VARIAC FOR TUBE AMPLIFIERS X 2 - electronics - by owner - sale
These is located near me. It appears that one of them would be sufficient. 120v, 10 amps. Is there anything I'm not aware of that would make one of these inappropriate?
These is located near me. It appears that one of them would be sufficient. 120v, 10 amps. Is there anything I'm not aware of that would make one of these inappropriate?
Certainly, these are high quality at a low price. Just properly fuse both the input and output.
The 136B would be the one to get.
The 136B would be the one to get.
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Cool, thank you very much. For fusing, would that be external inline fuses? I don't see any fuses in or on the unit.
Right, a Variac is very unforgiving when current overloaded, and can easily be damaged
and will not be repairable. Just use input/output fuses of around half its rating to be safe.
This is a piece of industrial lab equipment, so the user is expected to provide accessories.
Meter(s) for reading the input/output voltage are very useful, especially since the unit can
be wired to boost the input by around 15%.
and will not be repairable. Just use input/output fuses of around half its rating to be safe.
This is a piece of industrial lab equipment, so the user is expected to provide accessories.
Meter(s) for reading the input/output voltage are very useful, especially since the unit can
be wired to boost the input by around 15%.
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Very useful. I advise running it through an isolation transformer as well as a fuse. Or at least have the isolation trafo to hand for your measurement equipment if using the variac on the mains and need a floating measurement/ brave about such things.
Yes, please understand that this is a variable autotransformer, and does not provide
any isolation from the AC line.
any isolation from the AC line.
Can you elaborate a bit about the isolation transformer? I'm buying the variac for initial power up of my upcoming amp build, and also to keep on hand for powering on old amps. Where/when/why is isolation from the mains line important?
It reduces the chance of shock if you have poor habits. A large isolation transformer would be
rather expensive, and encourage bad habits. Better to have a differential high voltage probe,
which is both safe and useful.
rather expensive, and encourage bad habits. Better to have a differential high voltage probe,
which is both safe and useful.
Yes isolation is a wise precaution if working inside powered up equipment. If you only ever just power up with the covers on, a variac is fine, but that's not how things are, you will want to troubleshoot at some point with the covers off and that's when accidental contact with mains live is a hazard, espcially with old equipment that's not double-insulated with shrouded connectors and so forth.
I understand this thread takes from American safety standards - so following point of view may not be applicable for you.
Anyway: In my country (and I believe that to be the case as well in other european countries) it is mandatory to have a "fault" relay - at least in domestic installations.
This particular safety device trips a circuit breaker if there is an un-balance between neutral and live (used to be a 8mA threshold).
The result is that if a fault exceeds this "error" current or if you should happen to touch the live wire - you will get an electrical shock, but the current will be disconnected quickly.
If you use a variac with an isolation transformer you will actually disable above safety device and if you are clumpsy enough to get in touch with both wires from the variac, nothing will save you.
So (and this probably applies mostly to 230Vac countries) - be careful with isolation transformers as they may give a false sense of security.
But as previously noted: Develop good (and safe) pratices around high voltage power sources.
Cheers,
Martin
Anyway: In my country (and I believe that to be the case as well in other european countries) it is mandatory to have a "fault" relay - at least in domestic installations.
This particular safety device trips a circuit breaker if there is an un-balance between neutral and live (used to be a 8mA threshold).
The result is that if a fault exceeds this "error" current or if you should happen to touch the live wire - you will get an electrical shock, but the current will be disconnected quickly.
If you use a variac with an isolation transformer you will actually disable above safety device and if you are clumpsy enough to get in touch with both wires from the variac, nothing will save you.
So (and this probably applies mostly to 230Vac countries) - be careful with isolation transformers as they may give a false sense of security.
But as previously noted: Develop good (and safe) pratices around high voltage power sources.
Cheers,
Martin
A 'normal' transformer electrically isolates you (the secondary side) from the primary side. 😎
A variac (autotransformer) does not.😱
So regardless of the value set on the variac you are still effectively connected electrically to the mains side of things. Things can messy for the operator.
If you keep your hands out of the mains side of the product you're testing via the variac then you should be fine.
Variac:
So it's perfectly ok for slowly ramping up a mains product , but just remember there's zero primary/secondary isolation unlike like a 'normal' transformer, because the secondary is effectively the primary also.
Spot the difference:
An externally hosted image should be here but it was not working when we last tested it.
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So are you saying that, with a normal transformer, if I shocked myself on the secondary side I'd only get hit with the secondary voltage, but with a vairac, I would take the full voltage coming out of the wall?
Depends what you set the variac voltage to 😉, it's the full current capabilities of the wall side that you're constantly exposed to, i.e. it will not limit the current.
There is zero galvanic isolation with a variac unlike a 'normal' transformer where you are fully isolated from the primary side.. It's a crucial distinction.
Study the two diagrams again.
A Variac is not intended to replace the DUT equipment's internal power transformer.
It is a method to adjust the AC line voltage when developing or repairing electronic devices.
And is no more and no less dangerous than not using a Variac, with AC line operated equipment.
It is a method to adjust the AC line voltage when developing or repairing electronic devices.
And is no more and no less dangerous than not using a Variac, with AC line operated equipment.
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Ok, I think I'm clear. I was never under the impression that using a variac would increase my safety, so I don't think I'm at any more risk that I would have assumed.