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Lm338k

LightfootLightfoot Posts: 228
edited 2007-09-25 14:06 in General Discussion
I built a power supply using this regulator to power 4.53 amps worth of vacuum tube filaments at 6.3 volts. I set up the supply and ran 12 volts into the input. It gets too hot even with a very large heatsink. I have solved this problem by putting less input voltage into the regulator. The question is, in this case, what is the minimum voltage I am able to put into the LM338K to get 6.3 volts under load.

Note, with the supplies I have I was only able to test it with a 1.8 amp load.

Thanks

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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-24 03:50
    This is something you can determine yourself from the datasheet. Look for the dropout voltage spec at your current draw. Add this to 6.3V (and maybe another half volt for good measure) and you'll have your minimum input voltage.

    -Phil
  • LightfootLightfoot Posts: 228
    edited 2007-09-24 04:18
    What does the temperature have to do with this?

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  • RDL2004RDL2004 Posts: 2,554
    edited 2007-09-24 12:57
    Volts in minus Volts out times current out (in amps) = Power Dissipation (in watts)

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    - Rick
  • LightfootLightfoot Posts: 228
    edited 2007-09-24 14:13
    Would 9 volts be the minimum voltage? This is just a rough estimate. I do not know how to find the min voltage based on dropout voltage chart. How do you do this, with the temperature and all?

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  • pwillardpwillard Posts: 321
    edited 2007-09-24 14:46
    Drop out on the LM338 is 3 Volts.· So if you are regulating to 6 then·supply 9, the rest is heat.· The LM338 should be stable right up till you get close to 5 amps.· Are you sure your filaments do not exceed 5A draw?· To me, the difference between 5A and 4.3A is not a big enough margin of safety.

    I never considered that Filaments needed a strong regulated DC·supply.· Technically, a high current 6V Transformer creating AC to the filaments is all that is required. Are these filaments special?

    ·


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    There's nothing like a new idea and a warm soldering iron.
  • RDL2004RDL2004 Posts: 2,554
    edited 2007-09-24 15:49
    The chart on the data sheet is just showing how the drop out voltage changes with temperature. It never gets to 3 volts over the operational temperature range so you should be okay at 9 volts. Power dissipation is going to be close to 15 watts so you'll still need a hefty heat sink.

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    - Rick
  • LightfootLightfoot Posts: 228
    edited 2007-09-24 22:02
    Is 8 volts in too low for 6.3 volt regulated output?

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-24 22:34
    Yes, 8V gives you only 1.7V overhead, so it's too low. The dropout voltage, as pwillard pointed out, is 3V. So you'd need at least 9.3V.

    -Phil
  • LightfootLightfoot Posts: 228
    edited 2007-09-24 22:44
    What heatsinks out there will be able to dissipate 17 watts worth of heat? What mouser or digikey part will do the trick?

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-24 22:51
    17 watts is a lot of heat. Maybe you should consider a regulator with a lower dropout voltage, or give each filament its own local regulator. A switching regulator is another option. They run cool but can generate a lot of noise.

    -Phil
  • LightfootLightfoot Posts: 228
    edited 2007-09-24 23:04
    If I use an LM338K producing 1.6 amps (the current draw of a KT88 filiment) I will only have to deal with 6 watts of heat.

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-24 23:36
    The LM1084 has half the dropout voltage of the LM338, BTW. You could power it from 8V, where it would dissipate 2.7W driving the KT88 filament.

    -Phil
  • LightfootLightfoot Posts: 228
    edited 2007-09-25 01:44
    Are there any low dropout regulators like the LM1084 that are in a TO-3 package?

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-25 01:59
    You could easily do a search on DigiKey to find out.

    -Phil
  • LightfootLightfoot Posts: 228
    edited 2007-09-25 03:21
    I plan on using the LM1084 to drive all the filaments of the tubes at a total of 4.53 amps. 8 volts in and 6.3 volts out dissipating 7.7 watts. Will I need a heatsink here? If so what kind?

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  • RDL2004RDL2004 Posts: 2,554
    edited 2007-09-25 13:31
    http://www.aavidthermalloy.com/products/standard/index.shtml

    Maybe you can get an idea of what you need from the link above. You can buy Aavid heat sinks at Mouser.

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    - Rick
  • pwillardpwillard Posts: 321
    edited 2007-09-25 14:06
    I may be nuts here, but I still say that the KT88 Heater wants 6.3V --->AC not DC. I guess if you want to create regulator that controls INRUSH current to modern filamants you could go the voltage regulator route... but I'm not clear on why you want to regulate. Is it because you don't have a 6.3VAC 5A transformer?

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    There's nothing like a new idea and a warm soldering iron.
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