After watching the discussion of misdrawn diagrams for the new Rev Electronics set, I decided to look through all of the diagrams and specs of our electronics. The catch I found tonight was that the Rev RPM is outputting 18V passive POE. However, the radio is spec’ed for 24V passive POE or 48V POE 802.3af. AndyMark’s notes on the radio (see link) claim that it can take 12-24V passive POE but that is not denoted on the radio itself or in any other documentation I can find. I’m hoping someone more knowledgeable than myself on POE can provide some insight into this. In past years plenty of teams have run 12V passive POE so I suspect this in itself is a non-issue.
However, in past years it has been commonplace for some teams to run power over both the barrel jack and POE for redundancy. Previously these were both 12V signals. However, I think there is some concern of supplying a 12V and an 18V power source to the radio. Depending on the circuitry of the radio, this has the capacity to cause issues. I’m quite curious to see what other people think of this potential issue and also hoping to raise awareness for teams that have run redundant power in the past.
A nominal rating of 12-24v input is not the same thing as an absolute minimum given some level of capacitance, and teams used a stabilized radio power source via the VRM in years prior to this one. There’s likely some internal buffer to deal with slightly under-spec power supplies anyways.
As to the redundant power with the RPM, it depends on whether the radio actively switches or is just… connected between/to the barrel jack and the PoE via the appropriate 8P8C. I don’t have internal photos, or schematics, or something that’d help infer better unfortunately. I’ll defer to someone who’s done a teardown.
First, the current rules do not allow you to power the radio with an additional power source if you are using POE from the Radio Power Module (RPM). The rule states “either” and “or” for using the VRM or the RPM. (This may be worth Q&Aing if someone wants to be able to do it)
Second, I have tested using the RPM with a 12v VRM source powering the radio from the barrel jack and it worked without issue. The redundancy worked as expected and when the 18v POE is connected there was only a small voltage output (1v to 2v) on the barrel jack connector when not connected to the VRM (my guess is a capacitor or something but definitely didn’t seem to be an issue).
This has led me to believe it’s perfectly acceptable to run a redundant supply when at home when practicing but it is illegal to do so during the competition and you should not pass inspection if your radio is connected to both a VRM and RPM at the same time.
If the current radio is like previous ones where the barrel jack power input is connected to the power pin of the RJ-45 connector by a PCB track, then they should only be connected to the one power source. This can be tested using a DVM set to measure Continuity and probing the center pin of the barrel jack and the power pins on the RJ-45 connector.
Connecting two different power sources to the barrel jack and to the RJ-45 connectors can cause the two power sources to “fight each other”, depending on the internal design of the power supplies. This could be true even if one is using two separate power supplies that are of the same type i.e. two of the CTRE VRM’s or two of the Rev RPM’s.
Power supplies are generally not designed to be connected in parallel with other power supplies and some designs may “misbehave” if connected to another power supply. It is possible that the misbehaviour damages whatever load is connected i.e. the radio. One would have to test thoroughly, with different loads while monitoring the output with an oscilloscope to ensure that there are no undesirable effects of paralleling two power supplies.
There are power supplies that are designed with additional circuitry to allow their outputs to be connected in parallel. Typically, this feature will be prominently advertised since some applications require this feature.
