Variac buy help

Hi I am thinking of buying a variac to slowly power up old vacuum tube ham radio transceivers capable of up to 200W or RF output power.
Now I do not have any other specs other than that so I need to find out roughly (allow for a safe margin) the power of the variac I need.
I have also seen digital variacs, obviously I am talking about a real autotransformer, cause I am going to test radios and noise matters.

Input voltage is 230-240VAC here. Some sellers specify KVA, some VA and some W. I know my specs may not be very clear, but could you give me a rough estimation of the values in these three units to look for to test this kind of gear?

Hi,

neazoi said:

Some sellers specify KVA, some VA and some W

Click to expand...

They should specify "max. output current". This is what determines the heating of the winding.
In best case they have a diagram "I_out_RMS vs V_out" or "VA vs V_out"

kVA and VA is the same, but with the multiplier "k" which means "1000". This is the typical specification for a fixed output voltage transformer.

W makes no sense at all.

****
Variacs come with true isolated output or not isolated (autotransformer wiring)
For safety reasons I recommend to choose the isolating one.

Klaus

KlausST said:

Hi,


They should specify "max. output current". This is what determines the heating of the winding.
In best case they have a diagram "I_out_RMS vs V_out" or "VA vs V_out"

kVA and VA is the same, but with the multiplier "k" which means "1000". This is the typical specification for a fixed output voltage transformer.

W makes no sense at all.

****
Variacs come with true isolated output or not isolated (autotransformer wiring)
For safety reasons I recommend to choose the isolating one.

Klaus

Click to expand...

Thank you. Could I somehow convert a 1:1 240v:240v isolation transformer to a backing transformer capable of reducing the voltage from 240v to 220v?
I am thinking that baybe two identical back to back transformers could be lossy enough to allow this small voltage variation to happen. Is that true?

You cant do that no.
You could just use the isol txfmr into a non-isol variac
I had a post on variacs here some time between dec 2021 and march 2022 sometime, variacs are terrible.....they just dont give the proper spec for many of them..the one i showed the picture of in my post thread was good for at least 800W.
I say W because i only used it for pfc'd things, so w was equal to va.

cupoftea said:

You cant do that no.
You could just use the isol txfmr into a non-isol variac
I had a post on variacs here some time between dec 2021 and march 2022 sometime, variacs are terrible.....they just dont give the proper spec for many of them..the one i showed the picture of in my post thread was good for at least 800W.
I say W because i only used it for pfc'd things, so w was equal to va.

Click to expand...

The voltage difference I am talking in my second question (apologize if being a bit out of topic) is only 10v (240v to 230v). I wonder if I can exploit the losses in a back to back transformer cinfiguration and have an output of 20v or even less for some of my radios to run cool (old transformers that run hot on them with 240v)

I dont see why you cant just put a heatsinked resistor in to take some voltage off the radio...if its not pfc'd then it woudlnt be quite as good as the peak input currents are high.....but yes, you can do it......i mean you can put in a chain of diodes to drop 10v..........do antil pllel diode chain so can handle to AC

neazoi said:

The voltage difference I am talking in my second question (apologize if being a bit out of topic) is only 10v (240v to 230v). I wonder if I can exploit the losses in a back to back transformer cinfiguration and have an output of 20v or even less for some of my radios to run cool (old transformers that run hot on them with 240v)

Click to expand...

One way to reduce the AC is to add a 220V to 10V transformer with the output in series with the 240v output but reversed phase.
The added transformer will, of course, need to be rated for the maximum load current.

neazoi said:

output of 20v or even less

Click to expand...

This is (or was) typical of model train transformers. Dial speed zero to full.

I had two as a child. They burned when I forgot to switch them off and neither worked again.

crutschow said:

One way to reduce the AC is to add a 220V to 10V transformer with the output in series with the 240v output but reversed phase.
The added transformer will, of course, need to be rated for the maximum load current.

Click to expand...

I found a great article in the web too. https://sound-au.com/articles/buck-xfmr.htm
It seems that only the secondary of the backing transformer needs to be rated at maximum current.

generally a 500W ( 500VA ) variac - would be large enough for all your radio kit ....

winding up slowly allows electrolytic caps to reform slowly and not just go bang ...