6
\$\begingroup\$

I know you're not supposed to rectify mains because it's "dangerous" but if I use a 1:1 transformer and ground it to earth how exactly am I any "safer"? I ask because it would save me $130 to simply rectify mains and get the same voltage.

\$\endgroup\$
  • 6
    \$\begingroup\$ Why would you ground it to earth? \$\endgroup\$ – PlasmaHH Jan 8 '18 at 16:44
  • \$\begingroup\$ @PlasmaHH you ground it in much the same way the incoming mains is in any distribution. You connect neutral (or the side of the 1:1 you want to define as Neutral) to ground to get normal 3 wire distribution. If you don't ground the isolation transformer then you can get in trouble with items (such as power supplies) plugged in where active devices such as overvoltage or current sensing compare neutral to earth voltage or current flows. \$\endgroup\$ – Jack Creasey Jan 8 '18 at 17:50
  • 1
    \$\begingroup\$ @JackCreasey: If you connect the secondary to GND you defeat the purpose of an isolation transformer, that is there not being a (resistive) path through GND, thus no (little) current flowing when you connect a resistor (human) across GND and one of the legs. What most of the time you want is to have the DC GND be at 0V/Earth/Moon/Vehicle/Plane/Spacecraft potential. \$\endgroup\$ – PlasmaHH Jan 9 '18 at 8:53
  • \$\begingroup\$ @PlasmaHH. I would agree with you under the special circumstance of repairing live (typically high voltage) equipment. However isolation transformers are used for many more purposes: noise isolation, N-E potential discrepencies etc etc. It's not a simple answer, and certainly in many configurations Earth is required. \$\endgroup\$ – Jack Creasey Jan 9 '18 at 18:46
6
\$\begingroup\$

What you are describing is an isolation transformer.

Since the mains lines are referenced to real ground, touching one wire with your feet or other hand electrically connected to ground completes a circuit with YOU in it. That tends to spoil your day.

If used properly, an isolation transformer isolates the power lines from the ground. As such, touching the live wire on the isolated side while touching ground with your other hand does not complete a circuit.

Of course, none of that matters if you touch both sides of the power on the isolated side.

If you end up grounding one side of the isolated side of the transformer, you make that situation worse. In fact, in that case you are in more danger than touching the primary side which should be protected by an "earth leakage" or "ground fault" circuit breaker. Unfortunately, an isolation transformer takes those out of the circuit.

The transformer does however limit how much current can be passed since it will saturate at some point. That is better than connecting your self direct to the multi-megaWatt grid.

\$\endgroup\$
6
\$\begingroup\$

Because you get isolation with a transformer. The insulation of the line shields you from the full current that the mains line can source. Since transformers usually have a magnetic core, this also saturates and can help limit current. Why would you want to limit current? In the event of a short.

So yes, because of isolation and current limiting a transformer is safer.

In many cases the transformer isn't just a good idea, it's required.

Enter image description here

Because of the isolation, the secondary is floating. You can still reference your circuit to another voltage, like ground and get the power limiting of the transformer.

Enter image description here

Source: Learn about Electronics

\$\endgroup\$
  • 2
    \$\begingroup\$ But he's asking about grounding one side of the secondary. The transformer may limit current, but that doesn't matter to shock safety. \$\endgroup\$ – Olin Lathrop Jan 8 '18 at 17:37
  • \$\begingroup\$ Isolation does matter for shock safety, which happens to be the first line in the post. \$\endgroup\$ – Voltage Spike Jan 8 '18 at 20:43
  • 4
    \$\begingroup\$ I didn't say otherwise. You are still missing the point that he is asking how grounding the output of the tranformer is safer than using the line directly. Your diagram doesn't ground the secondary circuit anywhere. Yes, that's safer. Nobody is arguing that. But, it's also not what the OP is asking about. \$\endgroup\$ – Olin Lathrop Jan 8 '18 at 20:46
4
\$\begingroup\$

If you're going to ground one side of the transformer secondary, then you're no better off safety-wise.

However, with a transformer, you can ground the negative side of the full wave bridge output instead. Now you have one side of the supply that is safe. The other still has dangerous voltage, but that will always be the case whenever you have such high voltage.

If you don't really need rectified line voltage, then you can have the transformer serve two purposes: Provide isolation and reduce the voltage to a safe level.

Either way, you have to start by thinking carefully about how the DC supply will be used. Will it be totally internal to the unit and well insulated from the outside? If so, no isolation may be required. Will its ground need to be connected to external ground or something the user can touch? If so you probably do need isolation.

\$\endgroup\$
  • \$\begingroup\$ It's for a tube amplifier. Isn't it standard safety procedure to earth ground such a thing as well as its chassis in case of a fault? \$\endgroup\$ – coinmaster Jan 8 '18 at 16:53
  • 2
    \$\begingroup\$ @coin: Then you'd ground the negative output of the rectifier, as I said, not one side of the secondary. That is exactly how lots of tube equipment did it. The transformer also provided the low voltage for the filaments via a separate secondary. \$\endgroup\$ – Olin Lathrop Jan 8 '18 at 17:39
  • \$\begingroup\$ Whoever downvoted this, what exactly do you think is wrong? \$\endgroup\$ – Olin Lathrop Feb 24 '18 at 18:01
2
\$\begingroup\$

Now we have the context that this is a valve amplifier it is possible to give a more useful answer.

In normal equipment both the mains live and mains neutral need to be regarded as hazardous. Even if your country reliably connects neutral to earth and uses polarised plugs there is still the worry of what happens if a wire breaks or a pin makes bad contact and the worry that volt drop in the neutral wiring can drive current where you don't want it to go.

We also must avoid too much leakage between live/neutral and earth since it would cause hazards in the event of a lost earth.

A valve amplifier generally has at least one connection to/from the outside world. These connections need to be safe to touch and they need to be referenced to one side of the supply. So we require one side of our power supply to be touch-safe. Normally this is achieved by using a transformer and than grounding one side of the rectifier output.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.