How to Measure the Power of a Transformer

Difficult to say. Is it a ferrite core transformer, or really a can with something in it?

You have nothing printed on it, or stamped? The size and weight of the core can give you an indication, but when it is a pot, how much fill product is in it? Now a 2 X 15 watts amp doesn't require a big transformer.

If you have 50 watts available, it should work properly. I would put e.g a 50 watts load on it, power it up and check the temperature of the cylinder. If it is becoming hand warm it is OK.

When it is getting hotter than 80 degrees Celsius, I should still use it, depending on how much power you apply to the speakers. If you can give the size of the can or the core I can be of more help to you, because the two cylindrical types may differ a lot (factor 3?)

stevem,

Clipping is the term I was looking for.

Thanks.

Your last point is for temperature rise due to saturation leading to harmonics production and energy loss. This will also generate some audible sound due to core vibration as part of the harmonics will generate audible frequencies.

One can hear that transformer is no longer capable to deliver power and has reached a saturation level. This technique is rather simple but effective one. Transformer hum sound is due to core saturation.In fact saturation core transformers create sound.

You have made a very good point here. Monitoring the primary current with increasing load until primary current stops increasing will easily tell the secondary have reached to its limit to deliver power and core is saturated now. Output waveform will also deform at peak voltage and its shape to be seen only in oscilloscope and AC current meter not to be taken for finding saturation point in this experiment.

Excellent answer.

Yes, that makes a lot of sense. Thanks. BTW the transformer is a cylinder almost 4 inches tall (95 mm) and three inches in diameter 75 mm. It reads ITC (brand name) D-7784. Do you think I can use a variac to change the load on the secondary?

If you are testing for inductive load capacity then a variac would work, just make sure the variac is large enough.

If yours weighs much less than the OEM spec unit of 5lbs, quoted in post #2, then it probably won't be sufficient.

If I were you, I'll check the gauge of the magnet wires used in the transformer, both in the primary and the secondary windings. Look up the closest area-wise dimension in cm on a wire table for magnet wires. The size of the magnet wires are generally directly proportional to the amperage or current carrying capacity and power a device can safely handle or provide. If I remember correctly, the wire size criteria I was using in my designs was based anywhere from 750 to 1000 cm (circular mil) area-wise per ampere of current capacity, depending on the design type that can safely provide rated power.

I wish I could. The transformer is sealed and all the terminals are at the bottom of the cylinder, each with his own ceramic insulator...

It seems you have a transformer that was sealed in tar or asphalt type of enclosure? The terminals at the bottom if visible to you are also where the transformer windings will be terminated, and soldered. If that's the case, an AWG gauge can be place close to each termination to estimate corresponding wire size?

Measure the cold resistances of ALL windings. Apply a load to both secondaries (start conservatively), apply juice, and heat soak for at least a half-hour, preferably more. Disconnect and re-measure ALL resistances. A 17% increase represents 40C rise in temperature, which is about enough for modern transformer windings. If you don't get that amount of resistance increase, put on a heavier load, and re-soak. Of course, as already pointed out, also listen for excessive hum/rattle and check output voltages, just in case you have stuffed up the loading, or the manufacturer has been unusually generous with the gauge of copper wire.

all i can say is that it is possible that the terminals can change everything if of course they are off standard enough.drastically different.homemade etc.no assumptions.im just a roofer.bill

Do not forget that for a tube amp a considerable wattage is pulled just for heating the tube filaments. How many tubes are you using in the amp? You can look up the power draw for the filaments for the types of tubes you are using. Is your 6volt winding heavy enough? It will also effect the current in the primary of the transformer.

I think this is the circuit he is using. Only one channel is shown:

Thanks you Anonymous Hero for posting the schematics. There will be six filaments total.

Today I weighted it. It is 1,648 grams, 3.1 pounds, way less then the five pounds of the original. Maybe I should consider buying the clone from Triode Electronics....Too bad because this one looks so cool!!! Well, it can power a preamp for sure.

The good thing is that I found the original output transformers on ebay so this amp should sound really awesome. Interesting schematics uh? Can't wait to see those tubes lighting up!!! Thank you all.

Yeah, I can identify with your project. I use mine every day. It has a certain charm about it.

You can't go wrong with the Triode Electronics transformer. It is wound to Dynaco specs using the latest materials. Better than the originals.

Same goes for the output transformers. My understanding is the modern day iron is better than the original stuff, which was of good quality.

If you want more help with the project check this forum out:

DIY Audio

You can start tests in the following sequence

1. Test the continuity of the primary and secondary coils and see if they are OK and do not have discontinuity

2. Measure the no load voltage output and apply desired current at specified voltage load preferably the one specified by the manufacturer and see the output if it is what it is supposed to be.

3. Overload the transformer secondary coil in increasing steps and feel it if it is becoming uncomfortably hot and is also try to hear if it is creating audible sounds. Stop at that point and remain below this limit in planning the transformer load limit.

