PSU volt drop

Discussion in 'General Electronics Chat' started by samuel.whiskers, Jun 15, 2014.

  1. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    95
    2
    I have a 0-30V 3A linear PSU.... I'm testing my MkIII rocket launcher circuit, using a MOSFET to supply power to an igniter.

    In the real deal the MOSFET will deliver the supply voltage (12.7V) into approx 1-2 Ohm, until the igniter burns like a fuse.

    In testing I was using a 10 Ohm resistor in place of an igniter, not using 1 Ohm as my bench PSU wouldn't be up to it.... it seems it isn't up to 10 Ohm either - it seems to be able to manage a measley 8V across 10 Ohm. If I put a 1K resistor in place, the voltage drop measures 12.4V.

    In contrast, a (small) 12V SLA battery manages 11V - and even puts 11V across 5.6 Ohm too.

    Is this power supply faulty?? or just weak??

    Lee
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    612
    120
    Daft question but you haven't got the current limit set have you?

    Sounds like a dodgy power supply otherwise.
     
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  3. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    If the powersupply could give 3 Amps, at 12 Volts the minimum load should be 12 Volts / 3 Amps = 4 Ohms.

    The 1 - 2 Ohms from the ignitor are way to low for the used power supply.

    Bertus
     
  4. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    95
    2
    Starting to think that.... No, current limit not used....
     
  5. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    95
    2
    Yes - that's why I was testing with a 10 Ohm load, thought it should be well within the specs of the PSU.
    In the field I use a SLA battery, the PSU is just being used to bench test a board....

    I think it can't keep up with the rapid change in current draw???
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    How old is the powersupply?
    It could be that the buffer capacitor is getting bad.

    Bertus
     
    samuel.whiskers likes this.
  7. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    95
    2
    2 weeks old.... :mad:
     
  8. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    95
    2
    Isn't this embarrassing.... :D
    I should learn to RTFM....
    I got thinking after paulktreg's comment, and low and behold the current limit dial was wound back a fair way!
    Corrected this and it is happily delivering 2.5A into a 4.7 Ohm test load at 12.5V....

    Thanks for the advice gents!
     
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