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  1. #1
    Turned my router control box on yesterday to be greeted by sound of fuse popping. After a bit of poking around, it looks as if the toroidal transformer for the main power supply has failed. It's a dual-output 68V/5V unit, but with all transformer secondary windings disconnected, it still blows the fuse. It's been running very happily for around 3 years off a 5A mains fuse so when it starts taking out a 13A fuse, that seems pretty conclusive. I've no idea what the primary winding resistance of a transformer like this should be, although 2.5R seems a bit on the low side. Anyone have any idea of what a 500VA toroidal transformer primary should measure?

    Unless anyone knows better, a new transformer from Airlink seems to be on the cards. They don't appear to have an exact equivalent of the nominal 68V transformer in the existing power supply, so I shall have to go for a 2x25V or 2x30V. I'm currently using m752 drivers (nominal 75V max) although the new machine will be using EM806 (80V). I would measure the actual secondary voltage and output voltage of my existing supply but that's not really possible any more. Any thoughts? I do know that more volts is better, allowing a bit of margin for magic smoke territory. I'm using a 24V supply at present just so I can get a job done, and I've had to slow the machine down enormously, even though it wasn't that fast in the first place. The only good bit is that I have an old PC power supply in the box which just runs a case fan and supplies switchable 5V/12V for the cooling pump, so I can use that for the 5V needed for the BOB.

  2. #2
    I have a transformer here 500Va 24-24V and the primary is about 2.7 ohms are you sure that it is the transformer taking the fuse out?

    2 x 30V will be a bit high when it is rectified
    The more you know, The better you know, How little you know

  3. #3

    Everything I've read about toroidal transformers seems to say that the primary winding resistance is pretty low and mine is about the same as yours. I wouldn't trust my digital meter to fractions of an ohm! However, with just the transformer connected to the mains (all secondary windings disconnected) it blows the fuse. Maybe a shorted secondary? I've checked the two bridge rectifiers in the PSU and they look OK, the smoothing caps look OK (or at least, not short-circuit), and the secondary side fuses haven't blown. Points to a transformer fault to me, but there's still that nagging doubt...

    Anyway, I've ordered a 55+55V from Rapid who have a better price than Airlink, free delivery, and probably arrive sooner as well. Two windings in parallel and that should do the job, and only a couple of volts down on the original.

    Thanks for taking the time to measure your transformer for me - much appreciated.

  4. #4
    Try this cheaper than Airlink.

    Edit just seen you ordered one. 55V is a bit close on volts with 80V drive. 50v would be safer.
    Last edited by JAZZCNC; 03-01-2016 at 10:46 PM.

  5. #5
    Neale Are you happy with 55V as you are getting close to the limit as that will give you about 77V DC. Just checking.
    The more you know, The better you know, How little you know

  6. #6
    Did you remove the secondary's from the rectifier as a dud rectifier will cause the primary fuse to blow!


  7. #7
    Agreed on the point made in the last post - I'd disconnect both secondary windings and switch it on. If the fuse doesn't blow, then measure the voltage on each secondary. If that's fine and there's no connection between any of the windings (i.e. the insulation resistance is still good), then I'd suspect the load to be the problem...

    Quote Originally Posted by JAZZCNC View Post
    Amusingly, if you subtract the ~18% discount Farnell give to students and academics, you arrive at within 1 of the price Rapid sells those transformers for. Says something about Farnell's pricing strategy...
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  8. #8
    As mentioned, I've already tried the "secondaries disconnected" test. Loud buzzing from transformer for 3-4 seconds and then the fuse goes. I've now spent nearly as much on fuses during testing as a new transformer costs...

    I went for 55+55 as this was the closest to the existing 58V without going over. That's been running a set of three M752 drivers for around three years. While the theoretical offload voltage may be 1.4x that, I'm not sure what the actual voltage is with the three drivers permanently connected, even with motor current zero. I'll measure it and report back once the new transformer is installed, for those curious about such things (as I am - I probably measured terminal voltages when I first set the thing up but I can't remember that far back). Looking at the rectifier/capacitor board, the smoothing caps appear to be rated at 68V, which might also be a bit marginal...

  9. #9
    Quote Originally Posted by Neale View Post
    Looking at the rectifier/capacitor board, the smoothing caps appear to be rated at 68V, which might also be a bit marginal...
    Not Marginal it's well below. Want +20% of running voltage. 80V would be marginal.!! . . . . You've been a Lucky boy with Drives and Caps you can't be pushing the machine hard enough. .

  10. #10
    My mistake - the caps are actually rated at 100V, which makes more sense. What I remembered seeing was a label which happened to be stuck on one of the caps that said 68V on it - but this was actually the nominal PSU output voltage (from 58V transformer, according to what is written on it).

    As for pushing the machine hard - this is my "it's just about hanging together still" 3-year-old MDF machine that is normally flat out at 900mm/min. It's currently downrated to about 350mm/min on the temporary 24V PSU. It's a bit of a joke really but there are household jobs on the list that must be seen to be making progress...

    I'm suffering from the biblical plague of PSU problems at the moment - my electric razor won't hold charge, my laptop power supply went bang just before Christmas, the router PSU is blowing fuses, and this morning my tablet battery seems to be going on the blink

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