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 Post subject: Re: Variac Problems
PostPosted: Mar Sun 25, 2018 3:16 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 372
Location: Cedar Rapids, Iowa
Leigh wrote:
BTW...
Capacitors do not isolate AC. They conduct AC. How much depends on the value of the capacitor.

- Leigh

Thanks Leigh - I do know this...I guess it was too early for me to have my thinking cap on. I don't have the schematic in front of me now but I think I meant to say that B- is isolated from the chassis with a capacitor.

Rich


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 Post subject: Re: Variac Problems
PostPosted: Mar Sun 25, 2018 3:19 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 372
Location: Cedar Rapids, Iowa
pixellany wrote:
There is no tool that will prevent shocks. And some shocks are more dangerous than others.

Safety starts with your work habits. If you are very disciplined in how you do things, you do not absolutely need an isolation transformer. To be sure, some test setups get more difficult if you have AC on the chassis of the DUT.

The isolation transformer allows you to ground the chassis of a transformerless set. In addition to enhancing safety, this also makes it easy to use all of your test equipment. There is still shock potential within the DUT...if you get yourself between the chassis and the B+, or the other side of the AC line. This highlights a very important safety procedure: only one hand in or near the DUT----with other hand not touching anything. If you do accidentally touch something " live", you do NOT want the current going through your chest cavity.

Thanks for the great advice Mark - I know all of this but it's good to get a reminder. It's difficult to discipline yourself to do certain things one-handed, but I need to make more of an effort to do just that if I want to live long enough to complete all the radios I have to restore ;)

Rich


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 Post subject: Re: Variac Problems
PostPosted: Mar Sun 25, 2018 4:05 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 1583
Location: Dallas, TX
The suggestion of putting a resistor across a meter when checking for a voltage has the similar effect to using a lamp, it determines if there is enough current to be dangerous.
I noticed from the pictures that it looks like you have a pilot lamp connected, that doesn't show on the schematic. I don't think the lamp would cause this problem, but I'm just pointing out the schematic looks fine but it might not represent what you built.
Another thing I noticed from the picture is that you are using a residential power outlet. I might of missed something in your posts but the mounting for these outlets are connected to the ground terminal of the outlet.
The ground hole on your socket may be connected to the grounded case unless you insulated it some way.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: Variac Problems
PostPosted: Mar Sun 25, 2018 4:32 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 372
Location: Cedar Rapids, Iowa
Thanks again Tim - This is a 2 prong polarized outlet, so has no ground terminal. And I have checked to be sure that the mounting screw does not connect to either of the existing terminals.

Yes, I did not draw the power light...it is connected to a tap on the variac that provides a constant voltage...28v I think. When I get everything reassembled I'll temporarily eliminate that and see what I see.

Thanks!

Rich


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 Post subject: Re: Variac Problems
PostPosted: Mar Sun 25, 2018 5:04 pm 
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Joined: Dec Tue 14, 2010 7:52 pm
Posts: 36
Location: Kuna, ID
TPAairman wrote:
Jim Mueller wrote:
So I can't really think of anything that you would put on the bench that would have a grounded case, but an insulated chassis.

Jim,
In most cases I would say you are correct. Although some computer and test equipment has the case grounded but the the chassis and all input/output connectors are, floated above ground, isolated by a small capacitor. HP 3325a is an example that comes to mind since I just finished some repairs to mine.

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Russ WA7MLW


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 Post subject: Re: Variac Problems
PostPosted: Mar Mon 26, 2018 4:44 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 372
Location: Cedar Rapids, Iowa
Here are the results of my latest tests. I reassembled everything into the cabinet and took the following measurements from one terminal of the outlet to earth. I used the casing of a bnc connector on my signal generator, as well as the green wire on the power cord for my isolation transformer/variac box. The green wire was not connected to anything...just hanging free. I took 3 readings...one with just my Fluke 77. Another with an 11k resistor across the multimeter leads, and a third one with a 100k resistor across the test leads. I set my variac to 121 volts.

No resistor: 121vac
11k resistor: 59vac
100k resistor: 7.5vac

I find this disturbing but want to get input from the experts, as well as advice on how to proceed from here.

Thanks,
Rich


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 Post subject: Re: Variac Problems
PostPosted: Mar Mon 26, 2018 4:52 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 372
Location: Cedar Rapids, Iowa
Oops...sorry, but I reversed two numbers. Here are the actual readings:

No resistor: 121vac
11k resistor: 7.5vac
100k resistor: 59vac

Rich


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 Post subject: Re: Variac Problems
PostPosted: Mar Mon 26, 2018 7:32 pm 
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Joined: May Wed 18, 2011 2:40 am
Posts: 3930
Location: Littleton, MA
criageek wrote:
No resistor: 121vac
11k resistor: 7.5vac
100k resistor: 59vac

That's just a so-called "phantom" voltage from capacitive coupling. It's nothing to worry about. The coupling capacitance (probably between the primary and the secondary windings of the isolation transformer) is small, so the impedance of the voltage source is high. Consequently it can't provide enough current to be dangerous.

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 Post subject: Re: Variac Problems
PostPosted: Mar Mon 26, 2018 8:51 pm 
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Joined: Sep Wed 03, 2008 1:24 am
Posts: 1157
criageek wrote:
Oops...sorry, but I reversed two numbers. Here are the actual readings:

No resistor: 121vac
11k resistor: 7.5vac
100k resistor: 59vac

Rich

That's just slightly over the recommended AC leakage limit of 0.5mA, but, as Steve noted, unlikely to be of concern. That's not enough current to give you any sort of jolt, so it's not the cause of the shock you received the other day.

If you want to be extra careful, you can create the test circuit described here (although you can skip the need for the true RMS meter):

http://repairfaq.cis.upenn.edu/sam/safety.htm#saftes


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