[DML] Re: Headlight switch savers

["David Teitelbaum" <jtrealty@xxxxxxxxxxxxx>]

The engineering name for that is "inrush current". The bulb's resistance increases as it heats up reducing the current load. This as opposed to a coil which does the reverse, the load is fairly constant but when the contacts open the current continues to flow and causes arcing across the open contacts. If the contacts are not sized large enough they are pitted and eventually weld together. As I said in a previous post, ratings are very important but you need to understand the load so you can properly spec the correct size relay to handle the load to get a reasonable life expectancy. If you think it is complicated we are only discussing DC currents here, try polyphase AC and it can REALLY get complicated! Especially when you have to consider harmonics.
David Teitelbaum
vin 10757



--- In dmcnews@xxxxxxxxxxxxxxx, Joseph Kuchan <josephkuchan@...> wrote:
>
> 
> By the way, driving an incandescent light with a relay or switch is not as simple as it sounds. A cold filament in a light bulb (i.e. when the bulb is off)  will draw around 10 - 12 times the amount of current it draws when hot, i.e. "on". 
> 
> No joke. The surge current on an incandescent light is more severe than with an inductive load!
> 
> Joe Kuchan
> 
> 
> 
> To: dmcnews@xxxxxxxxxxxxxxx
> From: hmcelraft@...
> Date: Thu, 4 Jun 2009 14:11:32 +0000
> Subje



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