Author Topic: VARIAC with ungrounded chassis - fix and new digital meter  (Read 3819 times)

circuitous

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VARIAC with ungrounded chassis - fix and new digital meter
« on: January 02, 2014, 09:11:59 AM »
I had just opened up a VARIAC that I have used occasionally over the past couple of years, and noticed that the ground wire is not connected to the chassis.  I confirmed that the panel mounted electrical outlet is not electrically connected to the chassis either, the thick powdercoat is preventing any connection.

I added the ground connections to the chassis.  Removed the powder coat from around the outlet mounting screw and added a lock washer, that grounded the front panel.  Then connected a ground wire from the power cord to the chassis of the transformer, using a crimped spade lug and lock washer.

I also added a digital volt meter to it, which turns out to be surprisingly accurate.

I had expected this cheap variac to just have been painted, but instead it has a tough powder coat.  So, for fun I checked it with an insulation tester.

Here's my first short video. 
https://www.youtube.com/watch?v=vsnEMttQC_I

https://www.youtube.com/user/Corgitronics

I have noticed a variety of problems in the video, but rather than re-shoot it and eternally try for perfect, I'm just putting it out there to get feedback.  Please point out things to improve, lighting, angle, sound, whatever.
Thanks!
« Last Edit: January 02, 2014, 09:13:51 AM by circuitous »

SeanB

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Re: VARIAC with ungrounded chassis - fix and new digital meter
« Reply #1 on: January 02, 2014, 09:32:11 AM »
Powder coat can actually be cheaper if you already have the factory than setting up a paint line. Nice addition, if you want to have a display working down to zero at full brightness then you will have to reverse engineer that simple meter and separate the 115VAC power supply from the measuring resistor and then add an extra lead to run it off the 115 supply for power and simply have another for output reading.

With mine I wanted a range that was usable, so settled for a meter I had lying around. Rescaled for a range of 60-360VAC and used a DC supply I made in the case to give a 7V supply and a simple 500uA meter with a DC converter made from a half wave rectifier and a RC smoothing that was then reading the output. Got away without needing a power supply as the variac has tappings that gave 60VAC so this was used to drive a 723 to give the 7V reference. Simple and all fitted into the huge variac case. I had to add a input soft start as the inrush would trip breakers every so often. Output is from 0-340VAC depending on input voltage. Current is around 10A on the wiper or on the input. 15A fuse serves as a protection.

MJLorton

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Re: VARIAC with ungrounded chassis - fix and new digital meter
« Reply #2 on: January 09, 2014, 09:32:30 AM »

I have noticed a variety of problems in the video, but rather than re-shoot it and eternally try for perfect, I'm just putting it out there to get feedback.  Please point out things to improve, lighting, angle, sound, whatever.
Thanks!
As noted in my comment on your video....great post and a pleasure to watch. Great production.
Cheers,
Martin.
Play, discover, learn and enjoy! (and don't be scared to make mistakes along the way!)

circuitous

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Re: VARIAC with ungrounded chassis - fix and new digital meter
« Reply #3 on: January 09, 2014, 10:07:47 AM »
SeanB and Martin:  Thank you for the feedback.

I'll try to post more content in the near future... if I can come up with anything interesting!
I am more camera averse than Martin is... but I might try getting in front of it at some point.

Martin:  Keep up the videos, I have enjoyed your short takes.
SeanB: I always read your posts on the various forums, they're very educational.