Connect with us

Help with a simple LM338 circuit

Discussion in 'General Electronics Discussion' started by quirkymac, Aug 4, 2009.

Scroll to continue with content
  1. quirkymac


    Aug 4, 2009
    Hi there,
    I was hoping someone here could help me with this simple circuit. I have made the circuit listed on www dot

    to help me regulate the output voltage to a nichrome wire hot wire cutter - for cutting polystyrene foam wings for model aircraft.

    I have scavenged a couple of old computer power supplies and have a number of different input voltages to choose from (with amps to spare). The circuit appears to work perfectly and I now have an adjustable output voltage.

    My issue is that when I try and turn up the voltage the circuit appears to max out then go into a fluctuating state - when I hooked up the ammeter to see what current I was getting, it ramps up beautifully to 2.85amps then starts to alternate between 2.85 and about 1.5. Am I right in thinking that the LM338 should output up to 5 amps?

    If I was using the full rated 5 amps for the LM338 do I need a heatsink?
    Could it be that the LM338 is limiting the current due to overheating?
    Or could there be something else going on?

    Any help greatly appreciated!
  2. Resqueline


    Jul 31, 2009
    It'll put out 5A provided it has a voltage drop of 3-15V AND a proper heatsink.
    The device by itself in free air will not be able to dissipate more than around 1W before it shuts down due to overheating.
    Simple math: 3V drop times 5A equals 15W, minimum. Thats a lot of heat, and it has the potential to give off 5 times more..
    Look in the data sheet. You'll find the thermal resistance junction-to-case there. Add to that the case-to-heatsink mounting resistance (typ. 0.1-0.5 depending). Max junction temp is 125 deg C, and ambient is around 25, so you have a 100 deg C rise to play with. Take these 100 and divide them by the Wattage the device gives off, then subtract the result with the junction-to-heatsink resistance. The end result is the specification for the heatsink. A number lower than 0.5 means you have to use fan cooling to achieve, and if it's negative then you have to improve the mounting method and/or reduce the Wattage (= lower Voltage drop).
    So measure the Voltages involved for starters.
  3. neon


    Oct 21, 2006
    You realy do not need voltage what for ? you need current and regulate it. for that kind of power think abaout a bypass power transistor. heatsink becomes massive to dissipate if continious duty cycle. think of adding a small fan to the heatsink to get rid of heat..Most people when need power think of voltage but in essence is the product of the two that you want.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day