Friday, 25 September 2009

Second term at boarding school!

Just when you think you've got everything settled..........

.....My brother pointed out that just because I wasn't measuring 5V out of the voltage regulator whilst it was connected in circuit on the stepper motor driver board didn't mean it wasn't working; with a short-circuit anywhere in the 5V section of the circuit, the current would flow straight to ground, meaning no voltage to measure. However, there were no obvious shorts, no solder bridging two components and I had previously checked that all the resistors apart from the 0.25 ohm ones were sound. After Nophead's advive on checking low value resistors, I noted that my multimeter read 0.5 ohm when the probes were touched together on the lowest resistance range setting, 200 ohm. The resistors gave a reading of 0.8, meaning they are rated at ~0.3 ohm, which is about right allowing for slight inaccuracy of the meter at such low levels compared with range setting.
In circuit, there was no reliable way of measuring the capacitance of any of the small capacitors, so I was going on guesswork that the problem still lay, in fact, with the regulator.

I desoldered one pin on the regulator, and then the other.
I am getting on fine now using the solder braid with flux drawn onto it from a pen. This helps the solder to reflow. I placed the braid onto the top of the pin, acrosswise if possible, and applied the soldering iron tip on top of the braid for a few seconds. I found it easiest to position the braid and hold it in exactly the right orientation, with no twisting etc, with my hand, but once held in place by the soldering iron, I swapped to holding the braid with tweezers so that I didn't burn my hand when it got hot. Access was awkward, and I decided to desolder one of the RJ45 connectors to reduce the amount of melting and burning plastic where the large screws on the soldering iron shaft would catch! I still caught the edge of the Molex connector from time to time. I cut off the used end of desoldering braid holding the absorbed solder and repeated with clean and freshly-fluxed braid until the regulator's pins looked to be clear of solder. The pins still seemed tacked to the board, possibly with sticky burnt flux, so I cleaned around them and prised them up off the board. The pads below are fine.
Desoldering the through-hole pins of the connector was easier, still, after cutting the end of the braid into a point and using the fine iron tip, pressing the braid slightly down and into the hole as the solder wicked up.

Next I tried to desolder the regulator's large ground pad - that was just too tough! I managed to remove most of the solder, but there was some visible under the centre of the pad, but the gap between the part and the board was too thin (top to bottom) to be able to slide the desoldering braid in, and although I could melt some of it, because of the proximity of other components I couldn't use the big flat soldering iron tip, which would have been ideal to melt the lot all at once and allow the regulator to be released.
Eventually my partner went for the brute force option! and created more damage than was intended! Some of the board's contact pad lifted clean away, but the remainder still has contact to ground, and there is damage to the green coating over the track that leads from the 5V pin adjacent to the side of the regulator. There still seems to be continuity everywhere (there should be), though, so I have Araldited a thin protective layer over the shiny copper that is visible.
We tested the removed regulator - it is most definitely dead! Luckily, the minimum order for these had been 5, when I (thought I) needed 3, so there are a couple of spares after all (which is a relief because this was the part that came from RS, who were absolutely useless, and from whom I have no wish to order again)!

My brother rang just as all this was done, and said that I could test this board before putting another regulator on (just in case there is still a short on the board as well) by connecting the working board to power, and connecting a 5V contact on that board to a 5V contact on this board, and a ground contact on that board to a ground contact on this one. The regulator on the first board should easily be able to handle the current for both boards together. At least the power LED should come on for both boards. And if I do Nophead's thing of getting the other LEDs to work, too, I should get proper indication on both boards as well if this board is working. I need to look that up again, but meanwhile I'm letting the Araldite cure.


  1. wow. Sounds like you're having fun!

    Thanks for the detailed information. It's nice to have someone describing all the details. I've had to pack my stuff away for now, as we're trying to sell our house and move to a larger one - with a small workshop for my RepRap - yay!

    When I get my 'Rap working, I definitely need to upgrade to gen3 electronics, and eventually to a Mendel!



  2. To remove componenents with a large tab like the regulator. First clear the smaller connections so you can slide a piec of paper umder the pins and leave it there then you need a pair of pliers to grip the plastic body of the regulator. Heat the tab this time adding a little solder you will soon feel the device move and you can remove it.

    You then can clean up the pad that will have the extra solder using solder braid or a solder sucker (Maplin)I dont use solder braid very often I use a small solder sucker for SMT or a large solder sucker for things with bigger pads.

  3. Dave, shame you can't build and won't be blogging for a while then. Hope you sell/move quickly and smoothly. How did you manage to wangle agreement to have a dedicated workshop?!

  4. BodgeIt,
    I looked everywhere for advice on how to remove this part, and failed to find anything specific to large pads! So, thank you for this useful information.