Recently I made a transformer for my project which outputs 29VAC (required for project is 28VAC) and that works just fine with small load. The problem starts as soon as big load is attached.
I have a transformer core and bobbin for the same lying around which has slightly bigger dimensions than the older one. I want the transformer that has larger output current, 4.2A as per the components list.
I have had seen many videos on YouTube people removing the secondary of transformer replacing that with a thicker wire and thereby melting down metal objects.
What actual calculations are need to be done to get output above 28Vac at 4.2Amps?

 

hi rahul,
Check this link out.
http://engineerexperiences.com/design-calculations.html
E

 

This may help.
http://engineerexperiences.com/design-calculations.html
There are quite a few sites that can help you calculate the power a transformer core can supply.
Did you already rewind the secondary of your first transformer?

 

Did you already rewind the secondary of your first transformer?

I checked the continuity of primary it's not showing... So i need to unwind the primary too.
Thanks for help... I'll go through that site.

 

Maybe you could find another, like from an old tube style TV or a microwave oven?
But if you need to rewind the primary, count the turns you take off so you know how many to put back on. Also, look first to see if there is a thermal cutout or fuse in the transformer.

 

Hello,

What is the size of the current transformer?
This table might give you an idea about the size versus power:



Bertus

 

I like that and will keep a copy. Thanks Bertus

 

A general rule of thumb for EI cored mains transformer, is 6 turns/Volt per square inch of the central limb.
eg:
One sqr inch = 6 turns/volt, two square inches = 3 turns/volt.

 

Recently I made a transformer for my project which outputs 29VAC (required for project is 28VAC) and that works just fine with small load. The problem starts as soon as big load is attached.
I have a transformer core and bobbin for the same lying around which has slightly bigger dimensions than the older one. I want the transformer that has larger output current, 4.2A as per the components list.
I have had seen many videos on YouTube people removing the secondary of transformer replacing that with a thicker wire and thereby melting down metal objects.
What actual calculations are need to be done to get output above 28Vac at 4.2Amps?

This is a very complex theory. Its not that easy to change a transformer winding or calculate one. Please while using the transformer (new or old) leave it in a safe environment (box that can not burn or something similar) and keep your distance, so you do not burn the house or kill yourself!

 

A general rule of thumb for EI cored mains transformer, is 6 turns/Volt per square inch of the central limb.
eg:
One sqr inch = 6 turns/volt, two square inches = 3 turns/volt.

Yes, since the formulas are complex, a general rule of thumb is used.

 

Maybe you could find another, like from an old tube style TV or a microwave oven?
But if you need to rewind the primary, count the turns you take off so you know how many to put back on. Also, look first to see if there is a thermal cutout or fuse in the transformer.

I really don't know from where that transformer came from. I scavenged it during my childhood and I can't remember now.

 

hi rahul,
A transformer 28V at 4.2A, is very roughly 120VA, so I would wind for 150VA.
So using the table posted by @bertus, its finished weight should be ~ 5lbs.
What is the weight of you existing transformer.?
E

 

I like that and will keep a copy. Thanks Bertus

What does core value mean?
And what would be VA rating of my transformer which will output 29V and 4.2A I have read on QUORA that it's just a multiplication of voltage and current of secondary, only for hobbyist. There's a big calculation needed to be done to get VA rating. Is it?

 

hi rahul,
A transformer 28V at 4.2A, is very roughly 120VA, so I would wind for 150VA.
So using the table posted by @bertus, its finished weight should be ~ 5lbs.
What is the weight of you existing transformer.?
E

Sorry Eric, I don't have a weighing machine right now. But I'll manage it from somewhere and will let you know the weight.

 

hi,
Kitchen scales should cover upto 10 lbs +
Look at this link for wire gauge ratings.
https://en.wikibooks.org/wiki/Engineering_Tables/Standard_Wire_Gauge

E

 

hi,
Kitchen scales should cover upto 10 lbs +
Look at this link for wire gauge ratings.
https://en.wikibooks.org/wiki/Engineering_Tables/Standard_Wire_Gauge

E

My uncle owns a industrial motor repair shop, so I have access to all the guages of copper wire.

 

"VA" rating is very simply the product of the voltage and current at full power. It equates to watts at the output if the load is non-reactive (that is, the load is a resistance). A significant part of transformer rating is allowable temperature rise, and most of the loss in an iron core transformer operating at AC mains frequency is due to resistance of the windings. For a given resistance, the power is proportional to the square of the RMS current. If you rectify and capacitively filter the output of the transformer, the RMS current will be higher than the average current and the average power you can get from the transformer for the allowed temperature rise will be less than the VA rating. Because transformer manufacturers don't know exactly how a transformer will be used, rating in VA is the most sensible thing to do - of course you need to know the voltage and current, but VA gives you a quick comparison figure.