Buying an Isolation Transformer

[watchmaker]

I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally.

Now the question is size and manufacturer.  I am not averse to used and looking at new under $150 the stuff like Jameco and Alltronic look suspect and the BK looks underpowered.

I do not expect to do more than consumer type things (guitar amps, etc).

In terms of used, what else should I look at other than medical transformers like Torroid/Trip Lite (2 to 5 VA) or Hammond?  Other suggestions?  Am I wrong about Alltronics and Jameco?

Regards,

Dewey

[Jwillis]

The manufacturer is not really important, after all it's just a transformer. With that being said, you need to keep in mind that toroids have better efficiency, regulation, less losses, and lower audible harmonics that the conventional E core transformer.
Hammond ,Triad, Antek are all good transformers. Some may be more pricey than others depending on where they are made and how well they are built. I'm not familiar with Anatek/Altronics or CES transformers, so I have no opinion on those. Just to clarify, Antek and Anatek are not the same company.

The Real power of Watts is equal to the Apparent Power(VA) X Power Factor. So lets assume that the power factor is 1 for the sake of simplicity. A 300VA 120V transformer will give you around 2.5 Amps. A 500VA is around 4A. How many AC amps does a typical guitar amp or other consumer products need to operate properly? If it's going to be more than 4amps at 120V then you may need to get a 1000VA Isolation  Transformer. You'll typically need about 10 to 25% over head on output amps to cover any losses.

[max.wwwang]

I have an off-the-shelf isolation transformer that I bought second-hand. I also made another one by myself out of a pair of identical power transformers salvaged from AV gear, back to back, without modifying anything - simply from 230V to 80V then back to 230V, which gives me two stages of insulation (though each is not necessarily up to the standard for isolation transformer). Only a bit of loss of power efficiency but it's not a big deal.

[ArdWar]

after all it's just a transformer.

If you aren't that cash strapped at least get one with shield between the windings.

I do agree that almost any transformer (except auto duh) works just fine for most "casual" isolation use.

[Psi]

Transformers out of some types of UPS's have two separate mains voltage windings and so can be repurposed to make an isolating transformer.

From memory. One is just a basic 230V winding, and the other winding has tap at 215-230-245 for doing some basic mains stabilization. 
I can't recall if it had this feature to cope with the wide battery voltage range you get when you stack 3 SLA's in series. 
Or if maybe it was just to stabilize the incoming mains voltage a bit when its a bit low/high

[watchmaker]

Thanks all!

I just ordered a Toroid ISB 60 for $90 shipped and return privilege.

I will make sure the outlet ground pins are disconnected.  Man there is a fair bit of BS about what is fundamentally a simple application.

Checking what I thought I knew got me thinking about portable generators.  I am going to have the ground pin on mine.  Does it go to the frame?  Never thought about it.

Read the discussions on Class 1 and Class 2 devices and how Iso xfomrers for repair are an exception. I had no idea what pros must understand, and what civilians should understand.  Very much like chainsaws!

[Jwillis]

Medical grade isolation transformers are are not truly isolated. They are designed to filter undesirable noise from entering sensitive medical equipment like EEG's and are still tied to earth ground through the Neutral.

Here's really good video on isolation transformer types and experimentation.

[bdunham7]

I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally.

I wouldn't be so sure if I were you.  What exactly are you trying to do?  I would not disconnect the ground wiring internally, but rather use a 'cheater' plug if the situation warranted it.  The passing through of the earth ground is unrelated to the primary function of an isolation transformer which is to remove the ground reference from the LINE and NEUTRAL wires.

[DisasterNow]

Practically downstream of the isolation transformer we will have a floating AC line, with no reference to the earth.
I have an old transformer in the workshop, quite huge, removed from a UPS (230AC-12AC). If I were to find another one like it I could create an isolation transformer with two transformers.

[IanJ]

I managed to pick up an isolation transformer from the medical industry via Ebay.

I modified the wiring to give me the option of isolating or linking the input earth and the output earth……something you have to understand before using either way.

Worth noting that a good isolation transformer will have better isolation between the primary and secondary windings……..so depending on what you want general transformers may or may not suffice.

Ian.

[watchmaker]

I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally.

