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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> high pressure discharge lamp https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1238409376 Message started by stevenray58 on Mar 30^th, 2009, 11:36am

Title: high pressure discharge lamp
Post by stevenray58 on Mar 30^th, 2009, 11:36am

please can someone help ... when high pressure discharge lamp luminaries ar starting they... use less current ...or have higher than normal running currents ... or light as soon as their switch is turned on ... or loose all voltage when switched off. im suck on this in my on line electrical course many thanks steve

Title: Re: high pressure discharge lamp
Post by Zambezi on Mar 30^th, 2009, 11:38am

use more current than usual, inrush current.

Title: Re: high pressure discharge lamp
Post by Y3 on Mar 30^th, 2009, 8:52pm

Capacitor will sort that

Title: Re: high pressure discharge lamp
Post by Zambezi on Mar 30^th, 2009, 9:57pm

wrote on Mar 30^th, 2009, 8:52pm:

Capacitor will sort that

So then which answer is it? You can stick as many caps on it as you like but there is still going to be an inrush current.

Title: Re: high pressure discharge lamp
Post by Goodsparks on Mar 30^th, 2009, 11:52pm

Cap will help correct the power factor, only thing that will help the inrush current is a HF ballast.

Title: Re: high pressure discharge lamp
Post by LSpark on Mar 31^st, 2009, 12:49am

A HF ballast will also draw inrush current, and can actually be more of a problem if they are used in banks. They draw less overall than conventional electromagnetic ballasts though obviously.

Title: Re: high pressure discharge lamp
Post by Lectrician on Mar 31^st, 2009, 7:00am

wrote on Mar 30^th, 2009, 8:52pm:

Capacitor will sort that

A capacitor will not reduce the start up current, only imporve the power factor.

There is two types of 'startup current' to consider.

You will get an inherent inrush of current due to switching on an inductor (the balast). This presents a near short circuit to the supply for a fraction of a second until it sets its magnetic field up where it then 'chokes' back the current. This startup current lasts for a second or so.

While the lamp starts and comes up to full brightness there will be more current flow as the lamp heats up and vaporises the gases and metals and gets the electrons and stuff excited and moving. While this is happening more current will be drawn.

Once the lamp is upto full operating output, the current will have reduced and steadied. This can take several minutes depending on the type of discharge lamp and the wattage.

Title: Re: high pressure discharge lamp
Post by Zambezi on Mar 31^st, 2009, 8:12am

I had a mental block last night when I was trying to think what the caps were for and it was PFC, thanx for reminding me!

Title: Re: high pressure discharge lamp
Post by Y3 on Apr 1^st, 2009, 7:18pm

Lectrician wrote on Mar 31^st, 2009, 7:00am:

wrote on Mar 30^th, 2009, 8:52pm:

Capacitor will sort that

Thanx for that in depth description lec, very informative. Don't these lights use more current when the caps cease to work, I was told this in college. When we did a test we measured how much current was used with the cap and without, the results were that the current was more when the cap was out of the equation. This is where my original answer came from, I must have misunderstood the test in college and the question. They are both different.

Title: Re: high pressure discharge lamp
Post by Lectrician on Apr 1^st, 2009, 9:30pm

A capacitor will improve the power factor which will have the result of reducing the overall running current (not just the start up current).