Using a 400W MV bulb in 1000W system


Old 08-01-16, 06:35 PM
Thread Starter
Join Date: Aug 2016
Location: USA
Posts: 1
Received 0 Votes on 0 Posts
Using a 400W MV bulb in 1000W system

I have a power supply for a mercury vapor light. The writing on the step-up transformer states it is rated for 1000W. The ballast system appears to only have a step-up transformer from 120 or 240 to 450v and a large capacitor. I want to use a 400W bulb but the electrical supply store said I would either burn out the bulb or ballast by doing so. Is this true? Seems to me that while the system cannot supply more than 1000w, it should only draw the needed current for the installed bulb if a smaller bulb was used. Please clarify my obvious confusion.
Sponsored Links
Old 08-01-16, 08:10 PM
Join Date: May 2015
Location: USA
Posts: 3,138
Received 2 Votes on 2 Posts
You got good advice. The arc inside a MV lamp is very low resistance and if the current through the lamp is not limited by the ballast the lamp will happily destroy itself. A 1000 watt ballast will allow too much current to flow through the 400 watt lamp and will quickly destroy the lamp.
Old 08-02-16, 06:05 AM
Join Date: Mar 2010
Location: USA
Posts: 4,156
Received 87 Votes on 81 Posts
For mercury vapor (and also sodium and fluorescent and metal halide) the lamp (wattage and sometimes a few other characteristics) must match the ballast.

For incandescent (including halogen) lamps the lamp resistance increases as the filament heats up. Given the correct voltage, the current draw and temperature and light output level off at the correct values without the need for a ballast. The lamp draws what it needs regardless of the capabilities of the power supply.

At any moment in time in any portion of any circuit the current flowing equals the voltage applied divided by the resistance of that portion.

For MV etc. lamps the internal resistance decreases as the lamp interior heats up and in response the current draw would theoretically rise unchecked until something broke, possibly explosively. The ballast is needed to limit the current draw to the correct value (when the correct light output is attained). For the MV lamp the starting voltage applied is high enough at first to start the arc and lowers significantly as the current draw increases to the operating value. (For high pressure sodium lamps the starting voltage is not high enough to strike the arc and a separate circuit, called an igniter, gives momentary higher voltage bursts to start the arc.)

A MV lamp ballast may resemble a transformer. It works a little differently; its internal impedance (think: resistance but the voltage and amperes calculations are more complicated) is such that as the current draw increases the output voltage decreases significantly as intended to properly supply the MV lamp but not generating considerable heat or wasting considerable energy within the ballast.

Last edited by AllanJ; 08-02-16 at 06:30 AM.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: