Why my isolation transformer wasn't

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Why my isolation transformer wasn't

Post by tpaairman on Thu Apr 02, 2009 2:06 am

I'm going to post this on ARF as well. I figure it's a good lesson on checking EVERYTHING twice. I have what I call the Isovac 3000. It's simply a variac and isolation transformer all in one that I made. I actually used two matched 25.2 volt transformers from Radio Shack back to back (secondaries hooked together.) I had them laying around and figured I might as well use them, and it has the added benefit of dropping the voltage, so even though I have 123 volts from the wall, the most I can put out is 117 volts which is perfect for these old sets.

So anyway - last night I was using this as part of a makeshift battery eliminator, and when I hooked up the antenna ground I blew a resistor. And we are talking a flash of flames. Thank God it was only the one resistor, and after replacing it, the set is back as it was except for the addition of a bit of soot underneath. But this told me that I was NOT isolated. And tonight I found out why. When I made this, I picked up one of those outlet/switch combo's from Lowes. This is the deal that is a regular wall outlet, except one side is a switch instead. I know that some of you are reading this saying, that would do it. For those of you who are not, here's the problem. It turns out this thing is internally connected. So with the line cord connected to the switch, every time it was turned on, it sent power directly to the outlet. Now since the outlet was connected to the variac directly, it did still change the voltage, but it also meant that both ends of the isolation transformers were then connected to the line, which eliminated them from the circuit. So off to Lowes I go in the morning to get a switch and outlet.
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Re: Why my isolation transformer wasn't

Post by tpaairman on Thu Apr 02, 2009 11:07 am

Update - I've got to go to stop staying in the shop until about 2:30 in the morning.

I forgot to mention that there is a tab on the side that connects the hot side of the outlet to the switch. But here's where the lack of sleep came in. When I turned on the switch, the neutral side of the outlet and same side of switch were also showing continuity. That's why I thought it was internally connected. When I woke up it occurred to me why - I was getting a continuity through the variac.

The whole point is still valid - all because of that one little tab that I missed, it was bypassing the isolation.
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Re: Why my isolation transformer wasn't

Post by Bill Cahill on Thu Apr 02, 2009 12:18 pm

Ooops! Big BANG!!!!

Be more careful. You could have gotten badly shocked, or, even shorted the line.
I found that out when 30 years ago I had the same batteries, and, B supply connected to 3 20's battery radios, switched. I also switched antenna,thinking I was safe, and, grounded all three together.
When I switched one, my A battery instantly blew up, and, spat acid, and, smoke.
Oh, NO!
Two of the sets had Negative ground. But, the one had Positive ground! Embarassed Shocked

Bill Cahill

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Re: Why my isolation transformer wasn't

Post by Alfred Corbin on Sat May 15, 2010 12:09 pm

You lost me at the start. How do you get 117 volts out of 2 25 volt windings??

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Re: Why my isolation transformer wasn't

Post by denver on Sat May 15, 2010 8:03 pm

Icould see 100.8 volts if they were doubler transformers.
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Re: Why my isolation transformer wasn't

Post by Alfred Corbin on Sun May 16, 2010 6:53 pm

I guess you must be talking about 120 volts DC, if you rectify and filter the peak-to-peak 50 volts AC.

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