Isolation transformer .. #2

Thread Starter


Joined Dec 1, 2016
This question does pertain to isolation transformers and to an aspect mentioned in this thread but not I was not sure if you wanted me to post this question here or elsewhere.

My question:
Why is there a time delay on actuating the power to the primary of the isolation transformer? Why would
anyone have such a delay on the secondary?

My isolation transformer was originally a hospital isolation transformer. I want to build circuits but have
been afraid to use my new expensive (relatively) scope from my ignorance and mistakes. Hence I bought
this nice used hospital isolation transformer and altered it to protect my new scope.

i removed the PRIMARY ground connection to the secondary and the ground to the SECONDARY outlet ground pin
AND i removed the ground pin clamp as the ground assembly was part of the assembly which held the outlet to the
chassis of the isolation transformer (I had retained the ground to the chassis of the isolation transformer).

I apologize for adding all this miscellaneous info. I just wondered if my new use could take advantage of the
delay timer.

Thank you folks for taking your valuable time to help others (like myself).

William h Burling

Thread Starter


Joined Dec 1, 2016
I am disappointed with Wikipedia.

In my opinion their explanation is both flawed and potentially dangerous.

In electrical terms 'to isolate' means 'to disconnect from'.

An isolating transformer isolates the primary circuit from the secondary circuit.
That means they are electrically insulated from each other. Their only linkage is through their common magnetic coupling.

This is required for safety rather than signal reasons. It limits the effect that a fault condition in the primary (secondary) can have on the secondary (primary). It also means that the maximum current that may be supplied to the secondary circuit can be limited to a safe value as is done in bathroom shaver sockets. This limit is imposed by the magnetic characteristics of the transformer core.

Most transformers are used in this way and are, in fact, isolating transformers. This applies from large megawatt transformers in national power grids, to transformers supplying individual items of equipment in your home or workplace, to sniffer loops connected as inputs to electronic instrument.

Transformers that are not isolating may be simple autotransformers with only one winding. Here the secondary is actually a tap on the primary so there is clearly no possibility of electrical insulation or isolation. Bench variable transformers (variacs) are of this type.

Or the transformer may have two (or more) separate windings, but the designer (constructor) of the equipment has grounded (earthed) one end of both the primary and secondary windings. Since there is now a common electrical connection the isolation is lost.
Communication is incredibly different as a human tend to think the world thinks and addresses the issues as he/she does.

The word "isolation" means very different things to people who work in very different environments. You probably work in an environment in which safety to humans is of direct concern. In a hospital, there is so much interconnectedness, "safety" is not
only evaluated on an individual basis but also from a more broad point of view. Ie: a surgeon operating on a patient does not
want his equipment to suddenly be de-powered by a fault in equipment of one of his supporting attendents. It is my understanding (I could be wrong) that hospital isolation transformers are not there to electrically isolate the load from the primary but to isolate the secondary from the primary. Ie: a failure in the filtering in the circuit attached to the secondary coil
of the isolation transformer might lead to a "ground fault" condition. The hospital isolation transformer offers a way to isolate
the primary side (side that a few of the other operating room workers have access to) from failures of equipment on the secondary side.

This is why marriages and other relationships are so difficult. we strongly believe we know the meaning of a word, when in fact, we only know the meaning from a single perspective.


Joined Mar 14, 2008
I just wondered if my new use could take advantage of the
delay timer.
What type of delay timer is it?
What does it consist of?

If you referring to the 2A time-delay fuse in the TS's isolation transformer, that's also known as a slow-blow fuse, and only provides a delay when it blows, not when you turn the power on.

Thread Starter


Joined Dec 1, 2016
What type of delay timer is it?
Infitec TMS5.25w low cost delay-on-make timer. Spst NO solid state
INTENDED operation: upon application of power, timer begins. Upon completion of timing period, output energizes.
What does it consist of? Not sure i understand. It appears to be a timer and spst no solid state switch.
If you referring to the 2A time-delay fuse in the TS's isolation transformer, that's also known as a slow-blow fuse, and only provides a delay when it blows, not when you turn the power on.
see mfr spec. This timer appears to work
Upon every application of power and once timer load is energized, power is maintained until power is lost or turned off

thank you for responding


Joined Aug 27, 2009

Thread Starter


Joined Dec 1, 2016
The delay is for a soft-start circuit most likely. Due to core remanence and the magnetization current of the iron you can easily have a half-cycle or more that will trip the normal breaker circuit due to excess inrush current. Some have NTC thermistors for the current limiting effect.
YES !!!! Thank you !!!!! Wow !!!

You made me go back to look more carefully at the actual components. You are so correct.
There is a MS35 10018 inrush current limiter made by Ametherm across the contacts of the relay
that actually controls the current to the transformer. I am not sure why an inrush current should be
expected with a huge winding (i would expect a large inductance. is 2 H large?)

But a problem with an inrush current limiter based upon a thermister design is that the limiter consumes
current for the entire duration of equipment operation thus creating a voltage drop.

By including a delay timer.....for the duration of the delay, the inrush current limiter is determining the current
available to the transformer primary winding. Then after the delay the relay is actuated. As soon as the relay
is actuated the inrush current limiter voltage drop is shorted removing the voltage drop from the circuit and
reducing the current through the thermister to almost zero.

now....if i can understand why there is an inrush current.....I will read your they address this scenario.
Thank you so much....I am very grateful.

Brilliant. Love it.