power Schottky rectifier good or dead? help

Discussion in 'General Electronics Chat' started by daveyjones97, Nov 19, 2011.

  1. daveyjones97

    Thread Starter Active Member

    Oct 21, 2011
    hi, im trying to fix a razor e300 motor controller, please see the post on that.
    people tell me that the schottky diode is a likely suspect so i attempted to remove it from a dual layer board by using a heat shunt and laying the flat edge of my soldering iron across all 3 pins. the solder was melting but the bugger wouldnt come out. tried a few more times until the diode was too hot to hold. by then i figured it was trashed but thought id try for confirmation and advice on how i should have removed it. in the end i just broke it off by rocking it back and forth. my multimeter was on 20k ohms. when the red was on the central cathode the meter measured open circuit on both anodes. when the black was on the cathode both the anodes measured 4.50 approx.
    as an ameteur i need it confirmed that this is a good (STPS3045CT - a power Schottky rectifier) and how hot i can get it and methods for removing it? to reattatch im going to try and resolder it from the top side of the board.
    is the mosfet the next likely suspect in a motor control circuit that cuts out at half load approx?
  2. daveyjones97

    Thread Starter Active Member

    Oct 21, 2011
    oh, im aware snapping parts off in fustration is not a recognised method of circuit board disassembly...
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    If you need just that one part, you hold it in some large pliers to cool the diode and heat the crap out of the board until it lets go. Solder pen, hot air heatgun, propane torch....

    The most tricky part to desolder I have ever seen was a slider potentiometer from a mixer. The signal traces were small, but the biggest trouble were the four large thick pins soldered into thick plated vias which lead straight to a large ground plane on both sides. That took some time and brute force, but yet again I needed to place there a new pot, not save the old one..
  4. jimkeith

    Well-Known Member

    Oct 26, 2011
    Actually, cutting the pins and removing them one at a time is a good method that helps prevent circuit board damage. Sounds like you may need a hotter soldering iron. Also, this may be soldered with lead-free solder--really hard to repair--to reduce the melting temp a little, blob some real lead containing solder on the joint--helps a little.
  5. daveyjones97

    Thread Starter Active Member

    Oct 21, 2011
    thanks guys, i made sure the iron was as hot as it could get by leaving it on for 5 10sec bursts before laying it across all 3 pins, to try not to heat the component more than neccessary. if was hotter than anything ive soldered before, hence giving in and breaking it off rather than heat it til the magic smoke burst out of it...
    as far as i can tell from the net and my multimeter this schottky diode is actually ok? what to test (destroy?) next? that mosfet is giving me funny looks.
  6. crutschow


    Mar 14, 2008
    Sounds like you are using solder gun which is not good for PCB work since the heat is variable and uncontrolled. You need to use a soldering iron, preferably temperature controlled.

    If the forward and reverse resistance of the diode are significantly different, then it's probably OK.