Laptop Switched Mode PSU & 2200uf Cap

Discussion in 'General Electronics Chat' started by Tasp, Sep 24, 2011.

  1. Tasp

    Thread Starter Active Member

    Aug 10, 2008
    45
    0
    I have a project that uses a 13.8vdc 3A transformer PSU, I have a 2200uf capacitor after the PSU to hold a charge as my circuit uses a quick burst and this works ok, however I have now needed to uprate the power supply to 19vdc and have used a laptop SMPS, now the cap doesn't seem to hold the charge any more as when the quick burst occurs the power led (on the output side after the cap) dims where it never used to.

    Is there an issue using caps with SMPS?

    Searching the forums doesn't yield much help.

    Also I have a 0.1nf cap across the 2200uf as well if that makes a difference (for decoupling etc).

    TIA!
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Maybe it was time for the capacitor to fail or degrate.

    What is the voltage rating of the capacitor?

    Any additional useful information?
     
  3. colinb

    Active Member

    Jun 15, 2011
    351
    35
    Maybe the laptop PSU is trying to be “smart” and thinks the cable was unplugged, or possibly goes into a short-circuit protection mode when your massive 2200 uF cap tries to recharge — the instantaneous charging current might be too much for the PSU.
     
  4. Tasp

    Thread Starter Active Member

    Aug 10, 2008
    45
    0
    Cap appears fine it holds a charge when connected to a transformer based PSU.

    Cap is rated to 100v I believe.

    I don't think the SMPS is shutting down or cutting off as the rest of the circuit functions and is powered.

    As far as we know there is no issue using large caps with SMPS's?
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Post more information about the PSU and what the load is.
     
  6. Tasp

    Thread Starter Active Member

    Aug 10, 2008
    45
    0
    SMPS is 19v 3A typical laptop PSU.

    Load is typically around 30-50mA until the MOSFET output kicks a solenoid of approx 1-1.5A for approx 20-50ms.
    With the 1st transformer based attempt this works ok however needed the extra voltage to go further, this does work and the cap "bridged" the sudden burst needed and the power LED stayed steady with the SMPS the LED breifly dims, so was/am assuming the cap isn't holding the charge, and wondered if there was an issue charging caps using a SMPS.
    The circuit still works was just concerned about the dim.

    The original project this relates to is http://forum.allaboutcircuits.com/showthread.php?t=43557&page=2 however as this is a completely different question thought it best to start a new thread.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Put more capacitors in parallel to see what happens. The SMPS might not be able to charge the capacitor that fast to keep the voltage up.
     
  8. oldtech33709

    New Member

    Sep 24, 2011
    26
    0
    That large a cap is probably causing the output of the converter to current limit at turn on. Initially the cap looks like a dead short and supply is just protecting itself. The wall adapter you initilly used doesnt have that protection. An inrush limiting circiut would do the trick.
     
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