Do I need a 10, 15, 20 or 30 amp Variac for most of the DIY work I may do in the future? I am currently building my first amp P101@180wpc. and will be moving on to a variety of other projects in the future.
Thanks for the help ..........
Cheers,
David
Thanks for the help ..........
Cheers,
David
Hi Gavinator68,
Funny you should post this as I am currently wiring up my autotransformer. I bought 240V 8 amp one which I hope is big enough. My logic dictates that 8 amps out of the variac means a little greater than 8 amps in, and seeing my house wiring and switching is rated at 10 amps, its OK. I know some house circuits are 15 amp for 3 phase machinery such as air conditioners.
Most of my stereo amps have 240v 2 to 3 amp fuses so I know these amps draw less than that. In rush currents will be higher.
In the US, with 110v I guess that current rating must be more (double?).
Regards
Funny you should post this as I am currently wiring up my autotransformer. I bought 240V 8 amp one which I hope is big enough. My logic dictates that 8 amps out of the variac means a little greater than 8 amps in, and seeing my house wiring and switching is rated at 10 amps, its OK. I know some house circuits are 15 amp for 3 phase machinery such as air conditioners.
Most of my stereo amps have 240v 2 to 3 amp fuses so I know these amps draw less than that. In rush currents will be higher.
In the US, with 110v I guess that current rating must be more (double?).
Regards
Hi David,
I use a 2A model for testing. I rarely need the 10 amp model I also have. Unless you are building some real class A stuff, or really large tube amps, a 2 amp unit will do.
The advantage to the smaller variac is that you can mount it in a smaller panel or box with voltage and current meters. That and the smaller fuse will blow if something goes wrong with your back turned. And it will sometime.
-Chris
I use a 2A model for testing. I rarely need the 10 amp model I also have. Unless you are building some real class A stuff, or really large tube amps, a 2 amp unit will do.
The advantage to the smaller variac is that you can mount it in a smaller panel or box with voltage and current meters. That and the smaller fuse will blow if something goes wrong with your back turned. And it will sometime.
-Chris
Hi Chris,
Are you recommending additional fuses on the autotransformer?
My PSUs always have fuses (2 or 3 A) and obviously the power circuit has a 10 A circuit breaker.
Thanks
Are you recommending additional fuses on the autotransformer?
My PSUs always have fuses (2 or 3 A) and obviously the power circuit has a 10 A circuit breaker.
Thanks
Hi Greg,
Not necessarily. Many amplifiers I work on may have 6 to 15 amp fuses fitted. Since for initial power up and some testing you aren't running the unit at full power, a small fuse can save parts.
When powering up my own things that can be true as well. Some amps can break into oscillation only when reaching a higher heatsink temp. Then, wing! off it goes when my back is turned. 😉
Really it's your own preference. What works for me may not be what you want.
-Chris
Not necessarily. Many amplifiers I work on may have 6 to 15 amp fuses fitted. Since for initial power up and some testing you aren't running the unit at full power, a small fuse can save parts.
When powering up my own things that can be true as well. Some amps can break into oscillation only when reaching a higher heatsink temp. Then, wing! off it goes when my back is turned. 😉
Really it's your own preference. What works for me may not be what you want.
-Chris
Thanks Chris,
I can see how having a intermediate fuse would be handy for you. You don't need to worry about the customer's amp fuse rating because you have a small one in the test gear. For a hobbiest with relatively few amps I could just use a smaller fuse in the amp for testing.
I was going to have all the bells and whistles on my autotransformer but it ended up a major project that I would put off forever so I decide to use a small power board with an overload and sealed plug etc.
Regards
I can see how having a intermediate fuse would be handy for you. You don't need to worry about the customer's amp fuse rating because you have a small one in the test gear. For a hobbiest with relatively few amps I could just use a smaller fuse in the amp for testing.
I was going to have all the bells and whistles on my autotransformer but it ended up a major project that I would put off forever so I decide to use a small power board with an overload and sealed plug etc.
Regards
Consider how you will use the variac. 8 amps at 240v is almost 2000 watts. 2000 watts is a lot, and I don't care what your mains voltage is.
My 120v unit is fused at 8 amps. It is enough for anything I do. I will run a 4000 watt amp through it if I want to. I use the variac to test for excess draw without pulling the mains full up. I never use it to power the amp to full output. If a transistor is shorted or a rectifier or whatever, it will show up at idle, and the amp will draw heavily. I simply have no reason to dial the mains down while an amp is trying to produce 3000 watts.
Once the excess draw problems are sorted out with the help of the variac, I then power the unit from the mains directly. So a 2 or 3 amp unit should be fine for most applications in audio service.
My 120v unit is fused at 8 amps. It is enough for anything I do. I will run a 4000 watt amp through it if I want to. I use the variac to test for excess draw without pulling the mains full up. I never use it to power the amp to full output. If a transistor is shorted or a rectifier or whatever, it will show up at idle, and the amp will draw heavily. I simply have no reason to dial the mains down while an amp is trying to produce 3000 watts.
