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My little brother likes to salvage/experiment with old tech. Said old tech has to-date included tape recorders (probably nothing to worry about), slide projectors (these seem to blow fuses sometimes), and old speaker sets and amplifiers. The speakers I'm particularly concerned about because I know they can have large capacitors in.

My brother is an adult, and is likely to continue these activities no matter what I tell him. I'd like to buy him something that would allow him to continue in a safer manner. He is a chemist, so has some understanding of technical things, and given the right bit of kit, he might well be just fine.

If it makes any different to the question, he lives in the UK, and is currently powering everything using the household mains.

I came across an article here, which suggested to me that a "Variac" might be a good idea. It seems like the Variac would allow him to increase voltage slowly, up to the voltage that the historic object was designed for. Sounds like a good thing. Maybe he is less likely to burn the house down that way.

My questions are;

  1. Does a Variac allow someone to power a historic electrical device with greater safety than just wiring it into a UK mains socket?
  2. Would a Variac that operates at 6A max (like this one) be suitable for most devices?
  3. Is there anything else you think is particularly important to have for someone powering on an old speaker set?

(Looking around, I saw this question which made me think that maybe a Variac isn't appropriate for mains power? But the specs for the one I saw do seem to indicate the input is household mains.)

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    \$\begingroup\$ It appears that your brother might be more electronic-orientated than you so, why not discuss this with him and see what he thinks would be a suitable addition to his armory of equipment. I can't see a variac helping but your brother might (unless of course you invented the concept of a third person to ask this question on their behalf so as not to appear foolish maybe?) \$\endgroup\$
    – Andy aka
    Oct 14, 2023 at 17:32
  • \$\begingroup\$ @Andyaka no, I'm not into retro tech myself. His birthday is coming up, so the concept was generally to surprise him with something relating to his hobby. Which is always difficult, but I think I know enough about his activities and equipment to make a good attempt here. \$\endgroup\$
    – Clumsy cat
    Oct 14, 2023 at 18:36
  • \$\begingroup\$ Afaik Variacs can also transform the voltage output higher to a certain extent. \$\endgroup\$
    – MiNiMe
    Oct 14, 2023 at 19:34
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    \$\begingroup\$ From the TDGC3-2D model Note: Variacs are not compatible with most GFI (Ground Fault Interrupter) wall outlets. We have found that the inductive load that the Variac represents is not compatible with most GFI circuits and they will trip the breaker. \$\endgroup\$
    – MiNiMe
    Oct 14, 2023 at 19:34
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    \$\begingroup\$ The idea of using a variac for so called "Safety" is mainly for the safety of the device under test, not necessarily for the person doing the testing. For example if there is weak component in a piece of vintage equipment that would potentially go bang at full voltage, a variac slowly ramping up the supply voltage might initially cause a "sizzle" or a little "smoke" rather than going out with a "bang". (You would quickly switch things off at the first sign of smoke or a sizzle.) \$\endgroup\$
    – Nedd
    Oct 14, 2023 at 19:52

3 Answers 3

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You want to protect yourself from electrocution, starting a fire, and circuits blowing up in your face.

I highly recommend some type of series interface box. You can decide which of these features is important to you.

RCD/GFCI and/or an isolation transformer will help minimize the risk of electrocution. Many USA houses only have GFCI in the kitchen and bathrooms. But you can buy plug-in GFCI devices.

Fuses will lower the risk of starting a fire or circuits exploding, assuming that the fuses are sized well below the rating of the circuit.

A Variac can be useful if you use it to slowly bring up the voltage to a questionable device.

The series light bulb trick can also be useful. Start with the S2 open, if the light bulb doesn't light when your device is powered, then close S2. Of course, this assumes that the current draw of your device is less than the light bulb.

A Variac is more useful when you are designing your own circuit. Most Variacs will allow you to boost the voltage slightly. You want to test your circuit at higher and lower line voltages.

I believe that the UK uses 230 VAC with both lines hot. In the USA we use 120 VAC with one line hot, then you only need one fuse at the input.   enter image description here

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    \$\begingroup\$ Actually UK uses one line hot and one neutral, not both lines hot. I have to say your suggestion correctly takes into account that variacs don't generally isolate so a separate isolation transformer is a big plus for safety. \$\endgroup\$
    – Justme
    Oct 14, 2023 at 21:17
  • \$\begingroup\$ So if I understand this correctly, the idea is that provided the device is not shorting, the combined load of the device and the lightbulb is too much, so the light bulb will stay off. If the lightbulb lights up, the device is not providing resistance, and so probably has a short? \$\endgroup\$
    – Clumsy cat
    Oct 15, 2023 at 9:38
  • \$\begingroup\$ @Clumsycat - Yes. I haven't worked on mains in a while. But I have saved a few 200 W bulbs in case I do. \$\endgroup\$
    – Mattman944
    Oct 15, 2023 at 12:50
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Having worked on mains equipment for a few years in situations where I’m unavoidably dealing with live circuits, I’d say that an isolating transformer is the first piece of safety-enhancing equipment you’d want. In addition to that, a variac allows you to wind up the supply in a controlled manner, which can result in blowing stuff up less frequently but doesn’t much enhance safety from electrocution. In general the best option is to work carefully at all times, if you need to probe a part of a circuit it’s better to connect to it with the device powered off, double check everything, stand back and power on.

An RCD is a prerequisite these days, but I’m not convinced that a 30mA device would trip if you happen to touch a live terminal.

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Is there anything else you think is particularly important to have for someone powering on an old speaker set?

The single most important safety device after the fuse is the RCD (called in the US a GFCI): -

A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that quickly breaks an electrical circuit with leakage current to ground. It is to protect equipment and to reduce the risk of serious harm from an ongoing electric shock.

Maybe there is one fitted already in your "brothers" house. If not then urgently persuade him to have one fitted. And, preferably, if he uses some kind of bench area where "he" works, then have an extra one fitted on the bench for all his outlets.

Would a Variac improve the \$\boxed{\text{safety}}\$ of experiments with old tech?

A variac isn't a safety orientated device but, could prove useful in some circumstances.

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    \$\begingroup\$ Would you approve of the series light bulb trick? \$\endgroup\$ Oct 14, 2023 at 18:06
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    \$\begingroup\$ Not really having never needed to use either. My philosophy is double check everything at every design stage and apply the power. Be prepared to run for cover of course (LOL). \$\endgroup\$
    – Andy aka
    Oct 14, 2023 at 18:36
  • \$\begingroup\$ He lives in a flat share, so likely has the appropriate safety certificates. So we can safely assume an RCD. \$\endgroup\$
    – Clumsy cat
    Oct 14, 2023 at 18:40
  • \$\begingroup\$ @Clumsycat get them tested, get a proper earth test done. \$\endgroup\$
    – Andy aka
    Oct 14, 2023 at 19:04

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