[Ovmsdev] OVMS v3 First Board Layout

Michael Stegen michael at stegen.com
Fri Mar 31 17:03:14 HKT 2017

My recommendation for a step down converter:
The AOZ1280CI is cheap, has a wide voltage input range (3-26V), and can 
deliver up to 1.2A at 0.8-23V
Another good thing: it switches at 1.5Mhz, so the inductor can be kept 


Op 31-3-2017 om 7:30 schreef Mark Webb-Johnson:
> It seems that a dual voltage design would not be ideal.
>  1. We’d have to go 12V -> 5V, and then 5V -> 3.3V.
>  2. But then USB 5V is not necessarily 5V, so we would also have to
>     deal with boosting that if necessary.
>  3. We could use a dual-output buck converter 12V -> 5V and 3.3V. But,
>     then we still have to deal with the USB issue for true 5V.
> Reading what all you guys are saying (thanks to all that gave 
> feedback), it does seem that the concerns are unwarranted.
> Perhaps we can build the dev boards just pure 3.3V, and see how it 
> goes. We’d use a high range buck converter to bring everything down to 
> 3.3V. If we can find one that would also work in the USB 5V range, 
> then we could just use that. If not, we use a cheap and simple 
> 5V->3.3V device. I’ll ask the China guys what they can recommend based 
> on what they have available and popular in their market.
> Regards, Mark.
>> On 30 Mar 2017, at 10:33 PM, Collin Kidder <collink at kkmfg.com 
>> <mailto:collink at kkmfg.com>> wrote:
>> I'm unsure of why the 3.3v transceiver would be a problem. CAN wiring
>> is supposed to be differential and it isn't supposed to matter if it
>> is 20V and 18V relative to local ground or 4V and 2V. There is isn't a
>> requirement to keep a shared ground between two CAN devices so how
>> would a remote node even know what the "voltage" of CAN-L is? Voltage
>> is only defined between two points so if you don't share a ground
>> between two nodes then the only voltage the transceiver knows is CAN-L
>> to CAN-H which will range by the proper amount whether you use a 5V or
>> 3.3V transceiver. I've used the SN65HVD series transceivers in all
>> sorts of things and never had a single problem. I've also used the
>> ISO1050 isolated transceiver and also had no problems with different
>> devices. But, if the 5V transceiver is cheaper and you can get 5V
>> easily enough then maybe that's still the way to go. I'm just saying
>> in my experience the concern isn't terribly warranted. YMMV.
>> On Wed, Mar 29, 2017 at 9:07 PM, Mark Webb-Johnson
>> <mark at webb-johnson.net <mailto:mark at webb-johnson.net>> wrote:
>>> I’m trying to finalise the OVMS v3 final board layout, with the 
>>> factory in
>>> China. We have some questions and seek your opinions:
>>> CAN transceivers / power
>>> Overall, the OVMS v3 system runs at 3.3V. We have two power supply 
>>> sources:
>>> USB (where we use a 5V -> 3.3V regulator), and +12V vehicle power 
>>> (where we
>>> use a +12V -> 3.3V switching power supply, to be as energy efficient as
>>> possible). Diodes are used for reverse-polarity protection as well 
>>> as coping
>>> with the situation where both usb and vehicle power is applied
>>> simultaneously.
>>> Our problem is with the CAN transceivers. I’m used to the MCP2551 (been
>>> using it for a decade or more), but that is 5V so greatly 
>>> complicates the
>>> power supply arrangements at the +12V side. We can switch to 
>>> something like
>>> the SN65HVD233 transceiver that works at 3.3V.
>>> But, I am concerned about comments I am reading about 3.3V CAN 
>>> transceivers
>>> and their inability to meet the ISO11898 dominant condition 
>>> requirement of
>>> 3.5V. From my understanding, these 3.3V CAN transceivers get around 
>>> this by
>>> driving CAN-L to 1V, to still get the differential of about 2V 
>>> (recessive
>>> condition?). My concern is compatibility.
>>> What do people think about this? Any recommendations?
>>> External Connectors
>>> The idea is to retain the existing DB9 connector, with the same 
>>> basic pin
>>> arrangement:
>>> DB9-M   Signal
>>> 3       Chassis/Power GND
>>> 2       CAN-L (primary)
>>> 7       CAN-H (primary)
>>> 4       CAN-L (alternate CAN)
>>> 5       CAN-H (alternate CAN)
>>> 9       +12V Vehicle Power
>>> That leaves pins #1, #6, and #8 free for expansion uses. It gives us
>>> compatibility with existing OVMS cables.
>>> We would then add a second connector. The suggestions here are DB15 
>>> normal
>>> density, DB25 normal density, or DA-26 high density. My preference 
>>> is the
>>> DA-26 (as DB25 is the old parallel printer style connector and very 
>>> bulky).
>>> As well as power lines, expansion cards could wire to this connector to
>>> expose external inputs/outputs.
>>> What do people think about the DA-26 connector? I’m suggesting a female
>>> version (as power is carried there, and I don’t want the pins to get 
>>> pushed
>>> together for a short).
>>> Note that we’ve also got a micro-usb socket, as well as space for 
>>> GPS and
>>> GSM/GNS antennas.
>>> Other than that, we are good to go. Things have stabilised now with
>>> Espressif, so we can proceed with building developer boards.
>>> Regards, Mark.
>>> _______________________________________________
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>>> OvmsDev at lists.teslaclub.hk <mailto:OvmsDev at lists.teslaclub.hk>
>>> http://lists.teslaclub.hk/mailman/listinfo/ovmsdev
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