Discussion of AC mains isolation transformers

[Speedskater]

That's for Common Mode suppression!

a Hot to Neutral spike is Differential Mode.  A transformer can't suppress it and be a transformer.

[Speedskater]

Surge suppression should be dealt with where the AC power enters the building. Using a whole home suppression unit.

Make that a 'UL' rated/listed unit. Been reading on the electricians pages about unlisted units going up in flames.

[Speedskater]

With MOV's there is always a small possibility of failure. It was un-rated units that went up in flames.

Go with top shelf name brand units from an electrical supplier and replace them every decade or two. 

Everything in your AC power system should be 'UL' rated.

[Speedskater]

Everywhere (with the exception of the secondary of an isolation transformer) the only place that the Neutral and the Safety Ground/Protective Earth may be connected together is at the main breaker box and this connection is required.

[Speedskater]

Oops, with regard to:

(with the exception of the secondary of an isolation transformer)

This only applies if the transformer is wired as a Separately Derived System.

[Speedskater]

Some of the best AC power information is in this Middle Atlantic white paper:

 

"Integrating Electronic Equipment and Power into Rack Enclosures"
'Optimized Power Distribution and Grounding for Audio, Video and Electronic Systems'

 

https://www.middleatlantic.com/resources/white-papers.aspx

 

The part about wiring isolation transformers starts at page 19 or 20.

You do need to enter an e-mail address now.

[Speedskater]

On 4/14/2019 at 11:42 PM, One and a half said:

Yes the buzzing noise is caused by frequencies that are trapped in the windings with nowhere to go.

What in the world does that mean?

[Speedskater]

The advantage of a large Isolation Transformer (3000 to 5000VA) is:

It can be wired as a Separately Derived System. This means that the paths of the Safety Grounds/Protective Earths and the Neutral to the components and to each other can be kept very short.

[Speedskater]

In the USA/CAN there is another advantage to a large Isolation Transformer. The primary can be wired at 240V, with one or two 120V secondary circuits.

[Speedskater]

On 8/5/2019 at 9:06 PM, One and a half said:

Capacitance meters are often inaccurate and expensive.

I never found that to be true.

In the R&D labs, I measured some 1% capacitors on different expensive meters. At home the values were the same on very inexpensive meters.

But trying to measure anything in an AC power system in the low pF ranges is a total waste of time.

 

[Speedskater]

Check the continuity of all possible combinations.

Input plug:

Hot, Neutral & Safety Ground

Output Receptacle:

Hot, Neutral & Safety Ground

 

I think that there are 15 combinations.

Sperry makes some smart testers and some dumb ones. What model number?

[Speedskater]

If there is continuity from the plug Safety Ground to the receptacle SG, but no continuity from either SG to any H or N, that's a normal way that smaller Isolation Transformers are wired.

[Speedskater]

14 hours ago, One and a half said:

This filter reflects noise back to the source, so in essence it's a like to mirrors back to back (with a variable attenuation on either side for conducted noise.

What does that mean? I couldn't find that on the mfg's web page. Seems like a description of a long transmission line (which doesn't apply here).

[Speedskater]

2 hours ago, gererick said:

I can go up to 50 uF in capacitors across the + and - terminals to reduce transverse noise.

I hope that value was a typo.

The maximum line to ground value is 0.01 uF.

Typical line-to-line values are 0.1  to 2.0 uF.

Remember the AC power line capacitors need to be 'X' & 'Y' types.

[Speedskater]

4 minutes ago, gererick said:

Regarding 'X' and 'Y' types, I instead went with rated measurements provided by the manufacturer so that I could use better quality caps.  

Using only 'X' and 'Y' type capacitors is a safety requirement!

 

I think that you are getting into dangerous territory.

[Speedskater]

On 5/10/2022 at 12:08 AM, gererick said:

On Jan 17, One and a half showed a photo of a 7 uF Aerovox brand cap with a part # beginning with W43A250, as outfitted from the Topaz factory, wired from line to neutral at the outlet

That's from over four decades ago! Electrical safety has gotten more serious since then.

[Speedskater]

Interference expert Henry Ott (RIP) wrote in his big book on power line filters:

The performance of an AC power line filter is as much, if not more, a function of how and where it is mounted, and how the leads are routed, as it is of the design of the filter.

 

The filter should:

a] be mounted close to the point where the power line enters the chassis.

b] the wire grounding the filter to the chassis needs to be very short.

[Speedskater]

7 hours ago, gererick said:

a .0005 pF capacitance isolation transformer is “a very good surge suppressor as well. Most of the energy in high power surges is contained in high frequency components, which get suppressed by the low capacitance, thus it is quite an effective surge suppressor without needing any other special circuits to achieve this.”

Whether that transformer is a good suppressor on not, I don't know.

But that explanation is incorrect.  Nothing get suppressed by low capacitance.

[Speedskater]

When dealing with power transformer buzz, the first thing to check using a 'Kill-a-Watt' (or similar) meter is the AC line voltage. Over the decades AC line voltage has been slowly creeping upwards (the opposite of shrink-inflation). So an older transformer designed for a lower voltage can be very unhappy with high line voltage. 

[Speedskater]

It has a stepping function. It switches voltage taps as the voltage changes.

[Speedskater]

Power transformers often boost the secondaries a few volts.  If the primary and secondary are both the same voltage, you could reverse them (i.e. the input to the secondary) and get a few volts drop.

 

A 'buck/boost' transformer could be added, but boy that gets complicated real fast.

[Speedskater]

Well if you had old legacy/boutique equipment, 116 Volts would be good.

So:

a] how big is the transformer?

b] what is the total current or power that all the plugged in components draw?

(a little 'Kill-a-Watt' meter might be a handy thing to have when dealing with AC power.

 

Note that modern hi-fi components with a SMPS (Switch Mode Power Supply) don't care what the AC line voltage is.