How best to ground through a rectifier?

[toozie21]

I am using the Olimex PIC-MAXI-WEB board for a project and I got myself stumped. The input is "9-25V DC or 7-18V AC range, the bridge rectifier on the input allows both AC and DC adapters usage and makes power supply polarity problems go away."

The schematic can be seen here: https://www.olimex.com/Products/PIC/Development/PIC-MAXI-WEB/resources/PIC-MAXI-WEB_sch.pdf

But that rectifier is causing me some sleepless nights. I am only using DC (12V in my case), but I need to sample the ADC from a system running off of a different power supply (meaning I will want to tie my grounds together to keep everyone on the same potential). In an ideal world, I would tie the two power supply grounds together at the power supplies and then everything is happy, but it seems to be that the 12V's ground is not going to be the ground the uC sees on the board since it has to go through the rectifier. Short of modifying the board so that I tie the board's ground to the power supply ground (and not have it go through the rectifier), is there any way around this?

 

[audioguru]

Since you only use a DC input then simply do not use the rectifiers and tie together all the grounds.

 

[toozie21]

Since you only use a DC input then simply do not use the rectifiers and tie together all the grounds.

You mean by pulling the rectifier off of Olimex's board?

I am right to think that the ground going into the board (from the power supply) will not be the same as the ground on the board itself, right? It seems like an odd way to make the board when you show off the ADC connectors, yet the ground for powering the board won't equal the ground the uC will see....

 

[hexreader]

Connect the minus side of your 12V supply to ground, then connect the plus side of the 12V supply to either PWR 1 or PWR 2.

One diode within the bridge rectifier will conduct and pass the voltage through (minus about 0.6V) while the other 3 diodes will do nothing.

Take care though - you will have lost your reverse polarity protection. Might be a good idea to add a fuse inline with the +12V feed to get some degree (not 100%) of reverse-polarity protection back.

Better still - supply the Development board with it's own isolated Wall-wart as normal to PWR 1 and PWR 2 and don't use the wall-wart for any other purpose, since as you already pointed out - neither wall-wart connection will be ground.

I reckon either option should work.

 

[toozie21]

Connect the minus side of your 12V supply to ground, then connect the plus side of the 12V supply to either PWR 1 or PWR 2.

One diode within the bridge rectifier will conduct and pass the voltage through (minus about 0.6V) while the other 3 diodes will do nothing.

Take care though - you will have lost your reverse polarity protection. Might be a good idea to add a fuse inline with the +12V feed to get some degree (not 100%) of reverse-polarity protection back.

I went with this approach as it required the least amount of hardware changes and things seemed to work great. Thanks for putting my mind at ease!!!