order

Reading bus voltage feature can be added in next revision of the board. Do you have some ideas on how to implement it?
In any case neon bulb across main bus capacitors is must-have

Regarding the noise-immunity, I'm waiting for some time on laser cutter and I hope the coil will be finished soon
Inverter, electronics and secondary are all finished

TO-247 halfbridge inverter


Secondary


Primary


Primary supports

[Hydron]

Any interest in making this closed-loop by reading the DC bus voltage? One of the things this could provide is visual feedback for when the system is safe to handle after a run.

If this is added, please make it so that it is at least two different types of visulisation. LED bar graph and a neon bulb f.ex. so make it more fail safe.

A high brightness LED plus a suitably sized dropper resistor (probably a few watts rating) gives a good indication of remaining charge right down to a few volts, and will also slowly discharge the caps as an added safety bonus. If you don't have another alternative indication, you could put a couple on there (or one LED plus one neon - the neon isn't as safe as a primary indicator as it dies below 90V or something, but that's a lot better than 300V+ and is just acting as a backup anyway).

[acobaugh]

Of course I'm planning on having a neon or LED or panel meter as a secondary method of showing bus voltage.

I would take a look at the UD3.0 board, or possibly Fabricio's closed-loop auto precharge circuit, where he does DC bus sense with a simple opto-isolator and voltage divider: https://www.facebook.com/pg/franzolielectronics/photos/?tab=album&album_id=2694282280602139&ref=page_internal

[futurist]

Sorry for delay, I'm busy with work and I'm still doing house and electronics lab adaptation
I assembled the coil and so far it works very nice and I like bus precharge/fan speed controller board

Sneak peek

40N60 double halfbridge, 300 Apk @ 340V

I'll tune the coil in the next few days and post a thread with build materials and photos. And BOM for universal bus precharge & fan speed controller board

[futurist]

I thought I'll have more time when I finish uni and start working, but it has proved not to be the case
A year has passed and I still didn't post how it went with precharge controller and my small DRSSTC

In short, everything is working nicely, I posted a few videos of coils playing on Max's Syntherrupter thread

I devoted some time in repairing my medium DRSSTC, changing failed 24V chinese PSU, and in general making the wiring more organized. As part of that I'm planning to add my "new" precharge controller to that coil also

I thought about revising precharge controller's PCB a bit, moving a few parts and changing some parts which are no longer easily obtainable, and maybe changing small STM32 mcu for bigger one - STM32F103C8T6, one used in STM32 blue pill boards
However, when I looked to buy some STM32F103C8T6 chips, I was surprised to find that they go for 19 $ each! https://lcsc.com/product-detail/ST-Microelectronics_STMicroelectronics-STM32F103C8T6_C8734.html
Mouser and TME are out of stock. Before pandemic blue pill board with MCU could be bought for ~3$ delivered!

Now I don't know what to do
Any suggestions for this, and for possible new features are welcome