ECN Forum Forums Technical Discussion Electrical Theory and Applications Isolation Xfrmr: Floating Secondary

I think I'm gonna go for the "soft-ground" concept that JoeTestingEngr suggested, with a little indicator so I know if/when the secondary grounds-out.

Thanks for the helpful replies, gentlemen. I appreciate it.


-John

You're welcome John. Please forgive me for lusting after that transformer of yours. You might be able to use an unused primary to rig up a "press to test" function for your ground detection/clamping scheme. If you use a very small fuse and a current limiting resistor, you can test the function, and not just the lamp. I know that it might seem like overkill but since we're in a theory forum, might as well brainstorm.
Joe

John, One thing to consider is the fact that many indicators require more current than you are planning on sensing. For instance, you originally mentioned limiting your ground current to 1mA. An LED is normally biased at about 10 times that. Most lamps will be very dim if you're lucky at 1mA. So I was thinking that you might just try putting a neon bulb at your center point reference. I seem to recall that neons conduct at about 60 volts. Neon lamp assemblies contain current limiting resistors and flow currents more on the scale you are working with. A "T" network of 100ks across the top with a <100k (TBD) on the bottom going to ground through a neon might hold the winning ticket
Joe

[This message has been edited by JoeTestingEngr (edited 02-16-2006).]

And don't forget to do an insulation test on the windings.

primary to earth.
secondary to incoming earth.
primary to secondary windings.

In case of a 110 Volts transformer use 250 Vdc.
In case of a 240 Volts transformer use 500 Vdc.

A Megger reading at least gives you an idea of the overall condition of the insulation materials used in the TF.

Should be 50 M.Ohms or better for a 240 Volts TF.
Chhers Ray

I don't know what are you planning to do, but why do you want a floating secondary?

Regards,

Joe.-

I don't know what are you planning to do, but why do you want a floating secondary?

Regards,

Joe.-

Joe,
Things can get very exciting while troubleshooting TVs or other appliances with a hot chassis. The scope probe ground lead is at ground potential and the chassis ground isn't. Sparks will fly if you try to connect the two. The best bet is to float the appliance. I didn't have an isolation transformer at home. My scope had an input transformer and three prong plug. I would use the 2 prong adapter to deliberately float my scope, since I couldn't float my TVs. I realized that I had a shock potential if I touched earth and the scope at the same time. It's much safer to keep your scope grounded and float the appliance.

That said, I always liked the saying, "Whatever floats your scope."
Joe

[This message has been edited by JoeTestingEngr (edited 02-22-2006).]