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Old 21st August 2010, 02:59 AM   #1
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Default Protection of variac...

No, not the moderator. He's a big boy and does a fine job of that on his own...

I have a variety of salvaged transformers. I want to find if they are suitable for future projects. I'm using a variac to control line voltage. I'm doing this as a way to avoid any excitement (smoke) on my way to determining the secondary voltages.

The variac is rated @ 1.75 A and is fused. I had completed checking and recording voltages for several moderate sized transformers when I hooked up to one from a Peavey CS-800...a serious, large piece of iron...when I blew the fuse.

I have found 0.4 ohms on the primary & 0.3 ohms for each of the secondaries to CT. I find no shorts from windings to the case.

Is my variac too small for this large transformer?

If so, can I provide anything in-line to increase the protection or allow gathering measuremnents for large transformers?
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Old 21st August 2010, 04:00 AM   #2
Enzo is offline Enzo  United States
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Are you just flipping it on, or are you bringing up the test item from zero?

There can be a large starting surge powering up a transformer, but just sitting there with mains applied it should draw next to nothing. Is your fuse a slow-blow type then?

Due to the heavy gauge wire in that transformer, ohm readings won;t tell you much.

Look up "light bulb limiter" and make one and use it. My variac sits next to me at all times, but sometimes the simple trick is better. Especially for go vs. no-go.

And to detect shorted turns, go over the RG Keen's site New Page 1 and somewhere (in the tech tips, if I recall) is a VERY simple transformer tester you can make with a couple components. Make one of those real quick and use it too.
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Old 21st August 2010, 04:26 AM   #3
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Hi Enzo,

Quote:
Are you just flipping it on, or are you bringing up the test item from zero?
I have been starting from zero...and returning there before any disconnect.

Quote:
RG Keen's site
Found it, called "Transformer Short Tester" under the heading of "Tube Amp Tips"

I now have several reasons to build the Light Bulb Limiter.


Thanks
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Old 22nd August 2010, 05:20 AM   #4
Enzo is offline Enzo  United States
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If you are bringing up the transformer primary, that eliminates startup surge, and points to a bad transformer.

If your variac doesn't have one on it, you really should add a current meter to check what is being drawn from it. ON a service bench, a current metered outlet is a great thing to have.
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Old 22nd August 2010, 02:59 PM   #5
star882 is offline star882  United States
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You might just have the wrong connections or it is something other than a plain 60Hz transformer.
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"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 23rd August 2010, 11:21 PM   #6
Enzo is offline Enzo  United States
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It's a Peavey CS800 PT, so I'd believe the wrong connection, but not the esoteric freq spec.
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