LM317/337 Dual Power Supply Pot Sub?

Discussion in 'The Projects Forum' started by IowanChef, Sep 27, 2010.

  1. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
    I am about to dig into my bag of Thomas Henry literature after scoring an old Klipsch +/- 22v power supply at a thrift store. I was planning on using it in conjunction with the LM317/337 to create an adjustable power supply, but the only thing I'm missing are the 5k pots that are used in nearly every schematic I've seen for this particular project. Is subbing 10k pots a bad idea? Or am I better off placing another resistor in parallel with the 10k pot to knock it down in resistance, getting it closer to the recommended 5k?

    Gracias y'all.
  2. Wendy


    Mar 24, 2008
    Try posting a schematic of what you're wanting to do, and you'll get a good answer. You can not describe a circuit verbally and get quality answers.

    I have drawing up a core of schematics over time. For example, my standard LM317 looks like this.


    This design would need 2KΩ pots for R2 for a max of 22VDC.

    There are kludges you can use to get around it, but a core spec you must meet is the LM317 must draw 10ma minimum. You will also loose around 2V to 3V with insertion loss, leaving you with 19-20V out. The 10ma spec is usually met by R1 (1.25V/120Ω), but you can have an external load to meet this requirement.
    Last edited: Sep 27, 2010
  3. bertus


    Apr 5, 2008

    Can you post the schematic that you intend to use?
    The value of the potmeter will make the maximum possible voltage.
    A higher value can get it out of regulation.
    A lower value will get you a lower maximum voltage.

  4. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
    Here's the schematic I'm going off of, but again, not taking anything out of the wall socket, just using the pos/ground/neg lines from the Klipsch wall wart..

    Of course, I'm always open to suggestion...

    Thanks again everyone.
  5. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
    I should also add that I'm not looking to swing more than 15 volts in either direction any time soon. I just lucked out and found a supply in excess of what I actually need, so the drop in Vout isn't much of a concern..
  6. SgtWookie


    Jul 17, 2007
    If you look at the standard datasheets for the LM117/LM317 and the LM137/LM337, you'll commonly see 120 Ohms used for R1. This is to provide the minimum 10mA current flow so that the regulator will output with the guaranteed regulation specifications.

    If you want to use 5k pots for R2, then you can use 330 Ohms for R1, but understand that you will need to sink at least another 6mA current from the output before guaranteed regulation will be achieved.

    With +/-22v input and a light load, your output range will be ~+/-1.25v to ~+/-20v. There's a minimum 1.7v drop across the regulators. Note that you need a 0.33uF cap to ground on the input side of both regulators, and a 0.1uF cap to ground on the output of both regulators. If you omit them, you will most likely have problems.

    Your regulators are linear, and thus will dissipate a lot of heat; the worst case scenario is when you are attempting to power something with low voltage and high current. You will need LARGE heat sinks, and even that won't be enough if you try to output more than perhaps 200mA at low voltage.
  7. IowanChef

    Thread Starter Active Member

    Mar 6, 2010
    All very helpful information. I found a quick and cheap batch of the necessary pots online and will just put this off till they arrive and do this a little more to spec. Thanks again.