Why transformerless PS = death?

[cuallito]

Okay before you all freak out, I'm not planning to build a transformerless power supply.

However, I don't really understand why adding a transformer makes things necessarily safer. Is it because it'd keep the current going to infinity if there's a short?

Wouldn't a large resistor across the terminals accomplish the same thing?

[Andy5112405]

A transformer provides isolation between the primary supply and the secondary as they are not electrically connected together.

[cuallito]

Okay but what does "isolation" mean exactly in this case? I mean I understand what a transformer does and how it works more or less (I think.) If I stuck my fingers into the + and - directly from the wall I'd have more of a chance of dying than if I stuck my fingers on the + and - terminals of a 1:1 transformer connected to the wall?

[janneman]

Originally Posted by cuallito

Okay but what does "isolation" mean exactly in this case? I mean I understand what a transformer does and how it works more or less (I think.) If I stuck my fingers into the + and - directly from the wall I'd have more of a chance of dying than if I stuck my fingers on the + and - terminals of a 1:1 transformer connected to the wall?

You would have the same change of dying. There is the voltage step-down aspect of it, but there's another one.

If you have a transformer you have no direct connection to the mains. So you can ground one side of the secondary to the earth or chassis of your equipment, or things like the metal part of your RCA plug, to mention just one part. Then, when you touch your RCA, nothing bad happens.

Now take out the xformer. Depending on how you put the mains plug in you may end up with the live mains connected to your RCA shell or chassis. Definitely not a good idea!

jan didden

[stratus46]

Think of it this way. Without a power transformer the power supply diodes connect the '-' of your power supply to the most negative place on the AC cycle. Similarly for the '+' supply. BUT the AC is reversing 60 times a second (50 in Europe) so your power supply appears to be connected to BOTH the hot and the neutral so if you touch ANYTHING to the outside of your unit it goes poof. The transformer allows the secondary to 'float' relative to the power from the wall so you don't have the problem.

SO DON"T CONSIDER MESSING WITH IT EVER. OK? Stay alive.

G²

[Mooly]

Originally Posted by cuallito

Okay but what does "isolation" mean exactly in this case? I mean I understand what a transformer does and how it works more or less (I think.) If I stuck my fingers into the + and - directly from the wall I'd have more of a chance of dying than if I stuck my fingers on the + and - terminals of a 1:1 transformer connected to the wall?

Example 1... 240 volts AC from the wall socket.
Stick your fingers across it and you get a shock.
Touch the live and touch something earthed and you get a shock.

Example 2... 240 volts AC from a 1 to 1 transformer.
Stick your fingers across the secondary and the shock is identical to example 1
Touch either of the secondary terminals and ground and there is no shock as the secondary windings are "isolated" and have no reference to any other potential (such as ground)

In example 1, earth or ground is very much "related" to the live and neutral connections in the wall socket.
In example 2, there is no direct connection between the transformer secondary and earth so no current can flow.
It is "isolated"

[chrispenycate]

Old TV sets used to trust that neutral = ground, and ran without a transformer. Only on three pin plugs, obviously (so I wonder what they did in the states where they hadn't yet discovered grounding?)

This was relying on all power extension cords being wired correctly, and the electrician who wired your house not being French.

However, the only piece of touchable metal making it to the outside of the case was the aerial socket, and that was stood off with capacitors small enough that you couldn't kill yourself even if you peed on it. No video inputs, no headphone outputs, no external speakers. Only if the tuning knob fell off and you tried to change channels with pliers were you in any danger.

I once PA'd a concert where someone had built a guitar amp based on a television chassis. All worked well until the guitarist walked up to the microphone to make an announcement. If I had been using transistor gear the discharge through the mic input would have blown my front ends; as it was, the noise, and the screams from the musician were worth the ticket price, but he survived, somewhat burnt and very shaken.

[barrymagrec]

Originally Posted by chrispenycate

Old TV sets used to trust that neutral = ground, and ran without a transformer. Only on three pin plugs, obviously (so I wonder what they did in the states where they hadn't yet discovered grounding?)

This was relying on all power extension cords being wired correctly, and the electrician who wired your house not being French.

However, the only piece of touchable metal making it to the outside of the case was the aerial socket, and that was stood off with capacitors small enough that you couldn't kill yourself even if you peed on it. No video inputs, no headphone outputs, no external speakers. Only if the tuning knob fell off and you tried to change channels with pliers were you in any danger.

.

Back in the 50`s virtually all TV sets in the UK were "live chassis" and even if you had a 3 pin plug (not that common then) most had a reversible input connector. TV engineers always carried a neon screwdriver to check whether the chassis was connected to live or neutral and swap the polarity if required. Even then you couldn`t use earthed test equipment such as scopes - but hardly anyone had scopes anyway. Most engineers of the time survived though.

Barry

[hesener]

Touching one of the secondary connections of the transformer may still let you feel something (especially with bigger transformers), as there is capacitive coupling between the windings, can be several nF. Best is to not touch them, and enclose your applications in either a plastic case or grounded metal case.

BTW, an amplifier with 500W into 8 Ohms (as available e.g. with some class-D amplifiers) will issue 63V peak, you can feel it and it is also beyond safety limits (42V in Germany). And, with a low impedance so the current can be strong.....

Be carefull.....

[Th3 uN1Qu3]

Originally Posted by hesener

BTW, an amplifier with 500W into 8 Ohms (as available e.g. with some class-D amplifiers) will issue 63V peak, you can feel it and it is also beyond safety limits (42V in Germany). And, with a low impedance so the current can be strong.....

Yes but the human body is high impedance. 63 volts won't kill you unless you're touching it with both wet hands.

Btw i heard that some time ago (could be 2 or 3 years, can't remember), a blogger was killed when she dropped her laptop in the bathtub. The battery isn't more than 14 volts so you can clearly see the influence of humidity on what is considered "safe" voltage. And not sure what would you be doing with a laptop while taking a bath, but each one with their own.