Failed toroidal transformer

[Clive S]

Neile
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

[Neale]

Clive,

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.

[Clive S]

Neale Are you happy with 55V as you are getting close to the limit as that will give you about 77V DC. Just checking.

[ptjw7uk]

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

peter

[JAZZCNC]

Try this cheaper than Airlink.

http://uk.farnell.com/multicomp/vtx-...25v/dp/1675093

Edit just seen you ordered one. 55V is a bit close on volts with 80V drive. 50v would be safer.

[Jonathan]

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...

Try this cheaper than Airlink.

http://uk.farnell.com/multicomp/vtx-...25v/dp/1675093

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...

[Neale]

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...

[JAZZCNC]

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. .

[Neale]

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

[Clive S]

Neile I am a bit concerned here. You say the o/p is 68V DC that equates to a 50V (roughly) AC transformer. Does your transformer actually say 58V on it!!