IsoTransformers

Status
Not open for further replies.

Electroenthusiast

Active Member
How are they designed?
I know that these transformers have 1:1 turns ratio. How exactly do they limit the current? and how are they useful in driving sensitive electronic devices(microphones)? Do they work on DC (Transformers working on DC!)?
PHP: 
[I]An isolation transformer is a device that provides DC isolation from the power grid (or mains). At the same time, it provides an AC connection to couple in the power that you've connected to the grid to get. Recall that the power grid is an AC (alternating current) source, and we can use transformers to couple AC from one circuit to another while providing DC (direct current) isolation.

An isolation transformer is often a 1:1 device that neither steps the supply voltage up or down, but just couples it in. In most cases, the transformer will have a lower current output than the AC main circuit itself, but provides that DC separation from the main AC source. This will allow the devices or loads to `float`in a sense, and by doing that, references to earth ground can be eliminated. [/I]

Reference: http://wiki.answers.com/Q/Operation_of_an_isolation_transformer
 

hi RV,
Where does it say 'limits the current'.??
 
Hi eric,

If not, then how do they WORK?

Isolation transformers are not just for isolating electronics from mains voltages. it is also useful for separating parts of a circuit so that power is transmitted while protecting sensitive electronics. Chips and sensitive electronics require a small voltage; anymore can destroy the chip. In contrast, motors, solenoids, relays and many other components require larger voltages. These circuits may need to interact, and the device that protects the more sensitive components is the isolation transformer.

Reference: How Do Isolation Transformers Work? | eHow.com
 
Last edited:

hi,
Your post references 'Mains' isolation transformers.

Consider you have 230Vac mains supplies and you wind a mains transformer for 230Vac in and 230Vac out, ie; 1:1 turns ratio.

Assuming a 100% efficiency transformer, on load, say the primary current was 1amp, the secondary current in the load would be 1amp.
So there is nothing 'limiting' the current.

Also assume your mains supply NEUTRAL line is connected to Ground/Earth, which it usually is at the alternator.
If you touched the LINE/PHASE terminal on the transformer primary, you would get an electric shock, LINE thru you to Ground.
If only touched the LINE or NEUTRAL on the transformer Secondary you should not get a shock, because there is no direct return path thru your body to ground, hence Isolation.

I would add, usually on a good isolation transformer there is a metallic screen between the primary and secondary windings, which is grounded.
This screen provides capacitive isolation between the windings, on bigger transformer an unscreened one has sufficient interwinding capacitive coupling to give a shock.

If you ever [dont] touched the secondary LINE and NEUTRAL at the same time, you would get a serious shock.
 
hi RV,
If not then How do they WORK?


IMHO, this description of the purpose of a simple isolation transformer is pathetic.!
 
Thanks that was helpful...
So how would be used to guard sensitive electronic appliances(as above)?

I suppose, Power generated by Alternator is transmitted through series of transformer.
Power generated is at several kV, and is then stepped down to 230V. So why wont they work as isolation transformer?
 
Last edited:
r.vittalkiran said:
Thanks that was helpful...
So how would be used to guard sensitive electronic appliances(as above)?
Click to expand...

It doesn't.

I suppose, Power generated by Alternator is transmitted through series of transformer.
Power generated is at several kV, and is then stepped down to 230V. So why wont they work as isolation transformer?
Click to expand...

Because the neutral is connected to ground at the sub-station - all an isolation transformer does is remove that ground connection - giving you two wires, with voltage between them, neither neutral or live.
 
There are two types of isolation transformers:
Great big ones for isolating the mains, and little tiny ones for isolating signals.

Dont mix them up, if you drop one on the floor, if it is a mains isolator it could smash every bone in your foot, if it is a signal isolator you will have to search through the junk (if your workshop is untidy!)

JimB

PS, I concur with Eric:
IMHO, this description of the purpose of a simple isolation transformer is pathetic.!
Click to expand...
 

Hi again,
I need a clarification for this.
Whenever a use a tester(screwdriver tester) to test the mains voltage, it glows! Even when the current is drawn from Inverter(UPS). As said above, what will happen in case of UPS? 'UPS WILL NOT GROUNDED!'
 
ericgibbs said:
...
So there is nothing 'limiting' the current.
...
Click to expand...

Except core saturation in the transformer. Try drawing 100VA from a 1:1 power transformer that rated at only 10VA.

Audio isolation transformers are frequently used to eliminate a "ground-loop" between disjoint pieces of equipment (i.e. mic preamp to an audio power amplifier), which is a totally different application of "isolation transformers".
 