I wouldn't be so sure if I were you.  What exactly are you trying to do?  I would not disconnect the ground wiring internally, but rather use a 'cheater' plug if the situation warranted it.  The passing through of the earth ground is unrelated to the primary function of an isolation transformer which is to remove the ground reference from the LINE and NEUTRAL wires.

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

[watchmaker]

I managed to pick up an isolation transformer from the medical industry via Ebay.

I modified the wiring to give me the option of isolating or linking the input earth and the output earth……something you have to understand before using either way.

Worth noting that a good isolation transformer will have better isolation between the primary and secondary windings……..so depending on what you want general transformers may or may not suffice.

Ian.

Ian, I thought about this but do not think I will do it.  I have no idea who will have it next.  I do intend to mark the case as grounded and the outlets as floating only.

[bdunham7]

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

No, not the case.  The video in the post you mentioned has a reasonably clear explanation at about 15 minutes in that matches what I said if you understand it.  If you don't believe me, I'd suggest you review the design and operation of actual commercial isolation transformer units intended for electronics servicing.  BK Precision and Sencore have or had a good selection of them. 

The issue raised there and by Jwillis is not the pass-through of the earth ground, it is the bonding of the output neutral of the transformer to ground.  An isolation transformer by default will have an output that is almost entirely isolated--except for a bit of capacitive coupling--from the input and from ground.  Thus if you connect a meter or your arm between one of the outputs and ground, almost no current will flow and you won't be zapped.  The presence or absence of a separate earth ground wire is irrelevant to this issue and indeed some isolation transformers had two-prong plugs and sockets--no ground at all.  This is what I mean when I state that the primary function of an isolation transformer is to remove the ground reference from the LINE and NEUTRAL wires.

There are some additional features as well, as the isolation transformer will also reduce available fault current and may reduce noise and transients that are passed through, especially if it is a of a shielded or double-bobbin design.  Some devices may have site wiring detection that won't work on an isolated suppy or will set off alarms.  These devices, just like one of those little three-light socket testers, are expecting to see the neutral wire at about the same potential as ground and a definite voltage between line and both neutral and ground.  A specific setup to accomodate these devices is to bond the neutral of the output to ground so it functions just like a local service entrance.  You still get the other benefits, but there is no protection against a shock hazard or other issue due to the LINE wire still having full potential relative to ground.  This is the exact point the guy in the video was making when he demonstrated that the bulb did not light when he connected one end to his grounded green alligator clip.

Most isolation transformers will not have the output neutral bonded to ground.  If you are buying one originally intended for medical or other very specific usage other than electronic servicing, you would want to check that.  But this is an entirely different issue from disconnecting the actual green ground wire internally, something I strongly recommend you not do.  There are times when you do want to interrupt that ground, but as I said earlier I'd use an external cheater plug so that you can do it either way and never make a mistake about whether your ground is interrupted or not.

[watchmaker]

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

Most isolation transformers will not have the output neutral bonded to ground. If you are buying one originally intended for medical or other very specific usage other than electronic servicing, you would want to check that.  But this is an entirely different issue from disconnecting the actual green ground wire internally, something I strongly recommend you not do. There are times when you do want to interrupt that ground, but as I said earlier I'd use an external cheater plug so that you can do it either way and never make a mistake about whether your ground is interrupted or not.

Read what I wrote; not what you want me to have written.  This is all ado about nothing.  Thanks for your concern, but it is misplaced.

Merry Christmas.

[bdunham7]

Read what I wrote; not what you want me to have written.  This is all ado about nothing.  Thanks for your concern, but it is misplaced.

Here's what you wrote:

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

Perhaps you intend your words to mean something different than I understand them as?  If not, either your second statement is false or companies like BK Precision, Sencore, etc have been manufacturing and selling entirely useless products for decades.

[fmashockie]

Seems like you already purchased one, but I am privy to the Sencore PR-57 and BK 1653.  I like that they are both isolation and variac in one. 

[EHT]

As was noted above, it is worth testing your actual device especially if 2nd hand. I bought one on eBay. Someone had cut the Earth (PE) wire inside and not put any sticker on the case. My understanding of the most sensible way to use these is the neutral floating but PE connected.