Once the excess draw problems are sorted out with the help of the variac, I then power the unit from the mains directly. So a 2 or 3 amp unit should be fine for most applications in audio service.
I bought a 20A unit just because I got a good deal on it. The thing is, I can put a two amp fuse in it if I want to.
I always use a light bulb in series with it when I'm first powering up an amp. The light bulb will ususally tell me if there is something amiss. My variac also goes up to 140VAC output.
Blessings, Terry
I always use a light bulb in series with it when I'm first powering up an amp. The light bulb will ususally tell me if there is something amiss. My variac also goes up to 140VAC output.
Blessings, Terry
Hi Greg & Enzo,
That's about it. I still have my 10A unit, no voltmeter. It would be extremely difficult to mount one, as it is, the current meter is in a separate box. Greg, I can see why you put that on hold.
The smaller variac is permanently mounted in a rack mount plate on my bench. It them becomes very efficient to use. The larger one is a pain.
Terry,
The light bulb thing works well most of the time, but some supplies don't like that too much. Switchers may complain. I power them up with two supplies. One for the housekeeping, the other for the main supply. They will draw high currents at lower voltages. Sometime they will do strange things through a light bulb and you end up troubleshooting a non-fault. As long as you are aware of this, no problem.
-Chris
That's about it. I still have my 10A unit, no voltmeter. It would be extremely difficult to mount one, as it is, the current meter is in a separate box. Greg, I can see why you put that on hold.
The smaller variac is permanently mounted in a rack mount plate on my bench. It them becomes very efficient to use. The larger one is a pain.
Terry,
The light bulb thing works well most of the time, but some supplies don't like that too much. Switchers may complain. I power them up with two supplies. One for the housekeeping, the other for the main supply. They will draw high currents at lower voltages. Sometime they will do strange things through a light bulb and you end up troubleshooting a non-fault. As long as you are aware of this, no problem.
-Chris
Gavinator68,
I feel like I have stolen your thread, but at least I kept on topic for once. 😀
Terry,
I guess my 240V 8A is probably about the same size as your 110V 20A - 8.7kg. Mine goes to a bit over 270V and I was surprised how accurate the dial is. I'll going to try the light globe trick on my next amp.
anatech,
You're right, it is heavy and a pain to move. Having the power board on the output allows me to easierly plug my multimeter in.
An on indication light would be nice.
The autotransformer buzzes a little, is this normal?
Enzo,
Yeah, 2000 watts is much more than necessary, but I had the idea that I may make a high voltage PSU and would actually be using the autotransformer for more than testing. The weight, appearance and buzzing have probably put an end to that idea.
Everyone,
I have attached a picture of my autotransformer with cables added. The plug, cable and power board are rated at 240V 10 amps, but after all the amps I have made the 240V 10A wire seems awfully thin. I don't intend to power 4 amps at once but I probably could. 😉
Thanks
I feel like I have stolen your thread, but at least I kept on topic for once. 😀
Terry,
I guess my 240V 8A is probably about the same size as your 110V 20A - 8.7kg. Mine goes to a bit over 270V and I was surprised how accurate the dial is. I'll going to try the light globe trick on my next amp.
anatech,
You're right, it is heavy and a pain to move. Having the power board on the output allows me to easierly plug my multimeter in.
An on indication light would be nice.
The autotransformer buzzes a little, is this normal?
Enzo,
Yeah, 2000 watts is much more than necessary, but I had the idea that I may make a high voltage PSU and would actually be using the autotransformer for more than testing. The weight, appearance and buzzing have probably put an end to that idea.
Everyone,
I have attached a picture of my autotransformer with cables added. The plug, cable and power board are rated at 240V 10 amps, but after all the amps I have made the 240V 10A wire seems awfully thin. I don't intend to power 4 amps at once but I probably could. 😉
Thanks
Attachments
Thanks for the input guys..............I picked up a 0-140V / 20A from ebay. It sounds like I could have gotten by with a 5A model but only paid $20USD more for the 20A.
cheers
cheers
Hi Gavinator68,
Sounds fine, try to build it in if you can to save lugging it around when you want to use it. Also provide a spot for a fuse on the output side & meters if you want.
Hi Greg,
The autotransformer may buzz. I would think that's normal. My 10A does a bit, the 2A model does not.
-Chris
Sounds fine, try to build it in if you can to save lugging it around when you want to use it. Also provide a spot for a fuse on the output side & meters if you want.
Hi Greg,
The autotransformer may buzz. I would think that's normal. My 10A does a bit, the 2A model does not.
-Chris
Best to use is a variac with separate windings, regular variacs only have 1 winding.
With a variac without separate windings it is safer to put it in series with a separation transformer, that will limit the power output.
The ones with separate windings are a bit more expensive.
With a variac without separate windings it is safer to put it in series with a separation transformer, that will limit the power output.
The ones with separate windings are a bit more expensive.
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