pumping 3.3V to 3V rail

Hardware inside and outside of the ECU
Post Reply
puff
contributor
contributor
Posts: 2961
Joined: Mon Nov 11, 2013 11:28 am
Location: Moskau

pumping 3.3V to 3V rail

Post by puff »

It's not a secret that our 3.3V rail is actually not.
For my canbus-related exercises, the lack of those ~0.3V-0.4V was critical. So I decided add a small dc-dc board to ge it up to the nominal value, and here's what I got:
voltages.jpg
voltages.jpg (181.31 KiB) Viewed 4987 times
as a result, my canbus still doesn't work with a 5v tranceiver.
Is that voltage drop because of high-current, thin wiring, or why is that so? How much current to we expect to consume on a 3v3 rail?
puff
contributor
contributor
Posts: 2961
Joined: Mon Nov 11, 2013 11:28 am
Location: Moskau

Re: pumping 3.3V to 3V rail

Post by puff »

whoops! it,s 0.42-0.44A
User avatar
kb1gtt
contributor
contributor
Posts: 3758
Joined: Tue Sep 10, 2013 1:42 am
Location: ME of USA

Re: pumping 3.3V to 3V rail

Post by kb1gtt »

The 3V can only handle 0.15A. Your comment about 0.42A is alarming.

On the Discovery board, D3 is used to prevent an excess voltage on 3V from damaging devices on the 5V. AKA the commonly connected USB is via PC for your 5V. ST wanted to add a layer of protection to your expensive PC. You can bypass the D3 and get 3.3V, but if you connect 12V to a pin, and you are connected to the USB, you will damage both your discovery and your PC.
Attachments
STM32F4_3.3V_mod.png
STM32F4_3.3V_mod.png (37.5 KiB) Viewed 4981 times
Welcome to the friendlier side of internet crazy :)
puff
contributor
contributor
Posts: 2961
Joined: Mon Nov 11, 2013 11:28 am
Location: Moskau

Re: pumping 3.3V to 3V rail

Post by puff »

I think that was because of the external 5v to 3v power board left without power, buy still connected to 3.3 rail (I forgot to disconnect it when I decided to use my laboratory PSU to provide clear 3.3v to the board - what a pity it is a single-channel unit).
so, there was frankenstein, 5-to-3 psu, blue pill stm32 board and tja1050 can tranceiver connected to 5V of my disco.

btw, there was another scary moment when I saw only 2.4V on 5V rail, even when disco was connected to the USB port of my laptop - I thought I killed it (either disco or my laptop). But after a while it started working again - at first, on a bare disco board, then - connected to frankenstein. seems to be working fine. I decided that I need to source another 3v3 can transceiver - that would be simpler than making these two work together..
User avatar
kb1gtt
contributor
contributor
Posts: 3758
Joined: Tue Sep 10, 2013 1:42 am
Location: ME of USA

Re: pumping 3.3V to 3V rail

Post by kb1gtt »

I should probably encourage you to try the SN65HVD230. That is what we have used on Frankenso. They can be obtained on ebay for small $.
https://www.ebay.com/itm/SN65HVD230-CAN-Bus-Transceiver-Communication-Module-For-Arduino-LN/162421533418?hash=item25d113eaea:g:X-gAAOSwax5Yvin8
Welcome to the friendlier side of internet crazy :)
puff
contributor
contributor
Posts: 2961
Joined: Mon Nov 11, 2013 11:28 am
Location: Moskau

Re: pumping 3.3V to 3V rail

Post by puff »

yep. sn65hvd234 would cost me somewhat less than $2 locally, it allows sleep mode. just need some time to travel to that store.
puff
contributor
contributor
Posts: 2961
Joined: Mon Nov 11, 2013 11:28 am
Location: Moskau

Re: pumping 3.3V to 3V rail

Post by puff »

too lazy. don't want to spend 3-4 hours for that adventure so far.
here's what I'm thinking about: either cut away 3V rail to the tranceiver on the frankenstein board replacing it with a wire from a separate 5v-to-3v power supply, or (which is even simpler) cut the wire and solder it directly to 5V pin.
but here again, I'm facing some difficulties reading the datasheet:
https://www.waveshare.com/w/upload/3/3a/SN65HVD230-CAN-Board-Datasheets.pdf
it states in absolute maximum ratings that Vcc is in the range from -0.3V to 6V, but on the same page in the table below it states in recommended operating conditions that the supply voltage should be within the range of 3V to 3.6V. So, probably, supplying 5V from frankenstein is not that good idea?
Post Reply