Another problem which has not been mentioned is the inrush current. The one I bought to start with was 1kVA and had no inrush protection. FWIW it's an EU brand (Bronson) but made in China. The inrush on this huge transformer was enough to trip the whole domestic circuit. So I put some NTCs in series and measured the voltage across these with a scope. The peak dies away after about 300ms. I then added a time-delay relay to short the NTCs out after 300ms. All good apart from the time-delay relay's internal circuit seems to emit a frequency whistle (in the air, not on the line) which is a bit annoying...

Some of these transformers like the Toroid one in the video have NTCs in place but not the time-delay shorting. They've calibrated the value of the NTCs such that the inrush is about 10-15A, which is still not great, then also have the problem that the NTCs will then dissipate some heat.

BTW, I don't get the thing about the purpose of the medical isolation transformer being noise-reduction. Surely HF noise is passing through; why not just use a good filter, with MOVs etc?

[max.wwwang]

My understanding of the most sensible way to use these is the neutral floating but PE connected.

Unable to get your point here. Do you mean PE of the input side connected to the case (but if so what does "neutral floating" mean, if again it's the input side)? Or anything else?

[MathWizard]

I got a 500 or 1000VA 1:1 transformer, I believe it was the Hammond brand. It did have an Earth GND, on the input and output 3-prong plugs. I un-hooked it internally from a grounding screw.

That was a few years ago, I better open it up and see if it still makes sense to have unhooked. If for any reason the metal core/chasis become live, I'd still want the panel breaker to trip.

[RFDx]

Unable to get your point here. Do you mean PE of the input side connected to the case (but if so what does "neutral floating" mean, if again it's the input side)? Or anything else?

The PE wires on input and output are connected to each other and to the (metal) case. Do not cut the output PE wire as suggested by the OP multiple times.
The two main wires on the output plug are galvanically isolated through the transformer from L/N on the input.
Although your body is (normally) grounded, the isolation barrier keeps you now safe in case you are touching one of the output wires by accident (or on purpose).

[Jwillis]

BTW, I don't get the thing about the purpose of the medical isolation transformer being noise-reduction. Surely HF noise is passing through; why not just use a good filter, with MOVs etc?

The noise that is referred to is the transient voltage spikes that can occur over mains power. The secondary winding has significantly reduced voltage spikes present on the primary side. In the case with medical transformers, theirs also an added Faraday shielding between primary and secondary that shunts these transients further along with HF noise to ground. So no further circuitry or components are required. 
 

[watchmaker]

My understanding of the most sensible way to use these is the neutral floating but PE connected.

Unable to get your point here. Do you mean PE of the input side connected to the case (but if so what does "neutral floating" mean, if again it's the input side)? Or anything else?

Agree.  If something is earthed, it is not floating by definition.   FWIW, contrary to what one person claimed, OP NEVER said to disconnect the primary side from earth.  What was SAID was he disconnected the OUTPUTS from earth.  Otherwise, the outputs are NOT floating.


Most electrical circuits have a ground which is electrically connected to the Earth, hence the name "ground". The ground is said to be floating when this connection does not exist.[1]

"Conductors are also described as having a floating voltage if they are not connected electrically to another non-floating (grounded) conductor. Without such a connection, voltages and current flows are induced by electromagnetic fields or charge accumulation within the conductor rather than being due to the usual external potential difference of a power source."

Most electrical circuits have a ground which is electrically connected to the Earth, hence the name "ground". The ground is said to be floating when this connection does not exist.[1]"

https://en.wikipedia.org/wiki/Floating_ground

[vk6zgo]

I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally.

I wouldn't be so sure if I were you.  What exactly are you trying to do?  I would not disconnect the ground wiring internally, but rather use a 'cheater' plug if the situation warranted it.  The passing through of the earth ground is unrelated to the primary function of an isolation transformer which is to remove the ground reference from the LINE and NEUTRAL wires.

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

The only time "you have defeated the whole purpose" is if you connect one side of the secondary to ground.

If neither side of the secondary is connected to the external metalwork of the transformer, it is immaterial whether the metalwork is connected to the house ground or not.
The 115v/230v available at the secondary of a true isolating transformer is a floating supply.

