# my transformer is giving 17 vac output at secondary when primary is connected to 230 vac how can we

#### IKCTRAJAN

Joined Jan 6, 2016
40
my transformer is giving 17 vac output at secondary when primary is connected to 230 vac
how can we find out current withstanding capacity of this transformer theoretically & practically.
i want to use it for making dc power supply.

#### #12

Joined Nov 30, 2010
18,224
The practical method is to look up the part number and the manufacturer and ask them.
One fellow posted and deleted a suggestion about loading the secondary until you see a 5% drop in voltage, but I understand why he deleted. Transformer design is not very predictable from one purpose to another. Size has a lot to do with it. Tiny transformers will have more voltage drop under load than large transformers. The only thing I can contribute is that the basic principle where I worked at one time was that we had 20 watt cores, 50 watt cores, 500 watt cores, etc. I could order any set of secondary windings I wanted after I added up the wattage I needed. The weight of the copper wire would be the same for any given core, fully utilized. So, the weight is the clue. How much does it weigh? Do you have another which weighs about the same and has proper labels?

Joined Jul 18, 2013
28,528
On an unknown, I usually compare the core size to a known value one and also observe the secondary wire ga.
Max.

#### #12

Joined Nov 30, 2010
18,224
Hang on. Johan did not delete. This is a duplicate Thread. John in Tx already caught the mistake.

#### JohnInTX

Joined Jun 26, 2012
4,787
@Johann replied here. It was an earlier thread so if I move his reply to here, it will be the first in the thread - confusing.

#### GopherT

Joined Nov 23, 2012
8,009
I once read a formula concerning the weight of a transformer. Measure were gauge, measure DC resistance, then calculate wire length from the resistance and wire weight from total length of wire.

The remaining weight can be assumed to be the core weight (bobbins and mounting brackets are minimal vs. core weight in most cases.

Then there is a rule-of-thumb (something like X va/kg). Maybe 50?

#### #12

Joined Nov 30, 2010
18,224
Then there is a rule-of-thumb (something like X va/kg). Maybe 50?
It's different for different power line frequencies. A 100 watt core at 60 Hz will be larger in size by 60/50 for a 50 Hz power line.
And, most people assume the same quality of grain oriented silicon steel is used for the laminations.
And the Thread Starter has not said what country or what power line frequency.

Joined Jul 18, 2013
28,528
The problem is there is no definitive formulae out there for transformer design, I once read that many transformer manuf. use a general guide and then come up with the final spec from final empirical test results.
Max.

#### alfacliff

Joined Dec 13, 2013
2,458
measure the thickness of the wire on the primary and secondary. do not excede the current rating of the wire. some transfformers are "impedance protected" or have just barely large enough wire for the primary rating, acting like a fuse.

#### hp1729

Joined Nov 23, 2015
2,304
my transformer is giving 17 vac output at secondary when primary is connected to 230 vac
how can we find out current withstanding capacity of this transformer theoretically & practically.
i want to use it for making dc power supply.
Compare the size and what you do know with stuff available in a distributors catalog. If you can identify a manufacturer and part number go to the manufacturers web site.

#### R!f@@

Joined Apr 2, 2009
9,916
I just load the secondary until I get a 5% decrease from the unloaded voltage.
While measuring the current and voltage simultaneously.

#### IKCTRAJAN

Joined Jan 6, 2016
40
The practical method is to look up the part number and the manufacturer and ask them.
One fellow posted and deleted a suggestion about loading the secondary until you see a 5% drop in voltage, but I understand why he deleted. Transformer design is not very predictable from one purpose to another. Size has a lot to do with it. Tiny transformers will have more voltage drop under load than large transformers. The only thing I can contribute is that the basic principle where I worked at one time was that we had 20 watt cores, 50 watt cores, 500 watt cores, etc. I could order any set of secondary windings I wanted after I added up the wattage I needed. The weight of the copper wire would be the same for any given core, fully utilized. So, the weight is the clue. How much does it weigh? Do you have another which weighs about the same and has proper labels?
there is no identification mark of mfger nor its part no. on the transformer.
as regards to core size (nylon bobbin) is 1" x 1 1/2"

Joined Jul 18, 2013
28,528
there is no identification mark of mfger nor its part no. on the transformer.
as regards to core size (nylon bobbin) is 1" x 1 1/2"
The overall lamination size LxWxH ? Photo?
Max.

#### #12

Joined Nov 30, 2010
18,224
as regards to core size (nylon bobbin) is 1" x 1 1/2"
If you have the technology I used in 1974, that would indicate an EI core with a 1.5 inch stack of one inch (center leg) laminations...or the other way...a 1 inch stack of 1.5 inch laminations. Look for a similar transformer and read its label to find the wattage the core can carry.

#### IKCTRAJAN

Joined Jan 6, 2016
40
The practical method is to look up the part number and the manufacturer and ask them.
One fellow posted and deleted a suggestion about loading the secondary until you see a 5% drop in voltage, but I understand why he deleted. Transformer design is not very predictable from one purpose to another. Size has a lot to do with it. Tiny transformers will have more voltage drop under load than large transformers. The only thing I can contribute is that the basic principle where I worked at one time was that we had 20 watt cores, 50 watt cores, 500 watt cores, etc. I could order any set of secondary windings I wanted after I added up the wattage I needed. The weight of the copper wire would be the same for any given core, fully utilized. So, the weight is the clue. How much does it weigh? Do you have another which weighs about the same and has proper labels?
there is no identification mark of mfger nor its part no. on the transformer.
as regards to core size (nylon bobbin) is 1" x 1 1/2"
The overall lamination size LxWxH ? Photo?
Max.
L 3" X W 2 cm X H 2 1/2"

Joined Jul 18, 2013
28,528
A
L 3" X W 2 cm X H 2 1/2"
Mix of metric and Imp? I assume that is 3" x 1" x 2 1/2"?
Max.

Joined Jul 18, 2013
28,528
I have a Hammond on the shelf exactly the same dim's and it is 50Va.
Max.

#### #12

Joined Nov 30, 2010
18,224
L 3" X W 2 cm X H 2 1/2"
Congratulations on achieving step one. Now do step two. Find a similar size transformer with proper labels and discover the wattage it is designed to transfer from the primary to the secondary.