WBUS-DIP28 Design comment and warning

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WBUS-DIP28 Design comment and warning

Post by pythoncoder » Mon Apr 01, 2019 4:34 pm

Users familiar with the Pyboard should note that the name of the Vin pin is misleading. The grouping Vin, 3V3, Gnd is so familiar that you might be tempted to apply +5V to Vin. This is likely to cause a release of magic smoke as the pin is defined as an output. I appreciate it is documented but the choice of name seems bizarre.

In my opinion Vin should be renamed Vout.

In a subsequent revision of the PCB it would be useful to have pads for a 2-pin header offering a true Vin and Gnd to provide a ready means of powering the board from a bench supply. As far as I can see the only convenient solutions are to butcher either a USB lead or a JST battery lead.
Peter Hinch

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Re: WBUS-DIP28 Design comment and warning

Post by P@T » Mon Apr 01, 2019 5:51 pm

Hi,

I don't understand, from this document
https://micropython.org/resources/PYBD_ ... matics.pdf (page 4)

VIN is a input of U5 and U10 (LD39130 regulator => input voltage 1.4 to 5.5V)
bye

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Re: WBUS-DIP28 Design comment and warning

Post by chuckbook » Mon Apr 01, 2019 7:16 pm

We use VIN as power input to the PYBD in basically all of our designs. It will tolerate up to 5.5V, There is one caveat: If the board has a wireless module installed, the voltage shouldn't exceed 4.8 V. It is strongly recommended to use a reverse current protection diode anyhow. Even a Schottky diode would be sufficient.
It should also be mentioned that most of PYBD's voltage regulators are LDOs, there will be quite some power wasted if VIN is operated at 4.8V (or 5V).

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Re: WBUS-DIP28 Design comment and warning

Post by chuckbook » Mon Apr 01, 2019 7:41 pm

Another note about the DIP28. If you are one of the lucky guys (like me) that already got a WBUS-DIP28, please note that D1 isn't installed but bridged. For these boards I would recommend to power the board only via the USB connection on the WBUS-DIP28.

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Re: WBUS-DIP28 Design comment and warning

Post by pythoncoder » Tue Apr 02, 2019 10:32 am

I think you guys are pointing up exactly the confusion I was trying to identify. On the Pyboard D itself Vin is an input, but on the WBUS-DIP28 it's an output. The diode is indeed replaced by a link, and Vin is connected to the output of U3 see schematic.

To quote from the docs
The VIN pin is output only on the WBUS-DIP28 and will be supplied by either the USB FS or battery.
I think it is confusing to use the same name for an input on one board and an output on another.
Peter Hinch

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Re: WBUS-DIP28 Design comment and warning

Post by Jurjen » Sat Apr 25, 2020 10:31 pm

Hello,

Some of my future WBUS-DIP28 applications are intended to run on a breadboard (that is the reason why I bought this adapter board) that has its own power supply based on non-rechargeable batteries (3.6V lithium or 4.5V = 3*AA Alkalines), nothing else.

I interpret the outcome of the discussion in this thread as "there is no way to power the PYBD + WBUS-DIP28 combo other than through USB or rechargeable battery" (but correct me if I am wrong).

However, after studying the datasheet of the U3 chip on the WBUS-DIP28 adapter (BQ24074: Standalone 1-Cell 1.5-A Linear Battery Charger) and the schematics of the adapter and the PybD, I have the impression that one can connect and external battery (with a voltage roughly in the range of Vin on the PybD, i.e. 3.4V - 4.8V) to the rechargeable battery input connector port (and not connecting anything to the USB-FS port of the WBUS-DIP28, of course). According to the BQ24074 datasheet, with no charging present, the Vin pin voltage (indeed an output function only on the WBUS28) would then represent the external battery voltage with a very slight dropout voltage (in the range of 50 - 100 mv at output (load) current of 1A and Vbat > 3 V), and this Vin then is directly fed to the PybD as its main power supply. The regulator on the PybD for the fixed 3.3v output (LD39130S) has a 300mV dropout voltage itself (at 300mA load) and is connected to Vin directly. Consequently, one should see a regulated 3.3V output at the 3V3 pin for power supplies at 3.6V (roughly) and higher.

So I think it is possible to use the WBUS-DIP28 + PybD combo with a local power supply quite well - by routing it through the rechargeable battery connector ... . Please correct me if I am wrong, and I will soon do a hardware test myself.

/Jurjen
Last edited by Jurjen on Sun Apr 26, 2020 8:58 am, edited 1 time in total.

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Re: WBUS-DIP28 Design comment and warning

Post by pythoncoder » Sun Apr 26, 2020 6:27 am

In my original post I said
As far as I can see the only convenient solutions are to butcher either a USB lead or a JST battery lead.
What I had in mind was to use a JST lead to connect a power source similar to a charged LiPo cell, which seems to be what you intend. As you say, this will work so long as you don't connect a USB power source at the same time, when it would put current into the power source with probable unpleasant consequences.

There was some discussion of a DIP32 adaptor. The DIP28 is really a rather specialised adaptor which people (self included) are using just to mount Pyboards on breadboards or PCB's. At one time @jimmo had a completed design for the DIP32 but sadly it never went into production.
Peter Hinch

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