3-Wire Ballasts

Hello

Has anyone had any experience using a 3-wire ballast with C-Bus dimmers? I have been searching the forums but most people seem to just be talking about DALI, DSI and 0-10v for dimming. We need to dim a Belfer Reflex 1320 and the manufacture has told me that they ship them with the Lutron Compact SE 5% ballasts (model # FDB CF-26-120). These ballasts use 3 wires: hot, neutral, and dimmed hot for dimming instructions.

Other posts have talked about ballasts remaining active when dimmed to 0%. If they can be dimmed, is it good practice to wire 3-wire ballats into a relay as well.

Thanks, and apologies for all the questions.

 

Dimming Lutron Ballasts

While it is possible to dim the Lutron 3 wire ballasts directly from a CBUS dimmer, I don't recommend it. Lutron makes a ballast interface called a PHPM-3F (aka GRX-FDBI-16A-WH) that takes in a single phase controlled dimming signal and converts to what the ballast is looking for. This interface is around $150US for the 120V version and $250 for the 277V version.

If you are still undeterred, here's how you do it directly. You will need two dimming channels (or one dimmer and one relay) to control the ballast(s) directly. You connect the first dimming channel to the DH (Dimmed Hot) input of the ballast(s). This is typically the orange or red terminal on the ballast. Trim the low end of this dimming channel to appoximately 30%(roughly 27VAC output). Connect the second dimming channel (or relay) to the SH (Switched Hot) of the ballast(s). This is typically the black terminal on the ballast. Trim the low end of this dimmer to 99% (basically on/off like the relay). Assign your CBus group to both dimmers and dim away. FYI, the current for the ballast is carried on the SH so it should to be at line voltage whenever the ballast is lit. The dimming signal carries no current and should vary between low end (27V) and high end (close to line voltage).

The reason I don't recommend doing the above procedure is that CBus dimmers are not air gapped internally so there will always be some leakage current available at the ballast on the DH (and the SH if you use a dimmer for this leg). The Lutron interface is air-gapped so when you turn it off, it completely isolates the ballast(s) and the circuit.