Using a DC Isolation Transformer

The purpose of isolation transformers is to isolate the power supplies of the target signals from the power supplies in the measuring equipment. They are probably there for a good reason. You can bypass them, but the odds of anything good coming from it are low, and the odds of something bad happening are high.

Possible bad happenings are:

Degradation of your measurements due to noise, and/or circulating ground currents.

Destruction of either your sensors or your meters, due to high common mode voltages.

We do some of this in the plant I work in. We do not use isolation transformers for 4-20mA. We used to for 120vac and now it really seems we spent alot of money for very little return. VFD's are noisey of course by nature but if well grounded and if a shielded(one shield end landed) cable is used I highly doubt there will be an issue.

We have a panel of ten little Allen Bradley 160 vfd's whose speed signal is controlled this way and it has been in place better than 5 years now. Correct grounding and shielding seems to be the key.

If it is working ok leave well alone.

I think 1,2,& 3 are correct. A well designed and installed system can have low noise and no stray currents. However it's possible that in the future someone (someone else, of course) may not be as careful, causing all sorts of problems. So in a real world factory it's best to over do the isolation , grounding and sheilding, it'll cost as a bit more but you'll get more sleep. Jeff

Any panels I design and/or build anymore that has a VFD or Servo gets a galvanized back panel to ensure full grounding of the components. Looks better than a bunch of grind marks through the paint.

What is the point using a transformer instead of a capacitor on the first place?

Is it the high voltage difference or a complete line isolation between the two different circuits?

A capacitor blocks DC, but maintains the same reference (ground) potential. A transformer completely decouples both the signal, and the reference. The voltage offset between the primary and secondary is limited only by the breakdown voltage of the insulation on the windings.

This is not DC Isolation Transformer but Isolated Power-Supply; used where more than 1 such Ccts are required; such as GDA s Gate-Drive-Amps used in Inverters; UPSs; Drives of all types. Because all the pionts of Feed of Signal are at different potentials ; you have to keep those all well isolated galvanically.

In control panels I don't have any experiece.

Dear Friend,

Pls. be clear in your question. First & foremost, there is NOTHING LIKE DC ISOLATION TRANSFORMERS. All transformers operate on AC. What you have used is a transducer which provides isolated 4-20 mA proportional to speed or current, as the case may be.

This signal is necessary if you are using the system in a closed loop system. In your case, as you have mentioned, you are only using it for indication purpose. Thus if you donot need to monitor the current or speed, as the case may be, you can safely keep it open. Personally I feel that if it is not interfering elsewhere then you can safely let it be as it is.

No... The transformer is ALWAYS a d.c. isolation device as it blocks the d.c. content of a signal...

Anyway we use such isolation transformers for (usualy low level) signals when we need an isolation of "the signal and the reference of the signal" between two different circuits which communicate to each other... We use it also for a "d.c. supply isolation" in devices with high safety requirements (such as medical equipment/safety from the 220V main voltage )...

For measurement purposes sometimes is enough to have a "capacitor d.c. block" (for example if you need to measure "a signal with d.c. content" in a spectrum analyzer... you have to remove the d.c. content in order not to destroy the analyzer)... But in some other cases a transformer maybe necesary for a specific measurement...

Dear Friend

George says transformers are ALWAYS DC. How many of you would agree/disagree to this? Though this is a digression from the existing topic, but is certainly worth a discussion on ELECTRICAL ENGINEERING FORUM as this is a VERY FUNDAMENTAL ELECTRICAL ENGINEERING QUESTION.

Thanks and regards

Ashok Toshniwal

I am no electrical engineer but did study a number of courses while studying mechanical engineering.

So far as my knowledge ( of 45 years ago) goes there can be no animal like D.C. Transformer. The transformers are supposed to function on the varying voltage (essentially AC) across their primary winding to give a corresponding output at the secondary, depending on the turns ratio.

On the other hand inductances have no resistance to steady state DC so only thing that is likely to happen is heat up the windings leading to other problems. The proper method to block dc components would be to use capacitors of properly selected dielectric.

No, you are incorrect. George said, rightly, that a transformer is always a DC isolation device, which it is, since it can only AC can pass from the primary to the secondary.

Bhankiii understood what I said... Of course, a transformer is not the proper device in order to block a d.c. voltage, due to the fact that a very large d.c. current is passed through his primary windings ( although is always possible to reduce this current by adding a resistor is series to the primary windings )... And, of course, a capacitor is the adequate component to block a d.c. voltage ( and not a transformer )... I only said that no d.c. voltage can pass through a transformer... That's all...

Dear George,

Just a difference in interpretation and understanding. You are right in your place and I am right in my place.

Peace, peace peace!!!!

Cheers and all the best.

Thanks and regards

Ashok Toshniwal, Bangalore, India

Dear Friends!!!

A transformer is not a DC [Transformer]; But a DC Isolating [Blocking] Device. or more correctly Galvanic-Isolation.

But mind that Only Isolated-Wound one NOT AN AUTO-Transformer which is a Tapped Transformer.

Please see Posting #8
above

I happened along this blog and wanted to state that being a controls engineer that works with technicians; what I see going on here is a convolution of terms. Technicians and electricians have slightly different terminology than what an EE would have. An isolation transformer is, strictly speaking, a device that magnetically couples two coils together with a core. In order to generate current in a coil you must have dV/dT or changes in voltage, since all real devices have impedance this causes changes in current. The fact of the matter is that all transformers offer the same kind of magnetic isolation. Isolation transformer is a term that is usually associated with a transformer with a 1:1 output to input ratio. With that being said it is common for people to refer to a DC : DC converter as an "isolation transformer". Please review this : http://www.haloelectronics.com/pdf/3wattdc-dc.pdf, it should cover some of the confusion. Using a transformer as a "DC blocking device" they do work since no DC will arrive on the secondary terminals of the transformer. This is the reason that they can be used as an isolation device; they can transfer signals from one floating medium to another without having them grounded at the same potential. The previous post that noted that a capacitor is a better device for blocking DC was right in the proper context; if you have a large DC offset with a signal added to the top of it, a cap is the right choice. If you have a DC offset from one system to another an isolation device is the correct choice. The post that I replied to seems to have the idea.

Pleases note thatan Isolation-Transformer is:

1. Generally 1:1 but there is a lot of difference in design of a

1.1)normal isolated-Windings wound transformer and

1.2)an "ISOLATION TRANSFORMER".

In the case of 1.1) no care is neccessary as kept while in case of 1.2) "ISOLATION TRANSFORMER" :

a. Last layer of the 1st winding is marked "N" so to connect to near "E" side of mains.

b. 1st wlayer of 2nd winding is Marked to be connected to the "Virtual Low" side.

c. A Screen foil is placed in between both windings and marked "E" & should be grounded to provide CAPACITIVE_Coupling between primary & secondary.

[covering the last-layer of 1st winding with an overlap of 1/4 turn BUT HIGHLY INSULATED OVERLAP not to make a shorted turn to aviod burning of transformer]

This is an interesting question!! Mainly because it illustrates a severe lack of understanding of how a transformer works. In fact there is no such thing as a DC transformer!!. You could make one work by converting the dc to ac a nd the output ac back to dc. A silly exercise. It was suggested that what is being considered is a dc transducer.. That makes more sense where LEDs could be the isolating media.