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Sizing a Regulator
Sizing a Regulator
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Old 16th May 2005, 06:35 PM   #1
cantskienuf is offline cantskienuf  United States
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Default Sizing a Regulator

Is there any down side to using a larger sized regulator chip? I need 1.2 amps for the heaters I'm planning to feed, which seems a bit close to the 1.5 amp limitation of the LT1086. Is it OK to use the 3 amp LT1085? I don't know if that means a larger heat sink requirement, or what, but I'll bet that somebody here does. Thanks!
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Old 17th May 2005, 12:31 AM   #2
Stocker is offline Stocker  United States
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You may need to waste a very small amount of electricity as heat in order to achieve minimum current required to regulate properly with your powered circuits idle. Do not worry too much about a larger regulator unless its datasheet says not to use it for low currents.
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Old 17th May 2005, 12:40 AM   #3
hermanv is offline hermanv  United States
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Default Heater Inrush

Seems I recall that vacuum tube filaments have very large inrush currents, so make sure the regulator you use is safely current limited. With current limiting heaters will take longer to stabilize but tube life will possibly improve.

Tube fans; someone verify my recollection of filament inrush current at 10 to 1?
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Old 17th May 2005, 12:47 AM   #4
Tom Bavis is offline Tom Bavis
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3 to one is more like it, though some may be worse... current limiting will slow warmup, may haelp life.
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Old 17th May 2005, 12:50 AM   #5
jackinnj is offline jackinnj  United States
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Sizing a Regulator
Dr. Ruth said that size doesn't matter but you probably aren't old enough to remember that. I think that Helen Gurley Brown had a different opinon.

Don't forget that the regulator has its own "overhead" -- some voltage which is going to get chewed and spewed out as heat -- depends upon whether it's low dropout regulator, or the garden variety 7812 etc.

Have you considered just using a beefed up zener (i.e. a zener on the base of the transistor and a little bias current) -- there really is no need for "regulation" per se since the load on the tube filament is invariant.
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Old 17th May 2005, 03:55 AM   #6
gingertube is online now gingertube  Australia
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An optional Plan B,
Allen Wright in his Tube Preamp CookBook recommends using separate LM317 as CURRENT regulator for each tube heater. A small, device mounted, heatsink will then suffice. No switch on surge to worry about. Having said that, I don't remember when I last changed a tube because the heater had gone open circuit so I don't believe switch on surge is a major problem for tube heaters. Don't worry about power tubes - just run them from the 6.3V AC.
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