I might get a retrofit sodium lamp, but they say they can't be run on CWA ballasts. I have a few questions:

1: What would really happen if you ran it on a CWA ballast?

2: If the CWA ballast would overdrive the HPS lamp, could I lower the wattage by substituting in a lower series capacitor?

Thank you for any help

If the HPS retrofit lamp you are talking about is the "penning-start" type such as Sylvania Unalux, then it will harm both CWA ballast as well as the lamp. This is due to those HPS retrofit lamps not being able to handle constant current from a CWA. I recommend running these on HX-HPF or reactor ballasts. Only problem is that HX-HPF ballasts are rarer than CWA.

On the other hand, there are HPS retrofit lamps such as Phillips Retrolux and EYE Iwasaki Super Ace that feature "glowbottle" ignitors built into the lamp. These can be ran on CWA no issues. Unfortunately, these "glowbottle" HPS lamps are generally more expensive and harder to find.

Last but not least, there are HPS retrofit lamps as well as metal halide lamps that retrofit mercury vapor. They are compatible only with HX-HPF and Reactor ballasts. They utilize a built-in "FEC" ignitor to start the lamp. The FEC ignitor looks like a round brown disk with terminals.

The ballast compatibility is published on the lamps datasheet. If the lamp is rated as compatible with a given ballast type, there should not be any issue.

Whether a penning mixture, or any other HPS lamp is compatible with a constant current CWA ballast does not have to always be related to the lamp chemistry itself, but also to the lamp construction, especially its thermal management.

The fact is the constant current ballasts combined with a saturated vapor lamp in its basic form does lead to thermal instability (hither temperature -> higher pressure -> higher voltage drop -> higher power -> higher temperature positive feedback), but manufacturers came with design tricks to eliminate that sufficiently enough so the lamp becomes stable even on a CWA ballast. One example is to put the amalgam reservoir (aka from the vapor pressure perspective the "cold spot" whose temperature dictates the operating pressure) further away from the active arc, so suppressing the thermal feedback, but that is not the only one.
So the question is, whether any such thermal feedback suppression is used in the lamp design and whether it is effective enough to make the lamp really stable so compatible with the constant current drive of a CWA ballast.

From marketing perspective, if a big portion (maybe even a majority) of MV ballasts on the target market are the constant current drive CWA type, I would be very surprised any lamp maker would make a lamp advertised as a "MV replacement" without making sure its design is compatible with the CWA ballast.

Of course very different situation would be when you import the retrofit lamp from a "230V" area where series chokes are 99% of the MV ballasts in use, so where the saturated vapor lamp is stable by itself, so without any such thermal feedback suppression feature. Then you have to really make sure the ballast has the same or similar characteristics as the original series choke (so e.g. the HX transformer or so).

The ballast compatibility is published on the lamps datasheet. If the lamp is rated as compatible with a given ballast type, there should not be any issue.

Whether a penning mixture, or any other HPS lamp is compatible with a constant current CWA ballast does not have to always be related to the lamp chemistry itself, but also to the lamp construction, especially its thermal management.

The fact is the constant current ballasts combined with a saturated vapor lamp in its basic form does lead to thermal instability (hither temperature -> higher pressure -> higher voltage drop -> higher power -> higher temperature positive feedback), but manufacturers came with design tricks to eliminate that sufficiently enough so the lamp becomes stable even on a CWA ballast. One example is to put the amalgam reservoir (aka from the vapor pressure perspective the "cold spot" whose temperature dictates the operating pressure) further away from the active arc, so suppressing the thermal feedback, but that is not the only one.
So the question is, whether any such thermal feedback suppression is used in the lamp design and whether it is effective enough to make the lamp really stable so compatible with the constant current drive of a CWA ballast.

From marketing perspective, if a big portion (maybe even a majority) of MV ballasts on the target market are the constant current drive CWA type, I would be very surprised any lamp maker would make a lamp advertised as a "MV replacement" without making sure its design is compatible with the CWA ballast.

Of course very different situation would be when you import the retrofit lamp from a "230V" area where series chokes are 99% of the MV ballasts in use, so where the saturated vapor lamp is stable by itself, so without any such thermal feedback suppression feature. Then you have to really make sure the ballast has the same or similar characteristics as the original series choke (so e.g. the HX transformer or so).

you say that, but the first generation of HPS retrofit lamps sold in the US where indeed incompatible with CWA ballasts, it was one of the big limiting factors when it came to the wide spread adoption


if you look up the GE E-Z-Lux and Sylvania Unalux, you will see how their entries in the catalogs and such like are plastered with warnings about being suitable for operation only on HX-Autotransformer or Choke type ballasts


the other main problem is besides general instabilities as your describe is that at the end of life the voltage of a HPS lamp rises and on a constant current ballast like a CWA ballast, this can lead to dangerously high power levels


it was not until unsaturated HPS lamps where devloped did CWA compatible HPS retrofit lamps become possible






dedicated CWA HPS ballasts I understand work around the issue by having a very low OCV, so the lamp arc voltage cannot rise that much before it exceeds what the OCV can support and the end of life lamp will thus safely cycle

Definitely what matters is what the thing is really rated for by the manufacturer, ANY assumption or expectation just by itself could be completely wrong...

Thanks for letting me know! I was ready to try it but now I won't. If only HX ballasts were more common... 

 I killed my Sylvania 800w Retrofit lamp by running it from a CWA ballast . It ran for about 10 min. before it started cycing .

HX ballasts are quite common, depending on the application.