Problems with transformers - Page 1
Forum section: electronics / beginners

author: txescientist
author_rank: Regular Contributor
timestamp: February 18, 2016, 10:44:06 am
content: Hi,After several years of using my bench psu, today I have noticed that the output of my toroidal transformer is floating at 80V AC above ground because ground terminal is not connected to mains ground (floating psu). Measured one EI trasnformer and metal frame is also floating at ~60V AC, but the low side output is at 1-2V respective to mains ground. That's not a problem, because the transformer core should be grounded, and the secondary side is ok, but i think that my toroidal transformer is faulty. What do you think?Is there a way to protect the output of a floating psu in case of the faulty transformer (failed isolation between primary and secondary side)? This is not a problem for a mains grounded minus (DC ground) terminal on non floating psu.Regards!

author: Ian.M
author_rank: Super Contributor
timestamp: February 18, 2016, 11:13:26 am
content: Large AC common mode voltages on a floating transformer secondary are not at all uncommon due to capacitive coupling to the primary. I would suggest isolating the transformer and doing a 1KV DC insulation test between the primary and secondary. That will give you confidence the insulation hasn't been damaged by overheating, impact or excessive clamping pressure.The only totally effective protection against a primary to secondary fault in a floating output linear PSU is to specify a transformer with an inter-winding screen that will be grounded. Any primary insulation fault will then blow the fuse. However secondary insulation failure is much much harder to detect, and would require a low voltage RCD between the transformer and the bridge rectifier.

author: Gyro
author_rank: Super Contributor
timestamp: February 18, 2016, 01:27:06 pm
content: It's quite common for toroidal transformers to have higher capacitive coupling between the primary and secondary.An EI transformer has an exposed core that can be grounded, and normally the primary and secondary are wound side by side on a split bobbin. By contrast the toroidal transformer has primary and secondary wound one on top of the other, with insulating tape layers between, hence the greater capacitance.In all probability the toroidal is fine. I'd suggest a resistor from one output to ground (say, 10k) and measure the AC voltage across it. This would be safer initially than directly measuring with a meter on current range. You should hopefully find that the leakage current is minimal, i.e. just capacitive coupling.P.S. Some toroidals include an inter-winding screen that can be grounded but it's not that common (normally custom ones for instrumentation etc.)

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 18, 2016, 02:03:27 pm
content: Remember that when you measure things like mains leakage voltages/currents, it is not valid to measure with a very-high impedance instrument. Because that doesn't represent Real World conditions. I believe there is a specified resistor and capacitor load network which is the standard for such measurements.

author: dom0
author_rank: Super Contributor
timestamp: February 18, 2016, 03:38:11 pm
content: Quote from: hibone on December 09, 2015, 06:12:07 pmQuote from: dom0 on December 09, 2015, 07:16:17 pm

author: txescientist
author_rank: Regular Contributor
timestamp: February 18, 2016, 07:49:16 pm
content: It is just static. Grounded the negative rail and it is working fine. Than you all for the fast replies, those were very helpful.