Confused by what the emonVs actually is

I’m considering replacing my emonTx3 with the newer Tx4, but I am confused by the emonVs.

According to the description it’s a precision voltage reference and 5V PSU. This backed up by the Pictureon the emonVs page.

However, the picture on the emonTx4 page shows an additional black box between the emonVs and the emonTx.

What is this extra box? Why is it needed? If the emonVS provides 5V, why doesn’t it connect directly to the emonTx?

More importantly - why the need for an additonal lead anyway? Why not feed the 5V down the voltage sensor RJ11 and avoid the need to route another cable?


Hi Gareth,

That black box is an emonBase: emonBase 433Mhz (RFM69SPI) - Shop | OpenEnergyMonitor. It is not part of the power supply for the emonTx.

The emonBase in this photograph is communicating with the emonTx via an USB cable. It also draws its power from the emonVs since it is close by for this use case. But the emonBase can also be located elsewhere, perhaps next to a router, and communicate with the emonTx4 via UHF radio, just as it did with the emonTx3; perhaps this is the setup you are familiar with?

Thanks, @Gwil

So, just to confirm - the emends supplies 5V to the emonTx via the RJ11?


The emonVs (a severe case of digitus erroneous or an over-enthusiastic spell-checker there) supplies 5 V d.c. and one or three 0.333 V voltage samples (depending on the version - single or 3-phase) to the emonTx4. It has sufficient current capability at 5 V d.c. to supply ancillary items, such as a Raspberry Pi incorporated in an emonBase, or the emonPi2 (presumably with a different output cable & connector).