VRM experience and rule R43

[Mark McLeod]

Just to reinforce, don't put anything other than the OpenMesh radio on any of the 12v VRM outputs, since they use a shared rail.

[pblankenbaker]

Thanks for the replies. Based on what you have both said, I will recommend to our team that we be conservative and:

• Limit the total current draw of the four 5V connectors to 1.5A.
• That we only use the 12V side to power the radio (leave all three of the other 12V connectors disconnected).

Thanks again - appreciated your help.

[Mike Copioli]

Originally Posted by Mark McLeod

Just to reinforce, don't put anything other than the OpenMesh radio on any of the 12v VRM outputs, since they use a shared rail.

While the 12 volt outputs share the same regulator they do not share the same output protection. The rail is split into two separate PTC's, 500mA and 2amps. While a short to the 2 amp rail will cause the radio to drop out, a short to the 12 volt 500mA rail will NOT interrupt the radio's supply rail.

The 12 volt LED will turn off anytime the 12v 500 mA rail is shorted, however this is not an indication of the 12 volt 2 amp rail dropping out.

So yes it is safe to connect secondary loads to the 12 volt 500mA rail.

[Mark McLeod]

I mistakenly thought that I remembered having been able to put a 500ma continuous load on the 12v/500ma and a 1.2ma continuous load on the 12v/2a outputs and after a few seconds browning out the whole 12v side (on the older v1.2).

Sorry for the misinformation.

[Mike Copioli]

Originally Posted by Mark McLeod

I mistakenly thought that I remembered having been able to put a 500ma continuous load on the 12v/500ma and a 1.2ma continuous load on the 12v/2a outputs and after a few seconds browning out the whole 12v side (on the older v1.2).

Sorry for the misinformation.

On the older revs of hardware (<1.4) that was only true on the 2 amp rail and the limit was actually 1.5 amps. So if you are unsure which rev you have keep the total budget below 1.5 and you are good to go.

The latest rev (1.4) can be identified by removing the bottom cover and identifying the solder mask color. A black solder mask is the latest Revision.