There may or may not be reasons to extend the house ground through to the output socket, but the usual reason for connecting the house ground to the transformer metalwork applies (blowing a fuse or tripping a RCD in the event of a breakdown between the incoming active ("hot") & the case/metalwork), unless your isolation transformer is double insulated.

[watchmaker]

I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally.

I wouldn't be so sure if I were you.  What exactly are you trying to do?  I would not disconnect the ground wiring internally, but rather use a 'cheater' plug if the situation warranted it.  The passing through of the earth ground is unrelated to the primary function of an isolation transformer which is to remove the ground reference from the LINE and NEUTRAL wires.

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

The only time "you have defeated the whole purpose" is if you connect one side of the secondary to ground.

If neither side of the secondary is connected to the external metalwork of the transformer, it is immaterial whether the metalwork is connected to the house ground or not.
The 115v/230v available at the secondary of a true isolating transformer is a floating supply.

There may or may not be reasons to extend the house ground through to the output socket, but the usual reason for connecting the house ground to the transformer metalwork applies (blowing a fuse or tripping a RCD in the event of a breakdown between the incoming active ("hot") & the case/metalwork), unless your isolation transformer is double insulated.

I NEVER proposed disconnecting the case from Earth.  I SAID I was disconnecting the outlet sockets fed from the secondary from the case Earth so they would be floating.  To leave them connected to Earth indeed would entirely defeat my purpose for using an isolation transformer.

To be cyrstal clear, here is what I acutally wrote in post one:

 "I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally."

The ONLY way it can be suggested I talked about removing the case Earth is if you take the last sentence out of context.  Which one respondent chose to do while others understood how to read the whole parargraph and addressed my request for manufacturer recommendations.  I wrote this off as a language difference.

Your statement is in precise agreement with what I have said  Thank you.

Sadly, some have made sport of putting words into my mouth  Not only is this rude, it is intellectually dishonest.  If something is ambiguous to you, ASK for clarification.  But do NOT put words into another's mouth.

FWIW, I have been struck by the imprecision of language in this thread.  A ground is an arbitrary voltage reference point.  Earth means ZERO potential energy by definition.

[vk6zgo]

I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally.

I wouldn't be so sure if I were you.  What exactly are you trying to do?  I would not disconnect the ground wiring internally, but rather use a 'cheater' plug if the situation warranted it.  The passing through of the earth ground is unrelated to the primary function of an isolation transformer which is to remove the ground reference from the LINE and NEUTRAL wires.

Read the post above yours.  Yes, you disconnect the output ground from the case ground  Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

The only time "you have defeated the whole purpose" is if you connect one side of the secondary to ground.

If neither side of the secondary is connected to the external metalwork of the transformer, it is immaterial whether the metalwork is connected to the house ground or not.
The 115v/230v available at the secondary of a true isolating transformer is a floating supply.

There may or may not be reasons to extend the house ground through to the output socket, but the usual reason for connecting the house ground to the transformer metalwork applies (blowing a fuse or tripping a RCD in the event of a breakdown between the incoming active ("hot") & the case/metalwork), unless your isolation transformer is double insulated.

I NEVER proposed disconnecting the case from Earth.  I SAID I was disconnecting the outlet sockets fed from the secondary from the case Earth so they would be floating.  To leave them connected to Earth indeed would entirely defeat my purpose for using an isolation transformer.

To be cyrstal clear, here is what I acutally wrote in post one:

 "I am pretty sure I understand their use and importance.  They protect my scope and might protect me.  The isolation receptacles should not return to earth.  Check by testing socket ground to earth ground, or visual.  If earthed, disconnect internally."

The ONLY way it can be suggested I talked about removing the case Earth is if you take the last sentence out of context.  Which one respondent chose to do while others understood how to read the whole parargraph and addressed my request for manufacturer recommendations.  I wrote this off as a language difference.

Your statement is in precise agreement with what I have said  Thank you.

Sadly, some have made sport of putting words into my mouth  Not only is this rude, it is intellectually dishonest.  If something is ambiguous to you, ASK for clarification.  But do NOT put words into another's mouth.

FWIW, I have been struck by the imprecision of language in this thread.  A ground is an arbitrary voltage reference point.  Earth means ZERO potential energy by definition.

Different terms in different countries.
Earth is normally the term used in UK & Australia, with ground being mainly a USA thing.