In a transformer, how can you tell which one is the primary or the secondary by just measuring continuity if there's no available labeling?

Continuity is measured only to know if coil is not burnt out or got disconnected in inside wiring. I

n real sense in transformers there is no primary or secondary but there are number of separate windings in which one is used as primary to feed-in power to the transformer or the one meant to be primary for the required purpose such as step up or step down or isolation is used as primary.

Primary is often inner winding in overlapping windings for various reasons. There are other types of transformers where windings are separated or located elsewhere and they are only magnetic flux linked to work as transformer.

Autotransformer use same winding as input and output and are more like variable resistance but have special capability to boost the voltage which resistance can not do.

Current transformer have only one winding done on the core and conductor inserted for measuring current acts like a primary winding with less than one turn effectively.

Electron or ion beam sensing transformer directly use charge sensing capability of current transformer and charged particle beam passing through transformer hole itself acts like a primary winding with actually no winding at all.

Some electrical engineer can give you more insight into power distribution transformers and other special transformers.

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I was going by your posted instructions labeled 1, 2, and 3. when I raised the question. In step 3, you labeled it as the secondary winding. It seems there is a contradiction in your statement? In applying power. as per your instructions, since there are several windings available on that transformer, to which winding should it be applied and how much? You may end up cooking the unit under test?

You can apply load on any secondary winding and it really does not matter how many windings are not used in measurement of power of the transformer.

Primary and secondary only means that in some coil you are giving input power and secondary means the coil you are loading.

You can load all secondary coils if they are separate but single secondary coil with multiple tapping's to be loaded at its ends where there is maximum voltage in the secondary.

Reason why small portion of secondary can not be loaded for power testing is that small portion of coil is often not wired to entire power through it and it may simply burn out at full power of the transformer. Total power of the transformer is through all coils loaded according to their ratings.

For valve transformers, filament coil can deliver high current at low voltages but coil meant for anode plate voltage can supply only limited current. Load does not mean a fixed resistance but a V*V/R selected for coil rating.

Assume that I have 220-0-220V 440W secondary coil rated for 1A. If I use only 0-220V portion for testing then I need to load it to 2A for which coil may not have strength to take that high current and it may simply burn out even though I am loading transformer only for rated capacity of 440W.

Testing through single secondary coil in multiple secondary coil transformer not a very correct way even though sometime it simply may work as coils may have thick wire to accept more than rated current, but that is not to be taken for granted as it may permanently damage the transformer if it is really not designed in that manner.

Some transformers designed for pulse voltages can accept very high voltage and high load for short time. Even on normal transformers if you try pulsed mode for testing then you can find saturation level easily without damaging coil and can go far beyond the rating for low frequency. Only limit will be the maximum frequency a transformer can accept. Unless you have proper equipment and have expertise in this area, don't try this method.

Do you still have a question?

Yes Prof, at the beginning you claimed there is no such secondary nor primary windings in transformers? Now in most of your statements you are refer to them as "secondary" windings? You also refer to "resistance" as in "V*V/R" are you referring to winding resistances or impedance of the device? You also mentioned "pulse transformers" where you can feed pulses? You also mentioned several types of windings, are you trying to cover the whole transformer subject or you are just trying to respond to the OP? It seems to me that you are trying cover the whole transformer subject? That is what actually made me more confused? Although there are several types of transformers, types that normally depends on their intended application. They can be made as an induction type or an auto-type transformer. Windings can also be interlaced as in the output type of audio application. There are those that uses ferrite cores for high frequency type, high voltage type, saturable core type of application, pulse transformers, etc. It will be less confusing for your readers if you can only limit your advise to what the OP wanted to find out? And my understanding was, he has a power transformer for a radio amplifier type of application that he wanted to test, to find out if its usable for his project. I, as a reader also wanted to learn if there are any other ways, besides the ones I already knew.

Resistance is the pure resistive Load.

I agree that primary should be able to take the entire load of the transformer drive capability and hence if there are many secondaries then they can not be used as primary as their rating may not be same to the primary which is often designed for extra load capacity.

I do use 12-0-12v output step down transformers to regenerate 230V by switch secondary and loading primary. This assumes that both primary and secondary are made for full load capacity. I am not sure if this is true all the time.

If there are more number of secondaries then there is a problem in making any one of them primary coil as any one coil has limited and partial drive capacity of the current.

In such transformers, you can not assume reversibility in full sense.I agree that there is some problem in this case.

Sometime secondaries are wired like autotransformer by connecting two coils and each having a purpose and different current capacity. In such cases also this problem exists that entire coil can not accept full current load as in real sense they are partially two different rating coils with one common reference point.

I agree that generalizing the idea to make simple does not go well with complex minds of engineers who know many types of problems that may exist in design so use your own brain in complex designs.