Don't Make This Common Mistake When Using an LTEM-PA/PV

A convenient option when using a 3G or CDMA radio on a VISTA panel was the ability to share a single transformer for both devices. This was possible because both units used AC power. The new LTEM-PA and LTEM-PV communicators use a DC transformer, so attempting to share will cause issues.

The Resideo LTEM-PA and LTEM-PV dual-path communicators are a part of the new ProSeries lineup. In addition to these communicators, the Honeywell Home PROA7 and Honeywell Home PROA7PLUS and the Resideo PROA7C and Resideo PROA7PLUSC help to make up the ProSeries. These products may seem to make strange bedfellows, but the modular design shared by the All-in-One (AiO) panels and the stand-alone communicators is no accident. With some modules, such as the LTE communicators being shared between the two product lines. This is one reason why the LTEM-PA and LTEM-PV use DC power, rather than AC.

It's a natural assumption to make, thinking that since the older communicator from a particular manufacturer was an AC step-down transformer, the new communicator would employ a similar design. But in this case, that assumption can get you into a lot of trouble. Attempting to keep the original transformer from the 3G or CDMA communicator in place, and just connecting the LTEM-PA or LTEM-PV in place of the older one, can damage the VISTA panel, the transformer, or both.

The symptoms seen by Resideo Tech Support, when this mistake has been made are as follows:

• The transformer is damaged. Once this occurs, the VISTA panel will show an AC loss condition. Naturally, the next step is to replace the bad transformer with a good one. This can lead to the second symptom that has been seen.
• The VISTA board is damaged. Once the damaged transformer is replaced with a new one, feedback on the panel's power circuit causes one or more of the diodes on the VISTA board (above terminals 1 and 2) to become damaged. This can cause a burning smell and may even cause the panel to catch fire in a worst-case scenario.

In the image below, you can see a damaged diode. This area of the printed circuit board is just above Terminal 1 and Terminal 2 on a VISTA panel.

It is possible for the LTEM-PA or LTEM-PV to receive power from the panel so that only one transformer is required in the setup. However, the power comes from the 12 Volt DC auxiliary power output, not the transformer. You can see how to properly configure that setup in this helpful FAQ.