How to determine transformer rating?

I've got a bunch of mains transformers lying around that give out 12V AC which is perfect for all my electronics projects.

How do I determine the VA of the transformer if it has no markings on it. I don't want to start using it and then have it melt down on me as I'm drawing too much current from it.

Would I need to put a load on it and then if it gets warm I'm overloading it? Sounds dodgy - there must be another way?

A rough way is to look up 12V transformers and note the physical size versus output current. That should give you a good estimate of the power your transformers can deliver. Do note the transformer temperature when you use it. All transformers get warm under normal used but if it gets too hot to hold a finger on it, then it likely is at or over its rating.

gregmcc said:

I've got a bunch of mains transformers lying around that give out 12V AC which is perfect for all my electronics projects.

How do I determine the VA of the transformer if it has no markings on it. I don't want to start using it and then have it melt down on me as I'm drawing too much current from it.

Would I need to put a load on it and then if it gets warm I'm overloading it? Sounds dodgy - there must be another way?

Click to expand...

What you are proposing is a reasonable scientific approach. A bit of understanding of Thevenin's theorm is required to determine how good your circuit will operate on your load.

1. Use a DVM to measure the open circuit voltage. write it down.
2. Load it with a power resistor rated to draw the expected current or the load itself.
3. Measure the voltage across the load when connected to determine what is being dissipated inside the transformer. This will be power= (Vopen - Vload) x (Vopen - Vload) / Rload
4. I would say (others will disagree) that if your transformer is dissappating >25% then it's not upto the job long term.

Reply if this is not clear.

Thanks for the explanation. I'll try that out.

Thumb Rule from the 60's.

Core Area 1 Sq Inch => 30 VA

VA is proportional to Area Square => Core Area 2 Sq Inch => 120 VA

Ramesh

hi Ramesh,

Another rule of thumb from the 60's

Core Area 1 sq Inch = 6 turns per volt.

Eric

I concur, simply compare the size of the transformer to ones available - similar sizes have similar ratings.

Years ago I had a BIG, BIG row with Sony over this. We had repeated failures of the mains transformer in a Sony amplifier which was rated at 100W RMS continuous per channel.

The toroidal transformer looked small in the casing, with LOT'S of available space around it. Comparing it's size to available transformers it looked like only a 60W transformer, ludicrous for it's 100+100W RMS continuous claimed power. However, after months of arguing with Sony the only response I got was that there had been hardly any failures of them.

Considering the space available, I presumed (and still do) that a smaller cheaper transformer had been introduced during production to save costs - but Sony wouldn't admit it.

I replaced it three times, originally under guarantee, and twice more forcing Sony to send me a free transformer - I was hoping it would come back again and I could scrap it, and put a decent sized transformer in for myself

I had something similar think it might have been a tuner amp, smally tranny in a big hole burnt out (or maybe a thermal fuse on the primary), I assumed the previous owner being a 'rig doctor' had bunged one in that was too small.
Maybe not by the sound.

dr pepper said:

I had something similar think it might have been a tuner amp, smally tranny in a big hole burnt out (or maybe a thermal fuse on the primary), I assumed the previous owner being a 'rig doctor' had bunged one in that was too small.
Maybe not by the sound.

Click to expand...

Maybe not, it could be a similar situation - the one I did was just an amp, and a really nice one, big heatsinks and everything - but a tiny transformer (for the rated output).

Hi,

I heard of this happening before too. Similar circumstances too with a toroid power transformer in a stereo system. I cant remember what we did to fix it though, aside from installing a screw into the front panel to stop the volume control from going all the way up (chuckle). Seriously though, cant remember what we did. I think i recommended a replacement transformer that did not come from the original manufacturer so the full power would be there. We just had to hope that they didnt do the same with the power transistors!

But i just recently saw another little replace it with a cheapie grabber. I ordered a graphics card and via the picture and model number on the manu's web site and the advertisement i first saw it in i could see a rather largish heat sink. So large it had to wrap around the whole card, not just on one side. Well, when i got the card i was very surprised to see a smaller heat sink, and it was very noticeable because this heat sink was all on one side of the card and was not nearly as gigantic.
Made me wonder, but it seems to work ok so i'll see how it goes. Made by XFX.

Hughs and Kettner make a couple of studio amps which are very nice, and sound good, they also have tiny transformers, except you can thrash them and they dont burn out.
Hughs say is because of their efficient design, the amp is linear not a switcher.

When I compare stereo receivers of similar power output rating, one of my odd criteria is the weight of the receiver. My reasoning is that most of the guts of the receivers are probably quite similar with comparable weight, so the receiver that weighs the most likely has the biggest power transformer and is thus likely to deliver more power to all channels simultaneously. In some cases I have seen significant differences in weight (like 10 pounds for high power receivers).

