Newbie Question - Power Transformer vs Isolation Transformer

[SparkyPete]

Hello All,

I am trying to learn up on some concepts before I take the plunge in building a small 1W amp. I have googled this question, but haven't come across a straight answer. Apologies if this is obvious, but this is a case where assumptions could be painful, so I will ask anyway and possibly help some future googler at the same time.

There is much advice online about using an isolation transformer in designs, for safety, and to allow you to use test equipment easily, etc. However, in most amp designs online, there is only a power transformer at the front end.

Am I correct in my surmise that these power transformers (e.g., 269EX) play the role of isolation transformer in these designs? It seems to me that they do indeed isolate the internal voltages from the line voltage, but at the same time (usually) step up voltages? In other words, one does not require both an isolation transformer AND a power transformer, for reasonable safety, correct?

Am I further correct that the warnings about adding a true (1:1) isolation transformer are generally directed at people working with amps which are directly rectifying line power to make DC?

Thank you.

 

[KatieandDad]

You don't need an isolation transformer and a mains transformer. The mains transformer will isolate your equipment from the mains.

 

[Osvaldo de Banfield]

You don't need an isolation transformer and a mains transformer. The mains transformer will isolate your equipment from the mains.

This is almost true, but not always. Some electronic sets have a small transformer for the stand by circuitry, but a SMPS for power stages (like some old TV´s), or definitively all circuitry live although a transformer for some part of the circuit, or the secondary of the transformer may be referenced to some high voltage, including any of the power poles.

Be careful.

 

[KatieandDad]

This is almost true, but not always. Some electronic sets have a small transformer for the stand by circuitry, but a SMPS for power stages (like some old TV´s), or definitively all circuitry live although a transformer for some part of the circuit, or the secondary of the transformer may be referenced to some high voltage, including any of the power poles.

Be careful.

He's building a simple amp. I don't think he will be building separate standby supply supplies and mains referenced SMPS supplies.

Point taken though. Be careful with some commercial designs.

 

[Stefkorn]

An isolation safety transformer has primary windings and secondary windings in two serarate chambers (also NOT wound on top of each other!) so in case the primary winding
Burns out it can't come in contact with the secondary winding. In most valve amps the windings are in one chamber, wound on top of each other, insulated by a layer of insulation
Material and an earther copper screen. Te latter type is smaller and cheaper in comparison with a safety insulation transformer.

Ceerio,

Stef

 

[Osvaldo de Banfield]

He's building a simple amp. I don't think he will be building separate standby supply supplies and mains referenced SMPS supplies.

Point taken though. Be careful with some commercial designs.

Hi Kati. You are right, I´m referring to a general case, because some newbie can understand (wrong) that all transformer equipments are isolated, and not always is this the case.

 

[indianajo]

Another useful use of an isolation transformer is the "PC oscilloscope" using sound cards in personal computers. The PC sound card is not isolated from anything and it makes the use of the scope probe and ground wire quite dangerous. However, even using an isolation transformers on the PC, the sound card and PCI bus on the main board are subject to being blown up by a faulty circuit you are examining.
Really old TV's (1954) had one side of the electronic chassis connected to one side of the power line (western hemisphere) insulated from the metal case by cardboard. I was tickled many times by current in bare feet on Mother's 1954 Philco. So the direct connected to line televisions with SMPS were following an old and disrespected tradition. Thanks to Osvaldo for point out that they are still doing this.

 

[M Gregg]

The bottom line is,

That any amp you build should have a mains transformer (Tx) unless you know what your doing with SMPS..The idea of isolation Transformer is to prevent direct connection to the mains supply under fault conditions...A mains Tx limits the fault current and helps protect you...without it (in the past) the chassis was connected to neutral which is at a lower potential..however if the mains plug was wired incorrectly then the live could be connected to chassis....fault current is limited by the fuse, however it can draw as much as the supply will give for a fraction of a second before the fuse opens...not a good situation if your on the end of it..

Remember that there will still be lethal voltages present on tube equipment and power supply caps can hold charge...

A mains Tx does give seperation from mains supply but the definition of isolation is as above separate windings isolated from each other and earth so they must not short togeather under fault conditions a normal Tx can..
Also your tube supply will be earthed in some way and this is not separation of supply..

So fit a mains Tx and build in the normal way with correct earthing..So a standard mains Tx is OK...Unless you have some special reason for using an isolation Tx.
Having said that don't use an auto transformer it does not have separate windings....You can use an isolation Tx if you can get the correct voltages and use it as a normal Tx with the earthing etc as in a normal amp..might cost more to buy..


Link:
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Regards
M. Gregg

 

[MR OHM 1970]

I Ran A VCR/AUDIO Business For Over 16 Years (1986- 2003) and I always Used a Isolation Transformer and a Variac to service all my Units!!! When I took Electronics in High School Our Teacher told us,,, Better to be Isolated than electrocuted!! This proverb has Stuck with me to this Very Day!!! I always Believe Safety First,,,Sorry To Disagree with anyone here..What a Interesting Topic we have going here!!

 

[Tom Bavis]

In the service business, you're powering up many units a day, all in unknown condition... so starting every test with isolation (and ideally a ground leakage test) is wise. I generally test unknown older equipment for insulation resistance using a cap tester before plugging it in. Often I find a failure due to a leaky capacitor from line to chassis - sometimes know as the "death cap" - which I will clip out before testing further.

Non-isolated circuits were common from the 1930s to 1960s, running directly off the power line line without a transformer. While (usually) safe when used as designed, they can dangerous to work on without the use of an isolation transfomrer.

ANY modern power transformer has been tested for insulation breakdown from primary to secondary using at least 1000V AC. Those specifically designed for safety isolation may get higher test voltage, some as high as 3000V. But remember that the transformer isn't the only possible point of breakdown - I have seen broken fuseholders that caused shorts to the chassis, screws that dug into power wires, burned secondary wires that shorted to AC wiring...

 

[Miles Prower]

There is much advice online about using an isolation transformer in designs, for safety, and to allow you to use test equipment easily, etc. However, in most amp designs online, there is only a power transformer at the front end.

"Isolation transformer" implies a PRI : SEC turns ratio of 1 : 1. Most power xfmrs for VT equipment are voltage step-up devices since VTs are naturally high voltage, low current devices. Even huge, high powered types might operate at 25KV, and 1.0A to get 25KW. Bigvolts, tinyamps.

You also want an xfmr design with separate primary and secondary windings i.e. not an auto-former.

Am I further correct that the warnings about adding a true (1:1) isolation transformer are generally directed at people working with amps which are directly rectifying line power to make DC?

Thank you.

Deriving DC directly from the AC mains is a really bad idea, especially for an amp. You might get away with it if designing a radio or TV receiver, as the end user isn't expected to connect anything other than an antenna, and you can isolate the primary side of an RF xfmr from the rest of the circuit. Still, you can't guarantee an end user won't replace a broken knob with a pair of pliers, and thereby potentially expose themselves to dangerous voltages. Otherwise, how many CD players, MP3 players, sound cards, tape decks, etc can take 120Vrms across the outputs and survive? Not very many.

Just don't do it! It was a bad idea back in "the day', and it's still a bad idea today. A very bad idea. The most costly, most expensive, isolation xfmr is a good deal less expensive than a law suit -- or a funeral.