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20 amp variac
- Thread starter Tweetybird
- Start date
Another of my maniacal pursuits is "tube guitar amps" where these things are used all the time. Yes, I think those are good and worth the money.
A tip if you get one...... AFTER a short story...
There are two ways of protecting yourself from having a device under test blow up from having a short:
1) a light bulb limiter, and
2) a variac
A light bulb limiter is an odd little circuit that takes a line cord in, takes the white neutral directly to an output socket's neutral pin. The black wire... goes to one side of a screw in light bulb socket. The other side of the light bulb socket is wired to the black hot side of the output outlet. If you are being really safe.... the green wire also goes direct from the input cord to the output socket. So what you have.... safety and neutral are wired straight to the output socket, hot is wired with a screw in light bulb (100W, 150W...but it MUST BE the old incandescent style. Old type bulb is a must!)
The idea here is that, if there is a short in the device under test, as soon as you turn it on and it starts sucking current, the light bulbs come on bright!!! The light bulbs are doing two things...... dropping the HUGELY MAJOR portion of the AC voltage...AND... limiting the current to a minimal amount that is MUCH less likely to blow up the device under test. WIth one of these wired right... you can lay a screwdriver across the output socket hot and neutral...the light will come on ...but you will NOT draw a spark.
That is one way.
The other is the variac. You bring the variac up slowly to limit how the AC voltage hits your device under test. Now HERE is the "but".......
If you are going to use one of these... you really need to have an AC ammeter (5A, 10A....?) so that you can watch the current!!!!!! This one has a voltmeter.....but does not have an ammeter. Not many that I have seen have one built in.
I do see that it has a 20A fuse....but....if that current SNAPS to 20 A before you know what happened.... your device is "toast".
I would be sure to find an analog ammeter ....5A or even 10A and build it in to an enclosure. The idea here is... you start with your variac all the way down and watch that ammeter. If you start seeing the current climbing fast...... with the voltage NOT CHANGING much..... shut er down... you have a short in your device. You have to develop a feel for this.
Using the variac allows you to do one thing that the light bulb limiter won't do: Slowly crank the voltage up .....continually.... all the way to mains voltage. Great for bringing up devices that have had a cap job, or major work done.
MANY of the guitar amp guys....use both!!!! the limiter is so cheap to make.... and is really safer....for the SHORTS test. So they will put their unknown amp on the limiter and fire it up. If the light comes on SUPER bright... they know it has a problem. If it kind of flares (while the capacitors charge) and then drops to a low glow..... life is good. Then they switch it to a variac .... to watch it all the way up.
But....in answer to your question.... yes I think that is a good one. Probably overkill really..... you could do with less current.... but it absolutely wouldn't hurt anything.... as long as you are watching that ammeter!!!!
Bob
A tip if you get one...... AFTER a short story...
There are two ways of protecting yourself from having a device under test blow up from having a short:
1) a light bulb limiter, and
2) a variac
A light bulb limiter is an odd little circuit that takes a line cord in, takes the white neutral directly to an output socket's neutral pin. The black wire... goes to one side of a screw in light bulb socket. The other side of the light bulb socket is wired to the black hot side of the output outlet. If you are being really safe.... the green wire also goes direct from the input cord to the output socket. So what you have.... safety and neutral are wired straight to the output socket, hot is wired with a screw in light bulb (100W, 150W...but it MUST BE the old incandescent style. Old type bulb is a must!)
The idea here is that, if there is a short in the device under test, as soon as you turn it on and it starts sucking current, the light bulbs come on bright!!! The light bulbs are doing two things...... dropping the HUGELY MAJOR portion of the AC voltage...AND... limiting the current to a minimal amount that is MUCH less likely to blow up the device under test. WIth one of these wired right... you can lay a screwdriver across the output socket hot and neutral...the light will come on ...but you will NOT draw a spark.
That is one way.
The other is the variac. You bring the variac up slowly to limit how the AC voltage hits your device under test. Now HERE is the "but".......
If you are going to use one of these... you really need to have an AC ammeter (5A, 10A....?) so that you can watch the current!!!!!! This one has a voltmeter.....but does not have an ammeter. Not many that I have seen have one built in.
I do see that it has a 20A fuse....but....if that current SNAPS to 20 A before you know what happened.... your device is "toast".
I would be sure to find an analog ammeter ....5A or even 10A and build it in to an enclosure. The idea here is... you start with your variac all the way down and watch that ammeter. If you start seeing the current climbing fast...... with the voltage NOT CHANGING much..... shut er down... you have a short in your device. You have to develop a feel for this.
Using the variac allows you to do one thing that the light bulb limiter won't do: Slowly crank the voltage up .....continually.... all the way to mains voltage. Great for bringing up devices that have had a cap job, or major work done.
MANY of the guitar amp guys....use both!!!! the limiter is so cheap to make.... and is really safer....for the SHORTS test. So they will put their unknown amp on the limiter and fire it up. If the light comes on SUPER bright... they know it has a problem. If it kind of flares (while the capacitors charge) and then drops to a low glow..... life is good. Then they switch it to a variac .... to watch it all the way up.
But....in answer to your question.... yes I think that is a good one. Probably overkill really..... you could do with less current.... but it absolutely wouldn't hurt anything.... as long as you are watching that ammeter!!!!
Bob
Thanks bob. I want to learn how to fix without creating bigger problems.like any tool you need to know when and how to use it. Not just have them
For my bench.... I looked in the local Craigslist FREE section.... And found a business giving away a heavy office desk for free.... Just COME GET It!
That is bizarre! (I am only commenting here....because it is HERE that I saw this!)
That last two posts in this thread.....from Bobcat.... that is ME.....but I came in through the other browser ( I think!). I didn't even know I had a Bobcat user on here.
I can see that I am on now as guitar_199 which is what I want.....
I'm going to have to go clear cookies on the other browser and see if THAT is what is going on.......
Hmmmmmmm.......
That last two posts in this thread.....from Bobcat.... that is ME.....but I came in through the other browser ( I think!). I didn't even know I had a Bobcat user on here.
I can see that I am on now as guitar_199 which is what I want.....
I'm going to have to go clear cookies on the other browser and see if THAT is what is going on.......
Hmmmmmmm.......