Following is a summary of a method that I posted on another forum:

As long as the internal temperature doesn't rise above 90°C with the intended load, then it should be okay. It's quite easy to determine internal temperature based on the change in DC resistance of the primary winding. The DC resistance of the winding changes with temperature, and for copper it's 0.0039 ohms per ohm per °C. So, simply measure the primary winding resistance before powering up the transformer and also make note of the room temperature. We'll call this the cold resistance Rc and cold temperature Tc. Then run the transformer under load for about 45 minutes, then disconnect power and measure the resistance again. We'll refer to this resistance as Rh. The internal temperature is then:

Th=Tc+Rh/(0.0039*Rc)-1/0.0039

For doing the test, it may be more convenient to determine in advance, the resistance value that corresponds to 90°C. So, then it's simply a matter of making sure that the winding resistance doesn't exceed this value. The formula is:

Rh=Rc*(0.0039*(Th-Tc)+1)

So, let's say you measure the transformer primary at 20°C (room temperature), and get a value of 150 ohms. Applying the above formula, Rh comes out to be 191 ohms. So, as long as the hot resistance of the primary never exceeds 191 ohms you should be okay.

To put it even more simply:
The hot resistance of any transformer winding should never be more than 1.27 times the cold resistance.

You'll want to start with a conservative load, and as long as the operating temperature stays below 90°C, then you can increase it, and run the test again. If you'd rather put the full load on it right away then you should take frequent resistance readings to make sure that the transformer doesn't go above a safe temperature.

I recommend measuring the primary winding resistance because its temperature rise will account for all losses, will usually be the inside winding which is most critical for temperature, and will generally have a resistance that's high enough to measure with reasonable accuracy.

I should also mention that 90°C is too hot to touch, and so you may consider that to be pushing things a bit too hard. However, this is referring to the internal temperature, not the surface temperature of the transformer, which should be somewhat cooler. However, if the surface is ever too hot to touch, then you may want to derate it a bit, even though virtually all modern transformer insulation is designed for a minimum of 90°C.

Hi, Bob,

We haven't corresponded about inductance for a while, but I see you have been doing some more research.

I can't speak for vintage transformers, but the lowest temperature rating for modern magnet wire insulation is 105°C. See the thermal classes here:

https://www.electro-tech-online.com/custompdfs/2013/06/Pg_g.pdf

The temperatures given are the hot spot temp, usually the middle of the inner winding.

Inferring winding temperature by measuring the DC resistance is a standard method. The other way is to bury a thermocouple in the winding, but that's a little hard to do after the transformer is wound. Another good reason to measure the primary rather than the secondary is that when the transformer is hot, trying to measure the rather low resistance of a winding of heavy wire can be substantially in error due to the thermoelectric voltage upsetting the ohmmeter reading. What I do if I must measure the secondary is to pass a regulated 1 amp through the secondary and measure the DC voltage drop across the winding--this voltage will be large enough to swamp out the thermoelectric voltage.

I designed class H transformers, and when you touched one of those after it had been operating at full load for a few hours, you got burned! We couldn't use ordinary nylon bobbins because they would melt; only expensive polyimide would do.

Nigel's advice to compare it to available transformers is good. Look up available transformers on the Stancor and Triad web sites.

I got the numbers for a lot of those commercially available trnasformers and plotted power capability vs. weight. These are not milspec or other high performance transformers, so they most likely are class A temperature rating--105°C hotspot max. The data show surprisingly little scatter. This graph will give a fairly good first cut at determining the power handling capability of a transformer. Just weigh the transformer and find the intersection of the straight line with the weight; read off the power handling capability. Then knowing the output voltage of the secondary, divide that voltage into the power rating and get the AC RMS current rating. Multiple secondaries share the total power. If using the transformer for a capacitor input rectifier circuit, then derate appropriately.

Impressive graph. That formula should come in quite handy.

I agree that 90°C is fairly conservative, especially when considering the transformer by itself, but if it's installed inside a chassis that's not well ventilated, I get a bit concerned about the effect it will have on other components, especially electrolytic caps and some power semiconductors.

BobW said:

Impressive graph. That formula should come in quite handy.

I agree that 90°C is fairly conservative, especially when considering the transformer by itself, but if it's installed inside a chassis that's not well ventilated, I get a bit concerned about the effect it will have on other components, especially electrolytic caps and some power semiconductors.

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Of course. The hobbyist will have to pay attention to that. The long time way of rating these things dates from the days when radios, TVs. etc., used vacuum tubes. The inside of cabinets got hotter in those days. The standard assumption was that the inside of a cabinet would reach a maximum of 40°C, and a 65°C rise above the 40°C ambient would put a transformer hot spot at 105°C. So all the other components were expected to be possibly as hot as 40°C if they didn't dissipate any power of their own. The transformer wasn't as great a heat source as the tubes.

The chart I posted assumes the transformer is convection cooled, sitting in the open. If a muffin fan is placed a couple of inches away from the transformer, providing a good blast of air, the allowable transformer dissipation will about double, providing about 1.414 times the output power.

An interesting and comprehensive approach.
Industrial motors are wound to a class of insulation, the highest in common use I see is class H which withstands 180 degree's, I dont know if commercial stuff is built to similar specs, but if a trans is marked with its insulation class it a mere job of looking up on the table what temp it'll withstand.
As well as the enamel on the copper/alu wire having to withstand whatever temp rating is specified, also the paper/melananine spacers between windings and windings/core have to be rated similar as well.
If you were to rely on a cooling fan then a thermal fuse would be a good idea.