Protection of variac...

Ed LaFontaine · 21st August 2010, 02:59 AM

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?

Enzo · 21st August 2010, 04:00 AM

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.

Ed LaFontaine · 21st August 2010, 04:26 AM

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

Enzo · 22nd August 2010, 05:20 AM

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.

star882 · 22nd August 2010, 02:59 PM

You might just have the wrong connections or it is something other than a plain 60Hz transformer.

Enzo · 23rd August 2010, 11:21 PM

It's a Peavey CS800 PT, so I'd believe the wrong connection, but not the esoteric freq spec.

21st August 2010, 02:59 AM	#1
Ed LaFontaine ...truth seeker... diyAudio Member Join Date: Feb 2004 Location: where the Appalachians rise from the Blue Grass	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? __________________ ...call me Ed...Special Ed... EnABL kit http://www.diyaudio.com/forums/swap-meet/119852-enabl-kit.html DCB1 parts http://www.diyaudio.com/forums/swap-...ml#post2361098

22nd August 2010, 02:59 PM	#5
star882 diyAudio Member Join Date: Mar 2007	You might just have the wrong connections or it is something other than a plain 60Hz transformer. __________________ "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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21st August 2010, 04:00 AM	#2
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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.

22nd August 2010, 05:20 AM	#4
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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.

23rd August 2010, 11:21 PM	#6
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	It's a Peavey CS800 PT, so I'd believe the wrong connection, but not the esoteric freq spec.

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