After reading about some collectors attempting to run 700W H35 mercury vapor lamps on 1000W M47 probe start metal halide CWA ballasts by derating the series capacitor value from 24 microfarads down to 18.5 microfarads with relatively good results, I am interested in trying to find an appropriate capacitor value that is appropriate for running the rare 425W H40-17 mercury vapor lamps on 1000W M47 probe start metal halide CWA ballasts.

For reference, the 425W H40-17 mercury vapor lamps have an arc voltage drop of about 265V and an operating current of about 1.7A and is NOT interchangeable with 400W H33 mercury vapor lamps; the 700W H35 mercury vapor lamps have an arc voltage drop of 265V and an operating current of 2.8A; and lastly, the 1000W H36 mercury vapor lamps and 1000W M47 probe start metal halide lamps have an arc voltage drop of 265V and an operating current of 4.0A. So far, my initial guess is a capacitor value somewhere around 10 to 12 microfarads for the 425W H40-17 lamp if I am not mistaken. I am interested in answering this question because I am looking for easy ways to help collectors safely run 425W H40-17 mercury vapor lamps just in case they find any in the future, but they are extremely hard to find as they appear to have only been produced during the 1960s and would like them to make sure that those lamps would run properly.

The source that I used to find the electrical characteristics of the 425W H40-17 mercury vapor lamps can be found on the link below on page 15 of the following document:

https://www.scribd.com/document/394833080/Westinghouse-Mercury-Lamp-Brochure-1961

I would love to see someone find a H40 425W mercury lamp, never seen one in a collection I dont think sadly, but you cant fudge a M47/H36 ballast this much, you can do it fine for a 700W mercury lamp, but there is a limit to how far you can take things.

your best bet for running a such a lamp like this in 60Hz land would probably be across the red and blue wires of a 2 lamp F96T12 VHO ballast, 1.5A is likely good enough to run the lamp up to near enough full power.


(interesting brochure/technical document BTW interesting to get some detailed info on the 6.6A series mercury lamp, years and years ago someone uploaded a period picture of a lady holding a tray of early westinghouse mercury lamps and there where many interesting types there PS35 shaped 175W sorts and the afromentioned H17 425W and H24 6.6A ~400W~ types, the later of which with its 70V arc voltage you could probably get away with running on a series choke off 120V)

So, in this case, would the 425W H40 mercury vapor lamp still be overdriven on a 1000W M47 probe start metal halide CWA ballast even if the series capacitor value is near zero microfarads or could the ballast suffer possible overheating?

Alternatively, it may seem that a last resort for running these lamps could be to source a step up transformer with a 480V secondary and possibly do a series of trial and error tests of series choke ballast combinations until the lamp is running at the proper 1.7A operating current and 265V arc voltage drop?

As was discussed here recently, it is not the best idea to run CWA ballast with seriously reduced capacitor as the impedance becomes closer to pure capacitive, which is harsh for a lamp. Not the thing you probably want to do to a rare/collectible lamp.