I just got a used Triad model N-55M isolation transformer. 115/115 @ 250va
It seems to be working alright but even when there is no load at all on the secondary the transformer is slightly warm. Is this normal?
Also the primary has exactly 119.5v on it and yet at the same time the secondary kicks it up to 124.2v ... Why is this I wonder?
The primary measures about 13 ohms ( seems to jump around a bit which I find curious too.. but when it settles down it stops around 12-13 ohms ) on my Radio Shack Digital multimeter and the secondary measures 1.5 ohms.
Is this all normal?

Peter, I think you're OK - assuming radios, etc. work properly when plugged into the secondary side.

We think of iso xfmrs as ideal 1:1 xfmrs. Well, there is certain voltage regulation thru any xfmr. I think some manufacturers try to account for this by increasing the ratio a little.

Your 250VA xfmr should have plenty of capacity for most any tube-type radio. I think mine is 500VA, but it's oversized for most of what I use it for.

Try this: plug a load into the secondary - a good-sized radio, a soldering iron, etc. Measure the primary and secondary voltages. They'll probably be closer than when the xfmr is unloaded.

Your transformer should be fine. The reason for the heat is that the wire that the transformer is wound with, or any transformer for that matter, has resistance. The heat is developed because of the resistance of the wire and the voltage drop through it. Your transformer will read higher on the secondary when there is no load. The voltage will drop when the transformer is loaded due to resistance of the windings and core losses. When you measure the resistance and get fluctuating readings the meter probe resistance and the quality of the contact you are making with the transformer leads comes into play. You may also be picking up some ac noise from some other source.

As Mark says, there are core losses in a power xfmr caused by continually magnetizing and remagnetizing the iron core. These core losses are the same no matter whether the xfmr is loaded or unloaded. So, even an unloaded xfmr will produce some heat.

Not to worry.

:As Mark says, there are core losses in a power xfmr caused by continually magnetizing and remagnetizing the iron core. These core losses are the same no matter whether the xfmr is loaded or unloaded. So, even an unloaded xfmr will produce some heat.
:
:Not to worry.

Okay you guys thanks a lot. I did put it under mild load (25w solder iron & 30w radio).. still 5v higher on 2ndary but I understand.
I was just a little concerned as it semms a bit 'toasty' to the feel.. with no load. In fact I just measured it.
( lol... after first blowing the fuse on my digital multimeter cause I had the probe on the LOW a.c. amp scale (4oo ma max) not the 10 amp a.c scale. scale...ooops)
Anyway...I see that it draws 200 ma with no load on the 2ndary.
[Now actually the meter should not have blown the fuse because the draw (200ma) is only half the max (400ma)for that scale..but I guess the inital jolt and some juicy back-EMF while making intermittent contact is what did it.]Well live and learn. Live being the operative word there.
Speaking of life and living... safety being the original goal here...I took things to the next step.
So while I was that far in to my 'mad-scientist' spark-making, fuse-blowing role.. I decided to see how much life-saving isolation it actually affords me.

So I measured each side of the 2ndary to ground. One side shows 40v ac and the other shows 120v ac.
40 volts I can see.. phantom or whatever but 120? Well that scares me a bit. So I decided to see how real it is.
So I dug out an old porcelean light socket and put a light bulb in there and put one side to the isolation xformer 2ndary and the other side to ground.

Well just like that sexy blonde in your dreams that disappears so quickly as you awake....sure enough... nothing... the bulb won't light and the voltmeter that was displaying 120 volts at that same point NOW disappears.. and it shows zero volts! Great!
Ahhh saftey achieved ... even if this self/training lesson cost me a fuse for my meter.
Hey...At least there was a nice new spare fuse tucked right in there near the holder.
I guess Radio Shack knows their audience!

Get yourself an auto-ranging Fluke DMM, and kiss fuses goodby.

:Get yourself an auto-ranging Fluke DMM, and kiss fuses goodby.
Thanks Doug... which model do you use? I saw a Fluke 27 here:
http://store.yahoo.com/webtronics/fluk27andmm.html

but it has the same 2 separate amp sockets both fused and rated the same as my RadioSchak model

Well, I don't know about that particular model. I've had my Fluke 179 for a couple of years. Through my customary personal negligence, I've had >450V on every function on the DMM - ohms, you name it - with no problem. There may be a fuse inside, but it's never blown.

:Well, I don't know about that particular model. I've had my Fluke 179 for a couple of years. Through my customary personal negligence, I've had >450V on every function on the DMM - ohms, you name it - with no problem. There may be a fuse inside, but it's never blown.

Ok great Doug:... I see now, as I look up the Fluke 179:
That although , just like mine it has 2 separate fused amp jacks: a 400ma and a 10amp.... however very much un-like mine your specs claim a 20 AMP overanging for 30 seconds!!! Wow that's the really great part. That's certainly gonna save the world's supply of edangered fuses that's for sure. lol