To learn more about HID lighting read our buyer's guide. HID Lighting Buyer's Guide. Conversion bulbs allow you to run HPS bulbs on a MH ballast, or a MH bulb on a HPS ballast. If you already have a MH or HPS ballast, but want to run the opposite bulb, then conversion bulbs are the least expensive option to get you growing. HPS ballast have an igniter and MH ballasts do not. That is the only difference. An igniter is an electrical component that ignites the sodium in the HPS bulb. All electrical ballasts can fire MH bulbs, yet only Switchable or HPS ballasts can fire HPS bulbs--unless you get a conversion bulb.