Step down transformers

Discussion in 'General Electronics Discussion' started by Karthik rajagopal, Jun 11, 2017.

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1. Karthik rajagopal

227
7
May 9, 2016
Hi all,
I have heard many saying that the input power in a transformer will be equal to the power on the secondary side of the transformer i.e the voltage will be stepped down with an increase in the current in order to maintain equal power on both sides. My doubt is that , many transformers that I have bought has both voltage and current rating on that for the secondary side whose product will not be equal to the power on the primary side say for example I have a transformer with 220v 10A rating on its primary side and with 6v 500mA rating on its secondary side. How will this be possible? Please explain

2. gurbir

12
0
Jun 11, 2017
it might be misprint..

3. Minder

3,018
640
Apr 24, 2015
Transformers are generally rated in (K)va and the rating applies to primary and secondary, The only other limiting factor is if the secondary winding is of insufficient gauge to support the current.
M.

4. duke37

5,364
769
Jan 9, 2011
The output power will be less than the input power because of heating in the transformer. Efficiency may be 80% or so.
Transformers can be rated in VA which is volts * amps taking into account power factor.
Watts = V * A * power factor
It is the current which is limits the transformer.

5. Karthik rajagopal

227
7
May 9, 2016
So does that mean the excess current is liberated as heat in order to maintain the constant current on the secondary side?

6. duke37

5,364
769
Jan 9, 2011
You will not get constant current if you increase the loading, the current will rise and the voltage will fall. The efficiency could well drop so there will be more heat in the transformer and you will not get the output current you want.

The efficiency of an unloaded transformer is zero. The efficiency will rise as the loading is increased to a certain extent and then drop again. With a short circuit load, the efficiency is zero

7. ChosunOne

379
98
Jun 20, 2010
Karthik, there may be some confusion about the transformers ratings and its actual operation. The power ratings of a transformer indicate the voltage at which it was intended to operate--220V and 6V in your example---and the currents at which it can safely operate, i.e., at which it won't overheat and possibly burn up (or more often, melt the secondary winding open somewhere, at which time the secondary current drops to zero).

In actual operation, if no completed circuit (e.g., if a power switch is open) is attached to the secondary side, no current flows in the secondary and its actual output (in that circumstance) is 6VAC and zero Amperes--but some current does flow in the primary coil, at 220V, and warms the transformer.
While the rating of the coils in your example xfmr are 220V and 6V, you could connect into North American household line voltage (110-120VAC) and it would still operate. The lowered voltage will push less current through the resistance of the primary winding, and the increased 60 Hz (as compared to India's 50Hz) will increase the reactance and further reduce the actual current flow. In that circumstance, I would expect the output to be around 3VAC.
The current capacity of transformer windings is governed by their wire diameters, not the voltage, so the safe operating currents in that circumstance would still be 10A and 500mA.

The whole point being that a transformer's ratings don't mean that the transformer actually operates at those values. In most applications, (at least, here in North America) the operating currents of transformers are considerably lower than their ratings, to maintain a margin of safety.

8. Karthik rajagopal

227
7
May 9, 2016
My doubt is that will the secondary side supply around 500 mA ? The reason for mentioning (it will be mentioned on the transformer) the current on these transformers is that to say that it can supply a maximum current of 500mA and not more than that. Here we don't have transformers based on VA ratings and we usually buy that mentioning the maximum voltage and the current to be supplied by the secondary side. I have bought these transformers from Maxine company.
Thanks

Last edited: Jun 12, 2017
9. Karthik rajagopal

227
7
May 9, 2016
I used the word 'constant' to say that it cannot supply more than 500 mA. My doubt is that when it supplies 6v with a current of 500 mA(approx), it will not be equal to the power in the secondary side.
As power on both the sides must be more or less equal, my transformer does not satisfy this condition , please help me to understand this.

Please provide explanation in an easier way so that I can understand it in a better way.
Thanks

10. Karthik rajagopal

227
7
May 9, 2016
I have attached a photo of a similar transformer that I have bought with the same ratings but from a different company (Maxine).
These were the ratings which were written on my transformer (center tap).

11. BobK

7,682
1,686
Jan 5, 2010
The answer was given in the first post. Those specifications are wrong.

Bob

12. Audioguru

2,999
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Sep 24, 2016
What is the question? The little 6V center-tapped transformer can provide a maximum continuous output of 6VAC at 500mA. Its secondary power output is 6V x 0.5A= 3VA. Its primary power input will be about 3.3VA.

13. Karthik rajagopal

227
7
May 9, 2016
How will you find out the VA for the primary side. Because having 220v and 1A( assuming a smaller value for current) will give you a rating of 220VA. Is the way that I have understood is wrong? But as per your calculations if I divide 3VA by 220v it gives a current of 0.01A, isn't it too less for the primary side of a transformer?

14. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
10mA is certainly in the ballpark. But probably closer to 15mA

15. BobK

7,682
1,686
Jan 5, 2010
The question is why the primary is rated at 240V 10A, which is 2400VA. IMH0 this is simply a mistake.

Bob

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16. Audioguru

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Sep 24, 2016
Load the transformer so it is producing 6VAC at 500mA into a 12 ohms resistor. Let it heat up for about 1 hour then feel its temperature. If it is very hot and smoking then it is using 20VA or more. If it is barely warm then it is using 3.3VA and its heating is 0.3W.

Most transformers are very efficient. But a microwave oven transformer is made as cheap as possible and gets hot because it is never used continuously and is cooled by its fan.

17. Karthik rajagopal

227
7
May 9, 2016
No actually it is given 220v , but I thought that it will consume the full current available but that's actually wrong. Got it now.

18. Karthik rajagopal

227
7
May 9, 2016
I thought that the primary side will atleast draw an ampere from the supply. I didn't realise that it would consume as less as 15 mA.
Hence got the answer for my question.

Thank you all.

Last edited: Jun 13, 2017
19. Audioguru

2,999
672
Sep 24, 2016
The battery in my car can produce 400A when cranking the starter motor and cold engine and its very thick oil in winter. Then the power from the battery is 12V x 400A= 4800W. But the clock in the car uses the same 12V battery and if it used 4800W then it would be white-hot and the battery will be dead in one minute.

My clock radio is plugged into my AC electricity all the time but does not use the 1500W that is available, instead it uses about 1W, 0.2W for heating its transformer and 0.8W for its display and heating its other parts.

20. Sanu

9
1
Feb 14, 2020
If the secondary voltage is greater than a primary voltage, a secondary current is less than the primary voltage. A transformer adjusts voltage as well as current in such a way that the input power of the generator is equal to that of the input power (less for transformer losses).