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LM338: LM338 high voltage regulator

Bill Wells
Prodigy 50 points
Part Number: LM338
Other Parts Discussed in Thread: LM317HV

I need to build a high voltage regulator for a vacuum tube audio circuit my company is developing.  Regulator input voltage: 360 VDC; Regulator output voltage:  variable, via a trimpot, from 325 to 355 VDC, at about 30 ma.  My reading of the datasheets for the LM338 says this is possible with just one LM338 and a few resistors.   The LM338 is a 3-terminal regulator, the 3 terminals being Vin, Adjust, and Vout.  To quote from the LM338 datasheets:

"Because the regulator is floating and receives only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded."

Since it seems I cannot attach a schematic, I will describe the simple circuit:  The Vin terminal receives 360 VDC.  At the Vout terminal, there are 3 resistors connected in series to my circuit ground, 1K, 200K, and a 100K trimpot (Bourns 1 watt, 22 turns).  The Adjust terminal is connected to the junction of the 1K and 200K resistors.  Varying the trimpot should vary the voltage between about 325 and 355 VDC.  Can you please confirm that this will work?  Or, if not, why not?    (P.S. If this circuit works, I should also be able to use an LM-117HV, since the current draw is very low.)  Thanks so much.

Bill Wells
Marlborough Recording Technologies
Brooklyn, NY

  • 0
    TI__Mastermind 23361 points

    Hi Brooklyn,

    At the bottom of the post, you will find "Insert Code, Attach Files and more...", if you click it,  you will be redirected to the full text editor and you will then be able to upload pictures.

    For your question on the circuit, how fast you plan to vary the trimpot? 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 50 points

    Jason,

    I can vary the trim pot very slowly if necessary.  It is just to optimize the performance of the circuit by "fine-tuning" the high voltage supply for the vacuum tubes.  

    Why do you ask?

    Thanks !

    Bill Wells

  • 0 in reply to Jason Song
    Prodigy 50 points
    Jason -
    I've tried reposting with a schematic but do not find the "Insert Code, Attach Files, and more...." at the bottom of the post. I am using Safari on my iMac, in case it might be browser-related. Thanks. Bill Wells
  • 0 in reply to Bill Wells
    TI__Mastermind 23361 points
    Hi Bill,

    If you are not turning it too fast, I am not seeing a problem. The device should work for your application. Next time, if you plan to add a schematic, you might need to install another browser, like Chrome or Firefox.

    Regards,
    Jason Song
  • 0 in reply to Jason Song
    Prodigy 50 points

    Hi Jason,

    Out of curiosity, what would tend to happen if the trim pot were adjusted too quickly?

    Many thanks.

    Bill Wells

  • 0 in reply to Bill Wells
    TI__Mastermind 23361 points
    Hi Bill,

    If you shift the output too fast, the device will need some time to catch up. If you are moving back and forth, the output may not get enough time to settle at the desired voltage before you move to a new target. Since you are doing the turning and try to set it up at a steady voltage, I am not seeing a problem with your application.

    Regards,
    Jason
  • 0 in reply to Jason Song
    Prodigy 50 points

    Hi Jason,

    It seems like the TI LM317HV might work even better for my application.  According to the TI datasheets, the maximum Vin to Vout voltage is 60 volts, as opposed to 40 volts for the LM338, so the LM317HV would have a somewhat larger range of regulation.  Also, the the lower current capacity for the LM317HV will not be a problem, as it's 1.5 A, and my circuit's drawing only 30 ma, approximately.

    I believe the circuit and component values should be about the same for both ICs.

    I'd appreciate your thoughts on whether the LM317HV might be better suited for my application.

    Thanks so much.

    Bill Wells

  • 0 in reply to Bill Wells
    TI__Mastermind 23361 points
    Hi Bill,

    Sorry for my late reply. LM317HV could be a better fit if you don't care about the extra current capability LM338 provides ( LM338 max 5A, LM317HV max 1.5A). For your application, both of them should work. Please check the other specifications of the two devices to see if they will fit your applications.

    Regards,
    Jason Song