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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
So I've seen these "variac" things - a transformer that plugs into wall voltage (120VAC in America) and then you turn a knob to transform it down to whatever value you want - You can turn the knob to take it from 20V up to the full 120V.
Does anyone know where I could get one of these cheap?
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
Oh, and I've heard that a variac isn't a "true transformer" and therefore doesn't provide proper isolation. Does that mean that I should put an isolation transformer between my rectifier and the variac? or between the variac and the wall plug?
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Yes, an isolation transformer is a good idea. The only issue here is its not idiot proof, if you contact H and N at the same time you will still get electrocuted. What it will do is offer some protection if you just contact H i.e. by touching the wrong part of the circuit.
A circuit breaker is also a good idea to protect equipment downstream, as is an RFI passthrough filter to protect other devices.
As with all high energy circuits the "one hand rule" is essential as is the use of the "buddy" system.
Registered Member #4762
Joined: Sun May 06 2012, 05:59PM
Location: Russia
Posts: 93
It's an autotransformer, which means it is galvanically coupled between input and output, unlike the regular transformer. No need for any special preparations with it, though. Can be used directly from wall socket to your curcuit.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
Conundrum wrote ...
A circuit breaker is also a good idea to protect equipment downstream, as is an RFI passthrough filter to protect other devices.
does a surge protector count as a circuit breaker? What about my house circuit breaker?
I wanted to use this to build a variable-voltage DC power supply where I take the AC through the variac and rectify it. I thought I might need an isolation transformer in there somewhere in case the DC power supply was shorted together, thus tripping the circuit breaker or possibly doing other bad things.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Yanom wrote ...
I wanted to use this to build a variable-voltage DC power supply where I take the AC through the variac and rectify it. I thought I might need an isolation transformer in there somewhere in case the DC power supply was shorted together, thus tripping the circuit breaker or possibly doing other bad things.
I use an industrial isolation transformer 230V/230V 10 KVA supplying the sockets on my bench, so everything - power tools, soldering irons, and experiments - is isolated from the mains. The transformer unit itself has a pop-out overload switch. It also has a switchable option of connecting the bottom of the secondary to Earth, or not. I keep both ends of the secondary floating, so I can put a full-wave bridge across it and connect the negative bridge output to Earth.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
zrg wrote ...
It's an autotransformer, which means it is galvanically coupled between input and output, unlike the regular transformer. No need for any special preparations with it, though. Can be used directly from wall socket to your curcuit.
no isolation needed? What if I accidentally short the two outputs together? will it trip my house circuit breaker then?
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Cheap/used variac (variable transformer)?
