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In Reply to: Current draw with lower filament voltage? posted by Ehien on November 21, 2002 at 11:39:15:
Whoever told you that the current increases was wrong. The current decreases but it doesn't follow Ohm's Law. As a rough rule of thumb, a ten percent drop in voltage will reduce current by 5%. If you think about it the negative resistance characteristic would make nonsense of current regulating a heater.Your supposition that the filament is not a constant resistance is correct. Tungsten like most other metals has a positive temperature coefficient of resistance of about 4.5 x 10 (-3) K(-1) which is a jargon laden way of saying its resistance increases by nearly half a percent for each degree (Celsius or Kelvin) temperature rise.
When you underheat the heater it reduces in temperature therefore the resistance drops.
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Follow Ups
- Positive temperature coefficient of Tungsten. - Mark Kelly 14:52:08 11/21/02 (20)
- Re: Positive temperature coefficient of Tungsten. - Ehien 15:58:30 11/21/02 (19)
- Cold resistances - Mark Kelly 18:47:39 11/21/02 (12)
- Re: Cold resistances (pretty much confirmed) - Steve O 22:30:35 11/21/02 (9)
- "Sovteks meas about twice the cold resistance of the US brands - JJ Analog 22:42:04 11/21/02 (8)
- Lower Purity W: Good? Bad? Neither? - Steve O 23:08:26 11/21/02 (6)
- Re: Lower Purity W: Good? Bad? Neither? - Ehien 23:15:36 11/21/02 (5)
- Re: Lower Purity W: Good? Bad? Neither? - Steve O 23:29:32 11/21/02 (4)
- Re: Lower Purity W: Good? Bad? Neither? - SBB 05:49:22 11/22/02 (3)
- Inrush limiter - Ehien 10:29:44 11/22/02 (2)
- Re: Inrush limiter - Steve O 11:37:42 11/22/02 (1)
- Re: Inrush limiter - Ehien 13:59:06 11/22/02 (0)
- some more cold resistance - Ehien 23:08:08 11/21/02 (0)
- Re: Cold resistances - Ehien 21:35:09 11/21/02 (1)
- Much less than 2 seconds - Mark Kelly 20:01:47 11/22/02 (0)
- Re: Positive temperature coefficient of Tungsten. - klaus 17:06:08 11/21/02 (5)