View Full Version : Failed toroidal transformer

03-01-2016, 05:49 PM
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.

Clive S
03-01-2016, 10:20 PM
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

03-01-2016, 10:40 PM

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.

03-01-2016, 10:43 PM
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.

Clive S
03-01-2016, 10:49 PM
Neale Are you happy with 55V as you are getting close to the limit as that will give you about 77V DC. Just checking.

03-01-2016, 10:58 PM
Did you remove the secondary's from the rectifier as a dud rectifier will cause the primary fuse to blow!


03-01-2016, 11:08 PM
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.


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...

04-01-2016, 08:10 AM
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...

04-01-2016, 11:47 AM
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. .:playful:

04-01-2016, 12:08 PM
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 :sorrow:

Clive S
04-01-2016, 12:47 PM
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!!

04-01-2016, 01:24 PM
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!!
The PSU in question was bought from Zapp and is the PS806-5, nominal outputs 68V and 5V. They don't list this one at the moment but it is probably near-identical to the current PS806-12, the only difference being that mine has an auxiliary 5V output and the -12 has a 12V auxiliary. The listing and the PSU itself both say 68V DC. However, looking more closely at the transformer, I might have made a boo-boo. It has a tapped secondary with both 58V and 48V outputs. I know that only two of the three transformer secondary connections are in use and without thinking this through properly, I assumed that these were the 58V connections. However, the 48V AC would indeed give a peak voltage of around 68V off-load. I would check the colours of the tap wires in use but they are in Chinese... I'll try checking with an ohmmeter later but measuring slight resistance differences in a low-voltage high-current secondary might not be conclusive.

In the meantime, Rapid are living up to their name and the new transformer is already in transit. Might be sending it back for exchange...

04-01-2016, 01:56 PM
Rapid are trying to stop delivery of previous order, and I need to order the correct item.

So, it is the view of the committee that I should get a 50V transformer (nominal off-load 70V DC) or drop to 45V (nominal off-load 63V DC)? I'm currently using drivers rated at 75V but shortly moving to 80V EM806. Would the lower voltage be noticeable in practice? It's certainly the safer option but I don't have enough experience of these things to know whether this will matter.

Clive S
04-01-2016, 02:20 PM
Neile Yes it will be too high and not worth the risks for the drive. I obviously did not get myself across sorry about that. 24 - 24V would be better or 48v or 50V secondary

edit just read your last post 24-24 or 48V is what you need.

05-01-2016, 09:05 PM
New transformer fitted and all working again. I bought a 2x45V 625VA toroidal from Rapid Online. Off load, it gives 50V AC per winding, partly because it has a 230V primary and my mains measures (today, anyway) 241V, and partly, I assume, to allow for voltage drop on load to bring it down to the nominal output value. Off load and connected to rectifier/capacitor, it gives around 68V DC. I haven't measured DC voltage on load as this is somewhat difficult to do, but I did connect the control box to the mains via a power monitor, just out of interest. I have two NEMA23 motors, one driving the relatively light MDF gantry and one for Y. Max load while cutting (light cuts in some old oak) and 500mm/min was around 120W, full rapid speed (but that's only 900mm/min) around 160W. Quiescent (nothing moving) is around 40W. I don't think that the transformer is being over-stretched, but the intention is to move this to Mk2 router when complete and it's been sized more for that.

Anyway, I thought that a few actual measurements might be of interest.

Clive S
05-01-2016, 10:45 PM
Neale glad you managed to change it, happy cutting