Discussion in 'General Electronics Chat' started by cowasaki, Sep 3, 2011.

  1. cowasaki

    Thread Starter Member

    Apr 3, 2011
    The simplest of components seem to be the bane of my life!

    I have just attempted to repair the power supply on my network switch, Ireplaced all the capacitors then tested it to find it giving the correct voltage. I then plugged it into the switch's main board at which point there was a bright flash and one of the resistors exploded! The switch's main board still works (hooked up to my bench PSU) luckily but the resistor is dead. Now looking at the resistor it has these bands:

    (brown or orange) (white) (silver or grey) (gold)

    and the resistor is quite a large fat one (probably 1-2w). I cannot get a circuit diagram and the resistor is totally open......

    So it is:

    0.19 ohms
    0.39 ohms
    1.9G ohms
    3.9G ohms

    So it isn't going to be the last two leaving 0.19 or 0.39

    It looks like it could be either but most likely 0.39 ohms

    My local supplier doesn't supply 0.39 ohms but has a 0.33 ohm 3w resistor which I will pick up whilst out later. (if the PSU works with this then great, if not I will just replace the entire PSU with one from china which is £8.90 inc postage and probably cheaper to buy than replacing many more of the components)

    What a pain, can I start a petition to change resistors so they have the value written on instead of these annoying bands that have been winding me up for 25 years. I had a colleague a few years ago who was great but colour blind, he had to constantly ask me and even though my colour vision is A1 they can still be a bit difficult to tell apart.

    I bought a load of cheap ones off ebay recently that were all colour coded as 1% 10K but were in bags labelled 220r, 1K, 3K and 10K - Now that's a great way to annoy someone down the line that tried to fix something I've built..... I have stashed them away so I don't use them for normal stuff.

    Rant over :)
    Last edited: Sep 3, 2011
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Yes it's 0.39R. If it's used as a current sense resistor, 0.33R may change switching behaviour slightly as it will sense less current then there is. But it should work.

    However, if this one exploded, then there is probably something else defective, maybe the switching regulator or the main switching transistor.
  3. cowasaki

    Thread Starter Member

    Apr 3, 2011

    The PSU worked fine and gave the correct voltage before it was hooked up so it would appear that it is not happy about being connected to a load. All the capacitors are new and the two transformers are tested and working as are the rectifying diodes. I will stick this in and test it and if it doesn't work I will order the £8.90 PSU from hong kong! Analogue and PSUs were never my strong point!