Isolation transformer - high secondary voltage

[grossbard]

I recently bought a Triad 250VA iso transformer. Both my Kill-A-Watt and Fluke meter showed around 133v on the secondary with no load. As I understand, this is normal with no load. However, even with a 32" CRT connected it was reading around 128-129v. Is this too high to safely operate CRT TVs or are they pretty tolerant of this voltage?

[old_tv_nut]

What is the input voltage to your transformer? Hypothetically the input and output should be the same with no load, in which case your local line voltage is that high (often the case these days). A 32 inch solid state with CRT set is new enough to have a well regulated switch mode supply, and should be no problem. To run a vintage all-tube set, however, I always use a variac as well as an isolation transformer so I can reduce the AC to the rated 117 volts, which also gives the rated B+ voltage inside the set.

[grossbard]

The wall voltage is usually anywhere from 122-124V. I guess I don't need to worry about anything as I don't plan on using this with any vintage tube stuff.

[ARC Tech-109]

I wouldn't worry about it too much, your modern 32" set probably has enough regulation room for the higher voltage. Does it have an external audio/ video input jack? If so I'd say it also has a switching power supply for the internal isolation and they're quite tolerant of wide voltage swings. Either way an isolation transformer is an excellent investment regardless of the cost.

109

[grossbard]

Okay, you guys have made me feel better. Thanks. All of my CRTS save for one are hot/cold isolated chassis. My only hot chassis set is a Sharp mfg in 1994.

[Electronic M]

There's a few ways to knock it down if you decide you want or need to. If you have a variac you could plug that in either before or after the isolation transformer and use that to dial the voltage. If the transformer is beefy you could plug in some 60W or higher incandescent/halogen bulbs to it's output to try and load it down.
If you have a 120V to 12V step-down transformer you could wire that as a bucking transformer... basically connect one 12V lead to one 120V lead, connect a plug across the 120V leads and connect an outlet to the 12V lead that isn't connected, and the 120V lead that isn't connected to the 12V winding...you may have to swap the 12V wires to achieve bucking (it will either buck line voltage 12V or boost it 12V depending on how you connect it). Google line buck boost transformer circuit to see a schematic before you wire it.

[grossbard]

Originally Posted by Electronic M

There's a few ways to knock it down if you decide you want or need to. If you have a variac you could plug that in either before or after the isolation transformer and use that to dial the voltage. If the transformer is beefy you could plug in some 60W or higher incandescent/halogen bulbs to it's output to try and load it down.
If you have a 120V to 12V step-down transformer you could wire that as a bucking transformer... basically connect one 12V lead to one 120V lead, connect a plug across the 120V leads and connect an outlet to the 12V lead that isn't connected, and the 120V lead that isn't connected to the 12V winding...you may have to swap the 12V wires to achieve bucking (it will either buck line voltage 12V or boost it 12V depending on how you connect it). Google line buck boost transformer circuit to see a schematic before you wire it.

Thanks for the info. I've actually thought about getting a variac because it seems like one of those things that would be nice to have around if needed. Can you recommend one that isn't too expensive? (under $100 preferably)

[Doug]

You Should have your general location with your member name.
Maybe member nearby would sell u one
Save on postage

[Electronic M]

Originally Posted by grossbard

Thanks for the info. I've actually thought about getting a variac because it seems like one of those things that would be nice to have around if needed. Can you recommend one that isn't too expensive? (under $100 preferably)

I always buy used from hamfests, antique radio club swapmeets, etc (the first one I bought was from the flywheeler antique engine show in Florida) and it's never been high enough to be close to $100.
It's nice to have one that can go from 0-140V both for gently starting equipment that has sat unused for years and for cases where you need to boost line voltage (say to compensate for voltage drop on a 100' extension cord or make a cheap soldering iron run hotter). VA is Watts so VA rating (or amps rating x 120V) should be atleast as high as the highest watt load you think you might connect.