Isolation transformer

[Richard Crowley]

If it really has a shield, it would be trivial to replace that 2-wire power cord with a proper 3-wire, grounded power cord with the green wire safety ground ("protective earth") connected to the transformer case.

[Frankentronics]

I never got that reply from Jameco, asking them to provide a schematic from the manufacturer that would show how the internal static shield is connected. But I did end up taking a $50 risk and purchased one unit. Here's what I found.

The power cord is not polarized.

The inside view does not offer any clue as to how a static shield might be wired. There's no third wire coming out of the transformer, so I think there is no static shield. And all the connections are only "glued together" with solder - no firm mechanical connections.





Further, without a load I measure 119V on the output, while the input is getting 121V.

My understanding is that all transformers have losses and for that reason the 1:1 transformers should not actually have actual 1:1 ratios. As I understand it I should be getting more than 121V on the output, without a load, so that I would get a 1:1 ratio once I plug in a load. Is that correct?

Since this transformer seems to be stepping down, slightly, should I just rewire it backwards?

Or should I just return this item and search for a better one?

Thanks...

[schmitt trigger]

A little hard to tell from the photo, but it appears that there is a red-color external winding, and a green-color internal winding. Am I seeing it correctly?
If so, and based only on the provided photos, it doesn't appear to have any shielding.

The shielding would consist of a single turn ** copper foil with kapton or other insulating tape between the two windings, with a grounding wire attached to it.

** Not a full turn, and definitely with its ends not touching. At all. Otherwise it would create a single-turn short.

[Richard Crowley]

I never got that reply from Jameco, asking them to provide a schematic from the manufacturer that would show how the internal static shield is connected.

Yeah, that was a long-shot.  I wasn't expecting that you would get anything.  How would they know?  Why would they care?
IF it does have a shield, and it doesn't appear to have a separate wire for it, then they probably connected it to the transformer core. (Which is a good place, IMHO)

Further, without a load I measure 119V on the output, while the input is getting 121V.

My understanding is that all transformers have losses and for that reason the 1:1 transformers should not actually have actual 1:1 ratios. As I understand it I should be getting more than 121V on the output, without a load, so that I would get a 1:1 ratio once I plug in a load. Is that correct?

Since this transformer seems to be stepping down, slightly, should I just rewire it backwards?

If the transformer is really wound 1:1 then "rewiring it backwards" would yield exactly the same results.
What exactly is the issue with a 2V drop?  What kind of drop are you seeing with a load?  1A?  5A?  10A?

Or should I just return this item and search for a better one?

What does "better" mean in this context?  Remind us what is the problem here that you are trying to resolve?

If it were me, I would put a proper 3-wire supply mains cord on it.  I would connect the green-wire PE (protective-earth) to the transformer chassis.
If the transformer has a shield, then it will be connected to the core/frame and the input source PE.

Your internal photos show a transformer that appears to be reasonably good for a hobby-level bench isolation application.
My only beef with it would be the lack of PE on the input/source wiring.  And that is trivial to remedy.

[Frankentronics]

I took a closer look. Here's what I can see without taking it apart beyond repair.

Both coils use red wire. What I think might show up as sort of green on the photos is what I think the manufacturer calls a "shield".

I see that the secondary coil is wound close to the core, then there is a "shield" made of the same material as the outside wrapping, which looks like some electrical tape. Then, after this "shield" there is the primary coil.

I think it doesn't have a real static shield, as advertised. My understanding is that a static shield will send the capacitance between the two coils to ground. But I don't know what that means in practical terms. What difference in performance will I see with or without a static shield?

While reading through numerous internet articles about isolation transformers and DIY isolation transformers I came upon some information that DIY isolation transformers are not quite the same as factory made ones. One reason being that there is loss in the coils and that manufacturers wind a few extra turns on the secondary coils to compensate for these internal losses. Thus, the output without any load should be slightly greater than the input voltage.

I quickly tested it under load, to answer the question.  I plugged in an old tube radio through it. The line voltage is 121.7V and under this load the transformer outputs 118.2V. I don't have a quick way to measure how much current my tube radio draws, but would 2A be a good estimate?

I only need this for hobby purposes and I guess it's OK for $50 as I've seen other isolation transformers tend to be considerably more expensive. My main use will be to have an isolation transformer for my variac. The variac will be used to test transformers, power supplies and audio amps.

[Richard Crowley]

Insulation material (like tape, cardboard, etc.) cannot form any kind of electrostatic shield so the preponderance of the evidence suggests that this transformer does not have an electrostatic shield.

The only significant benefit from having an electrostatic shield in a hobby-grade bench isolation transformer is to reduce the capacitive coupling between primary and secondary which caused your original misunderstanding.  However that was only a measurement/perception error and not any kind of practical problem in common and ordinary use of a power isolation transformer. 

Now, if you were designing a permanent power supply for some kind of sensitive gear and/or for use in an unusually noisy power region, then electrostatic shielding might be an important consideration.  But my perception is that it doesn't seem very important for your application.  The difference in "performance" may be that your device under test would be subject to slightly higher power-mains induced noise.  Although going through both the iso transformer AND another chunk of heavy iron (the variac) this doesn't seem like any kind of real-world significant issue.

The voltage variations don't seem any greater in magnitude than normal utility voltage variations that we all see from hour to hour, or daily or weekly, etc.  And if exact mains voltage is important to you, then you have a readily available solution by simply tweaking the knob of the variac.

[Frankentronics]

Thank you, Richard, for taking the time to explain.

I'll keep this transformer and when I have time I'll just make a new housing with an ON/OFF switch and a panel meter. Possibly put it in the same housing with the variac and have a separate output option bypassing the variac. Might as well fit it with a grounded power cord. I have very limited bench space so a tower might be my best option.

Thanks...