I have a 20 amp, 2000 watt Variac. It's not the greatest quality--about $100 imported from China--one of those red ones.
I also have a tubed preamp with a separate power supply. Genius that I am, I plugged the power supply into the variac, but forgot to attach the supply to the preamp box--hence a powersupply with no load. I then switched on the variac with it set around 50 VAC.
This blew the house fuse, but not the 20 amp fuse on the Variac. Apparently, it also blew the Variac. The switch indicates on, the fuse is good, but no AC output.
I assume the Variac is fried, but wonder if what I had done was really enough to blow one of these things. Or was it just that I had a cheap one?
I also have a tubed preamp with a separate power supply. Genius that I am, I plugged the power supply into the variac, but forgot to attach the supply to the preamp box--hence a powersupply with no load. I then switched on the variac with it set around 50 VAC.
This blew the house fuse, but not the 20 amp fuse on the Variac. Apparently, it also blew the Variac. The switch indicates on, the fuse is good, but no AC output.
I assume the Variac is fried, but wonder if what I had done was really enough to blow one of these things. Or was it just that I had a cheap one?
Variac might have taken a magnetic set
I've used and abused the original 20A Variac for years. It's unlikely that the winding wire would fuse at 20A. Did you measure conductivity across the winding? Check the graphite brush, too. I've had problems with the 20A Variac taking a magnetic set (saturation) depending on the current through it when it was last shut down. It retains this memory until next time you turn it on. When you power it up, current surges momentarily through it since its inductance is low and it pops the house breaker or fuse. It may not be your supply after all, although it still could be. Without saying more about your supply, it's not clear why not connecting the supply without preamp would have been bad. Perhaps your chokes could bounce with no load.
I've used and abused the original 20A Variac for years. It's unlikely that the winding wire would fuse at 20A. Did you measure conductivity across the winding? Check the graphite brush, too. I've had problems with the 20A Variac taking a magnetic set (saturation) depending on the current through it when it was last shut down. It retains this memory until next time you turn it on. When you power it up, current surges momentarily through it since its inductance is low and it pops the house breaker or fuse. It may not be your supply after all, although it still could be. Without saying more about your supply, it's not clear why not connecting the supply without preamp would have been bad. Perhaps your chokes could bounce with no load.
I've had trouble with big variacs and circuit breakers too. I regularly used to trip a 32A breaker with the switch-on surge taken by a 10A variac. The pre-amplifier is a complete red herring. Check that you haven't blown a fuse going to the variac. It's highly unlikely that there is any real damage.
Other Variac problems
This should be covered under Safety !!!
Variacs provide NO isolation from the mains (that is they are a single coil "auto-transformer").
When powering tube gear from a variac - in order to bring the HT up slowly etc. remember that the tube filaments will NOT be at operating voltage.
The blowing of the fuse may be related to inrush current - that is the magnetising current. Make sure you are using "Anti-surge" or "Slow Blow" fuses - even then they will "fatigue" after a while and require replacing.
Cheers,
Ian
This should be covered under Safety !!!
Variacs provide NO isolation from the mains (that is they are a single coil "auto-transformer").
When powering tube gear from a variac - in order to bring the HT up slowly etc. remember that the tube filaments will NOT be at operating voltage.
The blowing of the fuse may be related to inrush current - that is the magnetising current. Make sure you are using "Anti-surge" or "Slow Blow" fuses - even then they will "fatigue" after a while and require replacing.
Cheers,
Ian
also having problems with a Variac
Hi all
I'm having a problem with a variac too, and thought I could post it here. Well, I just bought it, but it is an old one a guy had at the basement for years. It is rated for 8A and the first time I used it I had two 1,25A slow blow fuses (in serie with main and neutral) but worked without problem (it had no load attached). Today I turned it on (without load) and the 1,25A fuses blew... I took two 5A fuses and it worked...than I turned it off and after a while on, and one blew...
In my situation i used a <200VA EI trafo to charge some capacitors, with paralleled bleeding resistors (220k) so almost no current drag through Variac and trafo...
I know the Variac is working and so I planned to put it straight into the mains...but it is dark now, and I don't want to break my back going downstairs in the dark to change fuses - if the Variac decides to blew a house fuse...
So, what shall the problem be? High inrusch current? Just plug the 8A variac straight into the mains, when there is a bit more light outside?
Erik
Hi all
I'm having a problem with a variac too, and thought I could post it here. Well, I just bought it, but it is an old one a guy had at the basement for years. It is rated for 8A and the first time I used it I had two 1,25A slow blow fuses (in serie with main and neutral) but worked without problem (it had no load attached). Today I turned it on (without load) and the 1,25A fuses blew... I took two 5A fuses and it worked...than I turned it off and after a while on, and one blew...
In my situation i used a <200VA EI trafo to charge some capacitors, with paralleled bleeding resistors (220k) so almost no current drag through Variac and trafo...
I know the Variac is working and so I planned to put it straight into the mains...but it is dark now, and I don't want to break my back going downstairs in the dark to change fuses - if the Variac decides to blew a house fuse...
So, what shall the problem be? High inrusch current? Just plug the 8A variac straight into the mains, when there is a bit more light outside?
Erik
Hello Erik,
I had the same problem as you with my variac (old Philips 30A).
Some tend to draw a lot of current at startup.
In the end I hooked up a 100W light bulb in series for startup.
A bypass switch, shorting the bulb, is used to connect the variac directly to the mains after the initial inrush current has gone.
Hope this helps
Johan
I had the same problem as you with my variac (old Philips 30A).
Some tend to draw a lot of current at startup.
In the end I hooked up a 100W light bulb in series for startup.
A bypass switch, shorting the bulb, is used to connect the variac directly to the mains after the initial inrush current has gone.
Hope this helps
Johan
Hi Butterylicious and Johan
Thanks for your reply. Today I will try Johan suggestion of the light bulb and in the longer run I will look at the GFCI. I know my house has one, but don't know if it works for my bench (affraid not, because it is an old house with only earth connections in the kitchen and for the washing machine).
Erik
Thanks for your reply. Today I will try Johan suggestion of the light bulb and in the longer run I will look at the GFCI. I know my house has one, but don't know if it works for my bench (affraid not, because it is an old house with only earth connections in the kitchen and for the washing machine).
Erik
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