Last edited:
MikeMl said:
Except core saturation in the transformer. Try drawing 100VA from a 1:1 power transformer that rated at only 10VA.
Click to expand...
It's a common misconception that high AC load current will saturate a transformer. It won't. A transformer core will be saturated only when there is a DC component to the current through it, or the voltage/frequency magnetizing value is such that the magnetizing current exceeds the core rating. Any AC signal current through the transformer primary is balanced by the opposite current flowing through the secondary, thus the magnetizing fluxes oppose each other and are canceled. (Think of the current directions through the transformer windings based upon the winding polarities. Current goes into the plus primary winding, but goes out of the plus secondary winding, thus leaving a zero net current flux).

The only thing that happens when you overload a transformer with too much current is that the IR winding drop will overheat the transformer. Thus a higher power transformer requires a larger core to accommodate larger wire, not because more core material is needed to prevent saturation. If you could wind a transformer with ideal superconducting wire, then it's current rating would be unlimited.
 
So Carl, have you ever taken apart a wall wart transformer? The kind that has a one-window core where the primary is on one side of the core, and where the secondary is on the other side (windings are not interleaved, or wound on the same bobbin). That type of transformer is designed specifically to prevent a fire in the event of a secondary overload, and core saturation is the means by which the current in the secondary is limited...
 

This is easy to test.

Let the flux in the core be Φ; then an elementary physics text will tell us that the voltage seen across a winding on the core will be dΦ/dt.

Take a small transformer, heavily load it and look at the voltage across a winding, integrating the voltage across that winding ( look at ∫ Φ dΦ/dt ). The integration can be performed by a simple RC integrator, or by a scope with integration as a math function, for example.

We must use a transformer with two secondary windings so we can heavily load one of them, and use the other, unloaded, to monitor voltage without that voltage being contaminated by the IR drop due to the large current in the heavily loaded secondary.

If we excite the primary with a sine wave, then the integral of the unloaded secondary voltage will give us a signal proportional to the flux in the core. This voltage should be a sine wave except when saturation occurs; when saturation occurs, the peaks of the sine wave will be flattened as the flux reaches a maximum.

The grid voltage in most parts of the world these days already has peaks that are somewhat flattened, so one will have to see if there is additional flattening to detect saturation. Or a perfect sine wave from an audio power amp could be used to excite the transformer.
 
Mike, the primary and secondary are wound on separate bobbins for safety so that you can not ever get a wire short from primary to secondary and have line voltage on the secondary. Whether they are wound on separate bobbins or the same bobbin has no significant effect on the resultant flux in the core.

I believe wall wart transformers are current limited simply by the resistance of the winding wire.

When you design a transformer, voltage and frequency are the only two variables that determine the number of turns required to avoid core saturation (Basically you need to keep the excitation current from the primary inductance below the point of core saturation). Load current only enters in when you need to determine the size of the winding wire.
 
MikeMl said:
AFIK, they have an air gap, too...
Click to expand...
An air gap in a core is normally used to prevent saturation in DC current inductors.

In any event, saturation of an AC transformer core would significantly increase primary current, not reduce it. That's why you never want a transformer core to saturate.
 
Ordinary line frequency transformers are usually designed to run somewhat into saturation with no load. When they are loaded, the flux density in the core reduces slightly due to the IR drop in the resistance of the primary winding. Contrary to the often heard opinion that heavy load will saturate the core, quite the opposite is true.

I've attached a couple of scope captures showing the B-H characteristic of a small (1 amp @ 12 volts) transformer in operation. This is a better method of observing core saturation than the technique I described in an earlier post.

The first image shows the hysteresis loop with no load. Observe how far the loop extends to the upper right and lower left.

The second image shows the loop with a load of 3.5 amps applied. Notice how the loop extremities have shrunk in the horizontal direction, indicating that the flux density in the core is slightly less. The loop has become a little fatter because the copper loss has increased under load.

The current in a small transformer (wall wart) is definitely limited by the resistance of the windings. However, if the construction method which separates the primary and secondary is used, the leakage inductance will be greatly increased, and that will also serve to limit current, perhaps more than the winding resistance.

Current limiting by the resistance has the effect that heating of the windings may be sufficient to damage the transformer. Limiting by means of leakage inductance has the advantage that there is no heating in the leakage inductance.

The transformers that power neon tube lighting are current limited and are designed to have a very large leakage inductance for that purpose.

Not all wall warts are current limited; many just have a thermal cutout to prevent a fire.

At any rate, saturation in the core doesn't occur under overload conditions, and that is not a mechanism to limit current.

Also, a gap in the core won't limit current either, and is not desirable in a transformer.

Core saturation can occur if DC is passed through a winding, but that's another matter.
 

Attachments

  • NoLoad.jpg
    9.7 KB · Views: 105
  • HeavyLoad.jpg
    9.7 KB · Views: 109
Last edited:
Status
Not open for further replies.
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…