Isolation transformer?

[Hammy]

I have a 450VA isolation transformer. The transformer is supplied from
a three prong outlet. I've taken some measurements with my DVM they
show a voltage between lines to ground on both outputs. The combined
voltages between both lines to ground don't even add up to the line to
line voltage.

Heres a schematic of the measurements.

http://i32.tinypic.com/x4ns47.png

I just got this transformer but I've already used it to test a few
line powered circuits. These measurements must be wrong or I would
have blown fuses when hooking up my scope to the circuits.

I use the isolation transformer on the test circuits not on my scope.
For example if I were to hook up a flyback to the isolation
transformer and use my scope (un-isolated) to take primary side
measurements according to those measurements I should be getting a
short through my scope ground lead. This hasn't happened both the
isolation transformer and the flyback are fused; neither blew.
Can anyone explain this?

 

[Paul E. Schoen]

I have a 450VA isolation transformer. The transformer is supplied from
a three prong outlet. I've taken some measurements with my DVM they
show a voltage between lines to ground on both outputs. The combined
voltages between both lines to ground don't even add up to the line to
line voltage.

Heres a schematic of the measurements.

http://i32.tinypic.com/x4ns47.png

I just got this transformer but I've already used it to test a few
line powered circuits. These measurements must be wrong or I would
have blown fuses when hooking up my scope to the circuits.

I use the isolation transformer on the test circuits not on my scope.
For example if I were to hook up a flyback to the isolation
transformer and use my scope (un-isolated) to take primary side
measurements according to those measurements I should be getting a
short through my scope ground lead. This hasn't happened both the
isolation transformer and the flyback are fused; neither blew.
Can anyone explain this?

If you want to do a simulation, add a small capacitor from the primary to
the secondary of the transformer, and remember that your meters have about
10 MegOhm resistance. The transformer has some capacitance from primary to
secondary due to the overlapped windings and the insulation.

A really good isolation transformer (such as medical grade) will have
separate bobbins for primary and secondary, as well as electrostatic
shielding connected to ground, which will greatly reduce this capacitive
coupling.

You can get an idea of this leakage current by putting a 1 k resistor
across your voltmeters. You might read a couple of volts, which indicate a
couple milliamps of leakage, which is nothing to be concerned about. The
threshold of sensation is about 2-5 mA, and dangerous levels are generally
20 mA or more. But you really should not be touching any part of the 120
VAC secondary circuit.

Paul

 

[Hammy]

If you want to do a simulation, add a small capacitor from the primary to
the secondary of the transformer, and remember that your meters have about
10 MegOhm resistance. The transformer has some capacitance from primary to
secondary due to the overlapped windings and the insulation.

Thanks. I was concerned about my scope when I seen those measurments.
When you start paying for your own instruments you stop takeing things
for granted.

A really good isolation transformer (such as medical grade) will have
separate bobbins for primary and secondary, as well as electrostatic
shielding connected to ground, which will greatly reduce this capacitive
coupling.

I'm using an inexpensive TEMA model. Safe to say not in the medical
grade catagory.

http://canada.newark.com/97C7982/power-products/product.us0?sku=TENMA-72-6670&_requestid=122388

 

[Jamie]

I have a 450VA isolation transformer. The transformer is supplied from
a three prong outlet. I've taken some measurements with my DVM they
show a voltage between lines to ground on both outputs. The combined
voltages between both lines to ground don't even add up to the line to
line voltage.

Heres a schematic of the measurements.

http://i32.tinypic.com/x4ns47.png

I just got this transformer but I've already used it to test a few
line powered circuits. These measurements must be wrong or I would
have blown fuses when hooking up my scope to the circuits.

I use the isolation transformer on the test circuits not on my scope.
For example if I were to hook up a flyback to the isolation
transformer and use my scope (un-isolated) to take primary side
measurements according to those measurements I should be getting a
short through my scope ground lead. This hasn't happened both the
isolation transformer and the flyback are fused; neither blew.
Can anyone explain this?

Your seeing the effects of the over laid windings which creates a
capacitance coupling.
Try placing a load on the output and then test it.


http://webpages.charter.net/jamie_5"