Problems with transformers

[txescientist]

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!

[Ian.M]

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.

[Gyro]

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.)

[Richard Crowley]

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.

[dom0]

Dear all

I put together a very simple power supply based on a "linear" transformer, a diode bridge and a cap.
Since I am meant to avoid noise, and ground loops, I need to shield the power supply the cable and all, and for that I tied the shield to the main earth line.

Problem is, the output being floating, it floats all over the place. If I measure AC voltage between the output and the shield, I read nearly  80Vac with respect to the shield, on the hot line and on the cold line as well. The output of the power supply is 60Vdc with no ac component whatsoever.

As such I'd like to prevent the AC stray voltage to reach all the way to the output, but I cannot tie the hot line nor the cold one to the earth line for various reasons, mostly related to the avoidance of ground loops and measurement issues when using a scope.

How can I solve this?

Should I put a 10+10 MOhm resistor series between the transformer leads and tie the center of the voltage divider to the earth line (thus before the rectifier)? 

Somehow it doesn't feel all that right to me.

Quality lab supplies have one (or more) electrostatic shields for this very reason in their mains transformer. This reduces the inter-winding coupling capacitance between primary (mains) and secondary (floating) to very low values, some transformers reach <1 pF.

You can either
(1) Shunt the secondary winding to ground with a small capacitor. This shunts some of the current flowing through the inter-winding capacitance to ground. On the other hand it increases ground-to-earth capacitance of the supply even further.
(2) Get a high-quality transformer with electrostatic shielding. These are quite rare used, but often go just as cheap as normal transformers. Obviously you can have a transformer custom made for you, but these shields cost extra and are quite a bit more expensive than a simple winding.
(3) Get a different transformer with less coupling capacitance. Standard toroidal transformers are very poor here (EI core with stacked windings are poor, too), while a EI core (or similar) transformer with separate winding chambers can be quite acceptable.
(4) Chain multiple transformers back-to-back. I have done this once for a supply, to decrease said capacitance and to increase insulation. Problem: Very uneconomic in time, space, cost and weight all at once, even for little power. So this is probably not an option for any lab supply.
(5) Cancellation by another transformer:

There's always the direct cancellation approach:  Use another small transformer (lowest VA possible) to provide a voltage opposite phase to the incoming mains, and of similar amplitude.  Couple that to output 0V via a carefully selected class Y1 capacitor to cancel as much of the stray voltage as possible.

[txescientist]

It is just static. Grounded the negative rail and it is working fine. Than you all for the fast replies, those were very helpful .