Hi Allen –
Metal Halide (MH) and High Pressure Sodium (HPS) lights require a remote ballast to work. When building your own plant grow light, it’s important to match the ballast with the wattage of the bulb you will be using. You must also match the type of light (MH or HPS) you’re looking for with the ballast. For example, a 400 watt MH bulb will only work with a 400 watt MH ballast (or 400 watt digital ballast).
The cheapest 400w ballast we offer is the Quantum Digital Ballast. However you will also need an BASSP adaptor to match the ballast and lamp cord sockets. I’ve provided links below.
https://www.planetnatural.com/product/quantum-ballast/
https://www.planetnatural.com/product/plug-adapters/
Hope this helps!